diff --git a/src/wasm/ast-decoder.cc b/src/wasm/ast-decoder.cc
index e2f6a04..b8a86c3 100644
--- a/src/wasm/ast-decoder.cc
+++ b/src/wasm/ast-decoder.cc
@@ -42,17 +42,6 @@
   WasmOpcode opcode() const { return static_cast<WasmOpcode>(*pc); }
 };
 
-// A production represents an incomplete decoded tree in the LR decoder.
-struct Production {
-  Tree* tree;  // the root of the syntax tree.
-  int index;   // the current index into the children of the tree.
-
-  WasmOpcode opcode() const { return static_cast<WasmOpcode>(*pc()); }
-  const byte* pc() const { return tree->pc; }
-  bool done() const { return index >= static_cast<int>(tree->count); }
-  Tree* last() const { return index > 0 ? tree->children[index - 1] : nullptr; }
-};
-
 // An SsaEnv environment carries the current local variable renaming
 // as well as the current effect and control dependency in the TF graph.
 // It maintains a control state that tracks whether the environment
@@ -72,19 +61,30 @@
     control = nullptr;
     effect = nullptr;
   }
+  void SetNotMerged() {
+    if (state == kMerged) state = kReached;
+  }
 };
 
-// An entry in the stack of blocks during decoding.
-struct Block {
-  SsaEnv* ssa_env;  // SSA renaming environment.
-  int stack_depth;  // production stack depth.
+// An entry on the value stack.
+struct Value {
+  const byte* pc;
+  TFNode* node;
+  LocalType type;
 };
 
-// An entry in the stack of ifs during decoding.
-struct IfEnv {
-  SsaEnv* false_env;
-  SsaEnv* merge_env;
-  SsaEnv** case_envs;
+// An entry on the control stack (i.e. if, block, loop).
+struct Control {
+  const byte* pc;
+  int stack_depth;    // stack height at the beginning of the construct.
+  SsaEnv* end_env;    // end environment for the construct.
+  SsaEnv* false_env;  // false environment (only for if).
+  TFNode* node;       // result node for the construct.
+  LocalType type;     // result type for the construct.
+  bool is_loop;       // true if this is the inner label of a loop.
+
+  bool is_if() { return *pc == kExprIf; }
+  bool is_block() { return *pc == kExprBlock; }
 };
 
 // Macros that build nodes only if there is a graph and the current SSA
@@ -157,30 +157,50 @@
     return false;
   }
 
-  inline bool Validate(const byte* pc, FunctionIndexOperand& operand) {
+  inline bool Validate(const byte* pc, CallFunctionOperand& operand) {
     ModuleEnv* m = module_;
     if (m && m->module && operand.index < m->module->functions.size()) {
       operand.sig = m->module->functions[operand.index].sig;
+      uint32_t expected = static_cast<uint32_t>(operand.sig->parameter_count());
+      if (operand.arity != expected) {
+        error(pc, pc + 1,
+              "arity mismatch in direct function call (expected %u, got %u)",
+              expected, operand.arity);
+        return false;
+      }
       return true;
     }
     error(pc, pc + 1, "invalid function index");
     return false;
   }
 
-  inline bool Validate(const byte* pc, SignatureIndexOperand& operand) {
+  inline bool Validate(const byte* pc, CallIndirectOperand& operand) {
     ModuleEnv* m = module_;
     if (m && m->module && operand.index < m->module->signatures.size()) {
       operand.sig = m->module->signatures[operand.index];
+      uint32_t expected = static_cast<uint32_t>(operand.sig->parameter_count());
+      if (operand.arity != expected) {
+        error(pc, pc + 1,
+              "arity mismatch in indirect function call (expected %u, got %u)",
+              expected, operand.arity);
+        return false;
+      }
       return true;
     }
     error(pc, pc + 1, "invalid signature index");
     return false;
   }
 
-  inline bool Validate(const byte* pc, ImportIndexOperand& operand) {
+  inline bool Validate(const byte* pc, CallImportOperand& operand) {
     ModuleEnv* m = module_;
     if (m && m->module && operand.index < m->module->import_table.size()) {
       operand.sig = m->module->import_table[operand.index].sig;
+      uint32_t expected = static_cast<uint32_t>(operand.sig->parameter_count());
+      if (operand.arity != expected) {
+        error(pc, pc + 1, "arity mismatch in import call (expected %u, got %u)",
+              expected, operand.arity);
+        return false;
+      }
       return true;
     }
     error(pc, pc + 1, "invalid signature index");
@@ -188,9 +208,13 @@
   }
 
   inline bool Validate(const byte* pc, BreakDepthOperand& operand,
-                       ZoneVector<Block>& blocks) {
-    if (operand.depth < blocks.size()) {
-      operand.target = &blocks[blocks.size() - operand.depth - 1];
+                       ZoneVector<Control>& control) {
+    if (operand.arity > 1) {
+      error(pc, pc + 1, "invalid arity for br or br_if");
+      return false;
+    }
+    if (operand.depth < control.size()) {
+      operand.target = &control[control.size() - operand.depth - 1];
       return true;
     }
     error(pc, pc + 1, "invalid break depth");
@@ -199,6 +223,10 @@
 
   bool Validate(const byte* pc, BranchTableOperand& operand,
                 size_t block_depth) {
+    if (operand.arity > 1) {
+      error(pc, pc + 1, "invalid arity for break");
+      return false;
+    }
     // Verify table.
     for (uint32_t i = 0; i < operand.table_count + 1; i++) {
       uint32_t target = operand.read_entry(this, i);
@@ -229,46 +257,49 @@
       case kExprLoadGlobal:
       case kExprNop:
       case kExprUnreachable:
+      case kExprEnd:
+      case kExprBlock:
+      case kExprLoop:
         return 0;
 
-      case kExprBr:
       case kExprStoreGlobal:
       case kExprSetLocal:
+      case kExprElse:
         return 1;
 
+      case kExprBr: {
+        BreakDepthOperand operand(this, pc);
+        return operand.arity;
+      }
+      case kExprBrIf: {
+        BreakDepthOperand operand(this, pc);
+        return 1 + operand.arity;
+      }
+      case kExprBrTable: {
+        BranchTableOperand operand(this, pc);
+        return 1 + operand.arity;
+      }
+
       case kExprIf:
-      case kExprBrIf:
-        return 2;
-      case kExprIfElse:
+        return 1;
       case kExprSelect:
         return 3;
 
-      case kExprBlock:
-      case kExprLoop: {
-        BlockCountOperand operand(this, pc);
-        return operand.count;
-      }
-
       case kExprCallFunction: {
-        FunctionIndexOperand operand(this, pc);
-        return static_cast<int>(
-            module_->GetFunctionSignature(operand.index)->parameter_count());
+        CallFunctionOperand operand(this, pc);
+        return operand.arity;
       }
       case kExprCallIndirect: {
-        SignatureIndexOperand operand(this, pc);
-        return 1 + static_cast<int>(
-                       module_->GetSignature(operand.index)->parameter_count());
+        CallIndirectOperand operand(this, pc);
+        return 1 + operand.arity;
       }
       case kExprCallImport: {
-        ImportIndexOperand operand(this, pc);
-        return static_cast<int>(
-            module_->GetImportSignature(operand.index)->parameter_count());
+        CallImportOperand operand(this, pc);
+        return operand.arity;
       }
       case kExprReturn: {
-        return static_cast<int>(sig_->return_count());
-      }
-      case kExprBrTable: {
-        return 1;
+        ReturnArityOperand operand(this, pc);
+        return operand.arity;
       }
 
 #define DECLARE_OPCODE_CASE(name, opcode, sig) \
@@ -281,7 +312,6 @@
         FOREACH_SIMPLE_OPCODE(DECLARE_OPCODE_CASE)
         FOREACH_ASMJS_COMPAT_OPCODE(DECLARE_OPCODE_CASE)
 #undef DECLARE_OPCODE_CASE
-      case kExprDeclLocals:
       default:
         UNREACHABLE();
         return 0;
@@ -298,11 +328,6 @@
         MemoryAccessOperand operand(this, pc);
         return 1 + operand.length;
       }
-      case kExprBlock:
-      case kExprLoop: {
-        BlockCountOperand operand(this, pc);
-        return 1 + operand.length;
-      }
       case kExprBr:
       case kExprBrIf: {
         BreakDepthOperand operand(this, pc);
@@ -315,15 +340,15 @@
       }
 
       case kExprCallFunction: {
-        FunctionIndexOperand operand(this, pc);
+        CallFunctionOperand operand(this, pc);
         return 1 + operand.length;
       }
       case kExprCallIndirect: {
-        SignatureIndexOperand operand(this, pc);
+        CallIndirectOperand operand(this, pc);
         return 1 + operand.length;
       }
       case kExprCallImport: {
-        ImportIndexOperand operand(this, pc);
+        CallImportOperand operand(this, pc);
         return 1 + operand.length;
       }
 
@@ -350,6 +375,10 @@
         return 5;
       case kExprF64Const:
         return 9;
+      case kExprReturn: {
+        ReturnArityOperand operand(this, pc);
+        return 1 + operand.length;
+      }
 
       default:
         return 1;
@@ -357,7 +386,6 @@
   }
 };
 
-
 // A shift-reduce-parser strategy for decoding Wasm code that uses an explicit
 // shift-reduce strategy with multiple internal stacks.
 class SR_WasmDecoder : public WasmDecoder {
@@ -368,55 +396,62 @@
         builder_(builder),
         base_(body.base),
         local_type_vec_(zone),
-        trees_(zone),
         stack_(zone),
-        blocks_(zone),
-        ifs_(zone) {
+        control_(zone) {
     local_types_ = &local_type_vec_;
   }
 
-  TreeResult Decode() {
+  bool Decode() {
+    base::ElapsedTimer decode_timer;
+    if (FLAG_trace_wasm_decode_time) {
+      decode_timer.Start();
+    }
+    stack_.clear();
+    control_.clear();
+
     if (end_ < pc_) {
       error(pc_, "function body end < start");
-      return result_;
+      return false;
     }
 
     DecodeLocalDecls();
     InitSsaEnv();
     DecodeFunctionBody();
 
-    Tree* tree = nullptr;
-    if (ok()) {
-      if (ssa_env_->go()) {
-        if (stack_.size() > 0) {
-          error(stack_.back().pc(), end_, "fell off end of code");
-        }
-        AddImplicitReturnAtEnd();
-      }
-      if (trees_.size() == 0) {
-        if (sig_->return_count() > 0) {
-          error(start_, "no trees created");
-        }
-      } else {
-        tree = trees_[0];
-      }
+    if (failed()) return TraceFailed();
+
+    if (!control_.empty()) {
+      error(pc_, control_.back().pc, "unterminated control structure");
+      return TraceFailed();
     }
 
-    if (ok()) {
-      TRACE("wasm-decode ok\n");
+    if (ssa_env_->go()) {
+      TRACE("  @%-6d #xx:%-20s|", startrel(pc_), "ImplicitReturn");
+      DoReturn();
+      if (failed()) return TraceFailed();
+      TRACE("\n");
+    }
+
+    if (FLAG_trace_wasm_decode_time) {
+      double ms = decode_timer.Elapsed().InMillisecondsF();
+      PrintF("wasm-decode ok (%0.3f ms)\n\n", ms);
     } else {
-      TRACE("wasm-error module+%-6d func+%d: %s\n\n", baserel(error_pc_),
-            startrel(error_pc_), error_msg_.get());
+      TRACE("wasm-decode ok\n\n");
     }
 
-    return toResult(tree);
+    return true;
+  }
+
+  bool TraceFailed() {
+    TRACE("wasm-error module+%-6d func+%d: %s\n\n", baserel(error_pc_),
+          startrel(error_pc_), error_msg_.get());
+    return false;
   }
 
   bool DecodeLocalDecls(AstLocalDecls& decls) {
     DecodeLocalDecls();
     if (failed()) return false;
     decls.decls_encoded_size = pc_offset();
-    decls.total_local_count = 0;
     decls.local_types.reserve(local_type_vec_.size());
     for (size_t pos = 0; pos < local_type_vec_.size();) {
       uint32_t count = 0;
@@ -425,9 +460,9 @@
         pos++;
         count++;
       }
-      decls.total_local_count += count;
       decls.local_types.push_back(std::pair<LocalType, uint32_t>(type, count));
     }
+    decls.total_local_count = static_cast<uint32_t>(local_type_vec_.size());
     return true;
   }
 
@@ -448,15 +483,12 @@
   Zone* zone_;
   TFBuilder* builder_;
   const byte* base_;
-  TreeResult result_;
 
   SsaEnv* ssa_env_;
 
-  ZoneVector<LocalType> local_type_vec_;
-  ZoneVector<Tree*> trees_;
-  ZoneVector<Production> stack_;
-  ZoneVector<Block> blocks_;
-  ZoneVector<IfEnv> ifs_;
+  ZoneVector<LocalType> local_type_vec_;  // types of local variables.
+  ZoneVector<Value> stack_;               // stack of values.
+  ZoneVector<Control> control_;           // stack of blocks, loops, and ifs.
 
   inline bool build() { return builder_ && ssa_env_->go(); }
 
@@ -508,53 +540,6 @@
     }
   }
 
-  void Leaf(LocalType type, TFNode* node = nullptr) {
-    size_t size = sizeof(Tree);
-    Tree* tree = reinterpret_cast<Tree*>(zone_->New(size));
-    tree->type = type;
-    tree->count = 0;
-    tree->pc = pc_;
-    tree->node = node;
-    tree->children[0] = nullptr;
-    Reduce(tree);
-  }
-
-  void Shift(LocalType type, uint32_t count) {
-    size_t size =
-        sizeof(Tree) + (count == 0 ? 0 : ((count - 1) * sizeof(Tree*)));
-    Tree* tree = reinterpret_cast<Tree*>(zone_->New(size));
-    tree->type = type;
-    tree->count = count;
-    tree->pc = pc_;
-    tree->node = nullptr;
-    for (uint32_t i = 0; i < count; i++) tree->children[i] = nullptr;
-    if (count == 0) {
-      Production p = {tree, 0};
-      Reduce(&p);
-      Reduce(tree);
-    } else {
-      stack_.push_back({tree, 0});
-    }
-  }
-
-  void Reduce(Tree* tree) {
-    while (true) {
-      if (stack_.size() == 0) {
-        trees_.push_back(tree);
-        break;
-      }
-      Production* p = &stack_.back();
-      p->tree->children[p->index++] = tree;
-      Reduce(p);
-      if (p->done()) {
-        tree = p->tree;
-        stack_.pop_back();
-      } else {
-        break;
-      }
-    }
-  }
-
   char* indentation() {
     static const int kMaxIndent = 64;
     static char bytes[kMaxIndent + 1];
@@ -605,11 +590,11 @@
     total_locals_ = local_type_vec_.size();
   }
 
-  // Decodes the body of a function, producing reduced trees into {result}.
+  // Decodes the body of a function.
   void DecodeFunctionBody() {
-    TRACE("wasm-decode %p...%p (%d bytes) %s\n",
+    TRACE("wasm-decode %p...%p (module+%d, %d bytes) %s\n",
           reinterpret_cast<const void*>(start_),
-          reinterpret_cast<const void*>(limit_),
+          reinterpret_cast<const void*>(limit_), baserel(pc_),
           static_cast<int>(limit_ - start_), builder_ ? "graph building" : "");
 
     if (pc_ >= limit_) return;  // Nothing to do.
@@ -617,49 +602,45 @@
     while (true) {  // decoding loop.
       int len = 1;
       WasmOpcode opcode = static_cast<WasmOpcode>(*pc_);
-      TRACE("wasm-decode module+%-6d %s func+%d: 0x%02x %s\n", baserel(pc_),
-            indentation(), startrel(pc_), opcode,
-            WasmOpcodes::OpcodeName(opcode));
+      TRACE("  @%-6d #%02x:%-20s|", startrel(pc_), opcode,
+            WasmOpcodes::ShortOpcodeName(opcode));
 
       FunctionSig* sig = WasmOpcodes::Signature(opcode);
       if (sig) {
-        // A simple expression with a fixed signature.
-        Shift(sig->GetReturn(), static_cast<uint32_t>(sig->parameter_count()));
-        pc_ += len;
-        if (pc_ >= limit_) {
-          // End of code reached or exceeded.
-          if (pc_ > limit_ && ok()) {
-            error("Beyond end of code");
+        // Fast case of a simple operator.
+        TFNode* node;
+        switch (sig->parameter_count()) {
+          case 1: {
+            Value val = Pop(0, sig->GetParam(0));
+            node = BUILD(Unop, opcode, val.node, position());
+            break;
           }
-          return;
+          case 2: {
+            Value rval = Pop(1, sig->GetParam(1));
+            Value lval = Pop(0, sig->GetParam(0));
+            node = BUILD(Binop, opcode, lval.node, rval.node, position());
+            break;
+          }
+          default:
+            UNREACHABLE();
+            node = nullptr;
+            break;
         }
-        continue;  // back to decoding loop.
-      }
-
-      switch (opcode) {
-        case kExprNop:
-          Leaf(kAstStmt);
-          break;
-        case kExprBlock: {
-          BlockCountOperand operand(this, pc_);
-          if (operand.count < 1) {
-            Leaf(kAstStmt);
-          } else {
-            Shift(kAstEnd, operand.count);
+        Push(GetReturnType(sig), node);
+      } else {
+        // Complex bytecode.
+        switch (opcode) {
+          case kExprNop:
+            Push(kAstStmt, nullptr);
+            break;
+          case kExprBlock: {
             // The break environment is the outer environment.
             SsaEnv* break_env = ssa_env_;
             PushBlock(break_env);
             SetEnv("block:start", Steal(break_env));
+            break;
           }
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprLoop: {
-          BlockCountOperand operand(this, pc_);
-          if (operand.count < 1) {
-            Leaf(kAstStmt);
-          } else {
-            Shift(kAstEnd, operand.count);
+          case kExprLoop: {
             // The break environment is the outer environment.
             SsaEnv* break_env = ssa_env_;
             PushBlock(break_env);
@@ -667,268 +648,535 @@
             // The continue environment is the inner environment.
             PrepareForLoop(pc_, cont_env);
             SetEnv("loop:start", Split(cont_env));
-            if (ssa_env_->go()) ssa_env_->state = SsaEnv::kReached;
-            PushBlock(cont_env);
-            blocks_.back().stack_depth = -1;  // no production for inner block.
+            ssa_env_->SetNotMerged();
+            PushLoop(cont_env);
+            break;
           }
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprIf:
-          Shift(kAstStmt, 2);
-          break;
-        case kExprIfElse:
-          Shift(kAstEnd, 3);  // Result type is typeof(x) in {c ? x : y}.
-          break;
-        case kExprSelect:
-          Shift(kAstStmt, 3);  // Result type is typeof(x) in {c ? x : y}.
-          break;
-        case kExprBr: {
-          BreakDepthOperand operand(this, pc_);
-          if (Validate(pc_, operand, blocks_)) {
-            Shift(kAstEnd, 1);
+          case kExprIf: {
+            // Condition on top of stack. Split environments for branches.
+            Value cond = Pop(0, kAstI32);
+            TFNode* if_true = nullptr;
+            TFNode* if_false = nullptr;
+            BUILD(Branch, cond.node, &if_true, &if_false);
+            SsaEnv* end_env = ssa_env_;
+            SsaEnv* false_env = Split(ssa_env_);
+            false_env->control = if_false;
+            SsaEnv* true_env = Steal(ssa_env_);
+            true_env->control = if_true;
+            PushIf(end_env, false_env);
+            SetEnv("if:true", true_env);
+            break;
           }
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprBrIf: {
-          BreakDepthOperand operand(this, pc_);
-          if (Validate(pc_, operand, blocks_)) {
-            Shift(kAstStmt, 2);
+          case kExprElse: {
+            if (control_.empty()) {
+              error(pc_, "else does not match any if");
+              break;
+            }
+            Control* c = &control_.back();
+            if (!c->is_if()) {
+              error(pc_, c->pc, "else does not match an if");
+              break;
+            }
+            if (c->false_env == nullptr) {
+              error(pc_, c->pc, "else already present for if");
+              break;
+            }
+            Value val = PopUpTo(c->stack_depth);
+            MergeInto(c->end_env, &c->node, &c->type, val);
+            // Switch to environment for false branch.
+            SetEnv("if_else:false", c->false_env);
+            c->false_env = nullptr;  // record that an else is already seen
+            break;
           }
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprBrTable: {
-          BranchTableOperand operand(this, pc_);
-          if (Validate(pc_, operand, blocks_.size())) {
-            Shift(kAstEnd, 1);
+          case kExprEnd: {
+            if (control_.empty()) {
+              error(pc_, "end does not match any if or block");
+              break;
+            }
+            const char* name = "block:end";
+            Control* c = &control_.back();
+            if (c->is_loop) {
+              // Loops always push control in pairs.
+              control_.pop_back();
+              c = &control_.back();
+              name = "loop:end";
+            }
+            Value val = PopUpTo(c->stack_depth);
+            if (c->is_if()) {
+              if (c->false_env != nullptr) {
+                // End the true branch of a one-armed if.
+                Goto(c->false_env, c->end_env);
+                val = {val.pc, nullptr, kAstStmt};
+                name = "if:merge";
+              } else {
+                // End the false branch of a two-armed if.
+                name = "if_else:merge";
+              }
+            }
+            if (ssa_env_->go()) {
+              MergeInto(c->end_env, &c->node, &c->type, val);
+            }
+            SetEnv(name, c->end_env);
+            stack_.resize(c->stack_depth);
+            Push(c->type, c->node);
+            control_.pop_back();
+            break;
           }
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprReturn: {
-          int count = static_cast<int>(sig_->return_count());
-          if (count == 0) {
-            BUILD(Return, 0, builder_->Buffer(0));
-            ssa_env_->Kill();
-            Leaf(kAstEnd);
-          } else {
-            Shift(kAstEnd, count);
+          case kExprSelect: {
+            Value cond = Pop(2, kAstI32);
+            Value fval = Pop();
+            Value tval = Pop();
+            if (tval.type == kAstStmt || tval.type != fval.type) {
+              if (tval.type != kAstEnd && fval.type != kAstEnd) {
+                error(pc_, "type mismatch in select");
+                break;
+              }
+            }
+            if (build()) {
+              DCHECK(tval.type != kAstEnd);
+              DCHECK(fval.type != kAstEnd);
+              DCHECK(cond.type != kAstEnd);
+              TFNode* controls[2];
+              builder_->Branch(cond.node, &controls[0], &controls[1]);
+              TFNode* merge = builder_->Merge(2, controls);
+              TFNode* vals[2] = {tval.node, fval.node};
+              TFNode* phi = builder_->Phi(tval.type, 2, vals, merge);
+              Push(tval.type, phi);
+              ssa_env_->control = merge;
+            } else {
+              Push(tval.type, nullptr);
+            }
+            break;
           }
-          break;
-        }
-        case kExprUnreachable: {
-          BUILD0(Unreachable);
-          ssa_env_->Kill(SsaEnv::kControlEnd);
-          Leaf(kAstEnd, nullptr);
-          break;
-        }
-        case kExprI8Const: {
-          ImmI8Operand operand(this, pc_);
-          Leaf(kAstI32, BUILD(Int32Constant, operand.value));
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprI32Const: {
-          ImmI32Operand operand(this, pc_);
-          Leaf(kAstI32, BUILD(Int32Constant, operand.value));
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprI64Const: {
-          ImmI64Operand operand(this, pc_);
-          Leaf(kAstI64, BUILD(Int64Constant, operand.value));
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprF32Const: {
-          ImmF32Operand operand(this, pc_);
-          Leaf(kAstF32, BUILD(Float32Constant, operand.value));
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprF64Const: {
-          ImmF64Operand operand(this, pc_);
-          Leaf(kAstF64, BUILD(Float64Constant, operand.value));
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprGetLocal: {
-          LocalIndexOperand operand(this, pc_);
-          if (Validate(pc_, operand)) {
-            TFNode* val = build() ? ssa_env_->locals[operand.index] : nullptr;
-            Leaf(operand.type, val);
+          case kExprBr: {
+            BreakDepthOperand operand(this, pc_);
+            Value val = {pc_, nullptr, kAstStmt};
+            if (operand.arity) val = Pop();
+            if (Validate(pc_, operand, control_)) {
+              BreakTo(operand.target, val);
+            }
+            len = 1 + operand.length;
+            Push(kAstEnd, nullptr);
+            break;
           }
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprSetLocal: {
-          LocalIndexOperand operand(this, pc_);
-          if (Validate(pc_, operand)) {
-            Shift(operand.type, 1);
+          case kExprBrIf: {
+            BreakDepthOperand operand(this, pc_);
+            Value cond = Pop(operand.arity, kAstI32);
+            Value val = {pc_, nullptr, kAstStmt};
+            if (operand.arity == 1) val = Pop();
+            if (Validate(pc_, operand, control_)) {
+              SsaEnv* fenv = ssa_env_;
+              SsaEnv* tenv = Split(fenv);
+              fenv->SetNotMerged();
+              BUILD(Branch, cond.node, &tenv->control, &fenv->control);
+              ssa_env_ = tenv;
+              BreakTo(operand.target, val);
+              ssa_env_ = fenv;
+            }
+            len = 1 + operand.length;
+            Push(kAstStmt, nullptr);
+            break;
           }
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprLoadGlobal: {
-          GlobalIndexOperand operand(this, pc_);
-          if (Validate(pc_, operand)) {
-            Leaf(operand.type, BUILD(LoadGlobal, operand.index));
+          case kExprBrTable: {
+            BranchTableOperand operand(this, pc_);
+            if (Validate(pc_, operand, control_.size())) {
+              Value key = Pop(operand.arity, kAstI32);
+              Value val = {pc_, nullptr, kAstStmt};
+              if (operand.arity == 1) val = Pop();
+              if (failed()) break;
+
+              SsaEnv* break_env = ssa_env_;
+              if (operand.table_count > 0) {
+                // Build branches to the various blocks based on the table.
+                TFNode* sw = BUILD(Switch, operand.table_count + 1, key.node);
+
+                SsaEnv* copy = Steal(break_env);
+                ssa_env_ = copy;
+                for (uint32_t i = 0; i < operand.table_count + 1; i++) {
+                  uint16_t target = operand.read_entry(this, i);
+                  ssa_env_ = Split(copy);
+                  ssa_env_->control = (i == operand.table_count)
+                                          ? BUILD(IfDefault, sw)
+                                          : BUILD(IfValue, i, sw);
+                  int depth = target;
+                  Control* c = &control_[control_.size() - depth - 1];
+                  MergeInto(c->end_env, &c->node, &c->type, val);
+                }
+              } else {
+                // Only a default target. Do the equivalent of br.
+                uint16_t target = operand.read_entry(this, 0);
+                int depth = target;
+                Control* c = &control_[control_.size() - depth - 1];
+                MergeInto(c->end_env, &c->node, &c->type, val);
+              }
+              // br_table ends the control flow like br.
+              ssa_env_ = break_env;
+              Push(kAstStmt, nullptr);
+            }
+            len = 1 + operand.length;
+            break;
           }
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprStoreGlobal: {
-          GlobalIndexOperand operand(this, pc_);
-          if (Validate(pc_, operand)) {
-            Shift(operand.type, 1);
+          case kExprReturn: {
+            ReturnArityOperand operand(this, pc_);
+            if (operand.arity != sig_->return_count()) {
+              error(pc_, pc_ + 1, "arity mismatch in return");
+            }
+            DoReturn();
+            len = 1 + operand.length;
+            break;
           }
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprI32LoadMem8S:
-        case kExprI32LoadMem8U:
-        case kExprI32LoadMem16S:
-        case kExprI32LoadMem16U:
-        case kExprI32LoadMem:
-          len = DecodeLoadMem(pc_, kAstI32);
-          break;
-        case kExprI64LoadMem8S:
-        case kExprI64LoadMem8U:
-        case kExprI64LoadMem16S:
-        case kExprI64LoadMem16U:
-        case kExprI64LoadMem32S:
-        case kExprI64LoadMem32U:
-        case kExprI64LoadMem:
-          len = DecodeLoadMem(pc_, kAstI64);
-          break;
-        case kExprF32LoadMem:
-          len = DecodeLoadMem(pc_, kAstF32);
-          break;
-        case kExprF64LoadMem:
-          len = DecodeLoadMem(pc_, kAstF64);
-          break;
-        case kExprI32StoreMem8:
-        case kExprI32StoreMem16:
-        case kExprI32StoreMem:
-          len = DecodeStoreMem(pc_, kAstI32);
-          break;
-        case kExprI64StoreMem8:
-        case kExprI64StoreMem16:
-        case kExprI64StoreMem32:
-        case kExprI64StoreMem:
-          len = DecodeStoreMem(pc_, kAstI64);
-          break;
-        case kExprF32StoreMem:
-          len = DecodeStoreMem(pc_, kAstF32);
-          break;
-        case kExprF64StoreMem:
-          len = DecodeStoreMem(pc_, kAstF64);
-          break;
-        case kExprMemorySize:
-          Leaf(kAstI32, BUILD(MemSize, 0));
-          break;
-        case kExprGrowMemory:
-          Shift(kAstI32, 1);
-          break;
-        case kExprCallFunction: {
-          FunctionIndexOperand operand(this, pc_);
-          if (Validate(pc_, operand)) {
-            LocalType type = operand.sig->return_count() == 0
-                                 ? kAstStmt
-                                 : operand.sig->GetReturn();
-            Shift(type, static_cast<int>(operand.sig->parameter_count()));
+          case kExprUnreachable: {
+            Push(kAstEnd, BUILD(Unreachable, position()));
+            ssa_env_->Kill(SsaEnv::kControlEnd);
+            break;
           }
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprCallIndirect: {
-          SignatureIndexOperand operand(this, pc_);
-          if (Validate(pc_, operand)) {
-            LocalType type = operand.sig->return_count() == 0
-                                 ? kAstStmt
-                                 : operand.sig->GetReturn();
-            Shift(type, static_cast<int>(1 + operand.sig->parameter_count()));
+          case kExprI8Const: {
+            ImmI8Operand operand(this, pc_);
+            Push(kAstI32, BUILD(Int32Constant, operand.value));
+            len = 1 + operand.length;
+            break;
           }
-          len = 1 + operand.length;
-          break;
-        }
-        case kExprCallImport: {
-          ImportIndexOperand operand(this, pc_);
-          if (Validate(pc_, operand)) {
-            LocalType type = operand.sig->return_count() == 0
-                                 ? kAstStmt
-                                 : operand.sig->GetReturn();
-            Shift(type, static_cast<int>(operand.sig->parameter_count()));
+          case kExprI32Const: {
+            ImmI32Operand operand(this, pc_);
+            Push(kAstI32, BUILD(Int32Constant, operand.value));
+            len = 1 + operand.length;
+            break;
           }
-          len = 1 + operand.length;
-          break;
+          case kExprI64Const: {
+            ImmI64Operand operand(this, pc_);
+            Push(kAstI64, BUILD(Int64Constant, operand.value));
+            len = 1 + operand.length;
+            break;
+          }
+          case kExprF32Const: {
+            ImmF32Operand operand(this, pc_);
+            Push(kAstF32, BUILD(Float32Constant, operand.value));
+            len = 1 + operand.length;
+            break;
+          }
+          case kExprF64Const: {
+            ImmF64Operand operand(this, pc_);
+            Push(kAstF64, BUILD(Float64Constant, operand.value));
+            len = 1 + operand.length;
+            break;
+          }
+          case kExprGetLocal: {
+            LocalIndexOperand operand(this, pc_);
+            if (Validate(pc_, operand)) {
+              if (build()) {
+                Push(operand.type, ssa_env_->locals[operand.index]);
+              } else {
+                Push(operand.type, nullptr);
+              }
+            }
+            len = 1 + operand.length;
+            break;
+          }
+          case kExprSetLocal: {
+            LocalIndexOperand operand(this, pc_);
+            if (Validate(pc_, operand)) {
+              Value val = Pop(0, local_type_vec_[operand.index]);
+              if (ssa_env_->locals) ssa_env_->locals[operand.index] = val.node;
+              Push(val.type, val.node);
+            }
+            len = 1 + operand.length;
+            break;
+          }
+          case kExprLoadGlobal: {
+            GlobalIndexOperand operand(this, pc_);
+            if (Validate(pc_, operand)) {
+              Push(operand.type, BUILD(LoadGlobal, operand.index));
+            }
+            len = 1 + operand.length;
+            break;
+          }
+          case kExprStoreGlobal: {
+            GlobalIndexOperand operand(this, pc_);
+            if (Validate(pc_, operand)) {
+              Value val = Pop(0, operand.type);
+              BUILD(StoreGlobal, operand.index, val.node);
+              Push(val.type, val.node);
+            }
+            len = 1 + operand.length;
+            break;
+          }
+          case kExprI32LoadMem8S:
+            len = DecodeLoadMem(kAstI32, MachineType::Int8());
+            break;
+          case kExprI32LoadMem8U:
+            len = DecodeLoadMem(kAstI32, MachineType::Uint8());
+            break;
+          case kExprI32LoadMem16S:
+            len = DecodeLoadMem(kAstI32, MachineType::Int16());
+            break;
+          case kExprI32LoadMem16U:
+            len = DecodeLoadMem(kAstI32, MachineType::Uint16());
+            break;
+          case kExprI32LoadMem:
+            len = DecodeLoadMem(kAstI32, MachineType::Int32());
+            break;
+
+          case kExprI64LoadMem8S:
+            len = DecodeLoadMem(kAstI64, MachineType::Int8());
+            break;
+          case kExprI64LoadMem8U:
+            len = DecodeLoadMem(kAstI64, MachineType::Uint8());
+            break;
+          case kExprI64LoadMem16S:
+            len = DecodeLoadMem(kAstI64, MachineType::Int16());
+            break;
+          case kExprI64LoadMem16U:
+            len = DecodeLoadMem(kAstI64, MachineType::Uint16());
+            break;
+          case kExprI64LoadMem32S:
+            len = DecodeLoadMem(kAstI64, MachineType::Int32());
+            break;
+          case kExprI64LoadMem32U:
+            len = DecodeLoadMem(kAstI64, MachineType::Uint32());
+            break;
+          case kExprI64LoadMem:
+            len = DecodeLoadMem(kAstI64, MachineType::Int64());
+            break;
+          case kExprF32LoadMem:
+            len = DecodeLoadMem(kAstF32, MachineType::Float32());
+            break;
+          case kExprF64LoadMem:
+            len = DecodeLoadMem(kAstF64, MachineType::Float64());
+            break;
+          case kExprI32StoreMem8:
+            len = DecodeStoreMem(kAstI32, MachineType::Int8());
+            break;
+          case kExprI32StoreMem16:
+            len = DecodeStoreMem(kAstI32, MachineType::Int16());
+            break;
+          case kExprI32StoreMem:
+            len = DecodeStoreMem(kAstI32, MachineType::Int32());
+            break;
+          case kExprI64StoreMem8:
+            len = DecodeStoreMem(kAstI64, MachineType::Int8());
+            break;
+          case kExprI64StoreMem16:
+            len = DecodeStoreMem(kAstI64, MachineType::Int16());
+            break;
+          case kExprI64StoreMem32:
+            len = DecodeStoreMem(kAstI64, MachineType::Int32());
+            break;
+          case kExprI64StoreMem:
+            len = DecodeStoreMem(kAstI64, MachineType::Int64());
+            break;
+          case kExprF32StoreMem:
+            len = DecodeStoreMem(kAstF32, MachineType::Float32());
+            break;
+          case kExprF64StoreMem:
+            len = DecodeStoreMem(kAstF64, MachineType::Float64());
+            break;
+
+          case kExprMemorySize:
+            Push(kAstI32, BUILD(MemSize, 0));
+            break;
+          case kExprGrowMemory: {
+            Value val = Pop(0, kAstI32);
+            USE(val);  // TODO(titzer): build node for grow memory
+            Push(kAstI32, BUILD(Int32Constant, 0));
+            break;
+          }
+          case kExprCallFunction: {
+            CallFunctionOperand operand(this, pc_);
+            if (Validate(pc_, operand)) {
+              TFNode** buffer = PopArgs(operand.sig);
+              TFNode* call =
+                  BUILD(CallDirect, operand.index, buffer, position());
+              Push(GetReturnType(operand.sig), call);
+            }
+            len = 1 + operand.length;
+            break;
+          }
+          case kExprCallIndirect: {
+            CallIndirectOperand operand(this, pc_);
+            if (Validate(pc_, operand)) {
+              TFNode** buffer = PopArgs(operand.sig);
+              Value index = Pop(0, kAstI32);
+              if (buffer) buffer[0] = index.node;
+              TFNode* call =
+                  BUILD(CallIndirect, operand.index, buffer, position());
+              Push(GetReturnType(operand.sig), call);
+            }
+            len = 1 + operand.length;
+            break;
+          }
+          case kExprCallImport: {
+            CallImportOperand operand(this, pc_);
+            if (Validate(pc_, operand)) {
+              TFNode** buffer = PopArgs(operand.sig);
+              TFNode* call =
+                  BUILD(CallImport, operand.index, buffer, position());
+              Push(GetReturnType(operand.sig), call);
+            }
+            len = 1 + operand.length;
+            break;
+          }
+          default:
+            error("Invalid opcode");
+            return;
         }
-        case kExprDeclLocals:
-        default:
-          error("Invalid opcode");
-          return;
+      }  // end complex bytecode
+
+#if DEBUG
+      if (FLAG_trace_wasm_decoder) {
+        for (size_t i = 0; i < stack_.size(); i++) {
+          Value& val = stack_[i];
+          WasmOpcode opcode = static_cast<WasmOpcode>(*val.pc);
+          PrintF(" %c@%d:%s", WasmOpcodes::ShortNameOf(val.type),
+                 static_cast<int>(val.pc - start_),
+                 WasmOpcodes::ShortOpcodeName(opcode));
+          switch (opcode) {
+            case kExprI32Const: {
+              ImmI32Operand operand(this, val.pc);
+              PrintF("[%d]", operand.value);
+              break;
+            }
+            case kExprGetLocal: {
+              LocalIndexOperand operand(this, val.pc);
+              PrintF("[%u]", operand.index);
+              break;
+            }
+            case kExprSetLocal: {
+              LocalIndexOperand operand(this, val.pc);
+              PrintF("[%u]", operand.index);
+              break;
+            }
+            default:
+              break;
+          }
+        }
+        PrintF("\n");
       }
+#endif
       pc_ += len;
       if (pc_ >= limit_) {
         // End of code reached or exceeded.
-        if (pc_ > limit_ && ok()) {
-          error("Beyond end of code");
-        }
+        if (pc_ > limit_ && ok()) error("Beyond end of code");
         return;
       }
+    }  // end decode loop
+  }    // end DecodeFunctionBody()
+
+  TFNode** PopArgs(FunctionSig* sig) {
+    if (build()) {
+      int count = static_cast<int>(sig->parameter_count());
+      TFNode** buffer = builder_->Buffer(count + 1);
+      buffer[0] = nullptr;  // reserved for code object or function index.
+      for (int i = count - 1; i >= 0; i--) {
+        buffer[i + 1] = Pop(i, sig->GetParam(i)).node;
+      }
+      return buffer;
+    } else {
+      int count = static_cast<int>(sig->parameter_count());
+      for (int i = count - 1; i >= 0; i--) {
+        Pop(i, sig->GetParam(i));
+      }
+      return nullptr;
     }
   }
 
-  void PushBlock(SsaEnv* ssa_env) {
-    blocks_.push_back({ssa_env, static_cast<int>(stack_.size() - 1)});
+  LocalType GetReturnType(FunctionSig* sig) {
+    return sig->return_count() == 0 ? kAstStmt : sig->GetReturn();
   }
 
-  int DecodeLoadMem(const byte* pc, LocalType type) {
-    MemoryAccessOperand operand(this, pc);
-    Shift(type, 1);
+  void PushBlock(SsaEnv* end_env) {
+    int stack_depth = static_cast<int>(stack_.size());
+    control_.push_back(
+        {pc_, stack_depth, end_env, nullptr, nullptr, kAstEnd, false});
+  }
+
+  void PushLoop(SsaEnv* end_env) {
+    int stack_depth = static_cast<int>(stack_.size());
+    control_.push_back(
+        {pc_, stack_depth, end_env, nullptr, nullptr, kAstEnd, true});
+  }
+
+  void PushIf(SsaEnv* end_env, SsaEnv* false_env) {
+    int stack_depth = static_cast<int>(stack_.size());
+    control_.push_back(
+        {pc_, stack_depth, end_env, false_env, nullptr, kAstStmt, false});
+  }
+
+  int DecodeLoadMem(LocalType type, MachineType mem_type) {
+    MemoryAccessOperand operand(this, pc_);
+    Value index = Pop(0, kAstI32);
+    TFNode* node =
+        BUILD(LoadMem, type, mem_type, index.node, operand.offset, position());
+    Push(type, node);
     return 1 + operand.length;
   }
 
-  int DecodeStoreMem(const byte* pc, LocalType type) {
-    MemoryAccessOperand operand(this, pc);
-    Shift(type, 2);
+  int DecodeStoreMem(LocalType type, MachineType mem_type) {
+    MemoryAccessOperand operand(this, pc_);
+    Value val = Pop(1, type);
+    Value index = Pop(0, kAstI32);
+    BUILD(StoreMem, mem_type, index.node, operand.offset, val.node, position());
+    Push(type, val.node);
     return 1 + operand.length;
   }
 
-  void AddImplicitReturnAtEnd() {
-    int retcount = static_cast<int>(sig_->return_count());
-    if (retcount == 0) {
-      BUILD0(ReturnVoid);
-      return;
+  void DoReturn() {
+    int count = static_cast<int>(sig_->return_count());
+    TFNode** buffer = nullptr;
+    if (build()) buffer = builder_->Buffer(count);
+
+    // Pop return values off the stack in reverse order.
+    for (int i = count - 1; i >= 0; i--) {
+      Value val = Pop(i, sig_->GetReturn(i));
+      if (buffer) buffer[i] = val.node;
     }
 
-    if (static_cast<int>(trees_.size()) < retcount) {
-      error(limit_, nullptr,
-            "ImplicitReturn expects %d arguments, only %d remain", retcount,
-            static_cast<int>(trees_.size()));
-      return;
-    }
+    Push(kAstEnd, BUILD(Return, count, buffer));
+    ssa_env_->Kill(SsaEnv::kControlEnd);
+  }
 
-    TRACE("wasm-decode implicit return of %d args\n", retcount);
+  void Push(LocalType type, TFNode* node) {
+    stack_.push_back({pc_, node, type});
+  }
 
-    TFNode** buffer = BUILD(Buffer, retcount);
-    for (int index = 0; index < retcount; index++) {
-      Tree* tree = trees_[trees_.size() - 1 - index];
-      if (buffer) buffer[index] = tree->node;
-      LocalType expected = sig_->GetReturn(index);
-      if (tree->type != expected) {
-        error(limit_, tree->pc,
-              "ImplicitReturn[%d] expected type %s, found %s of type %s", index,
-              WasmOpcodes::TypeName(expected),
-              WasmOpcodes::OpcodeName(tree->opcode()),
-              WasmOpcodes::TypeName(tree->type));
-        return;
+  const char* SafeOpcodeNameAt(const byte* pc) {
+    if (pc >= end_) return "<end>";
+    return WasmOpcodes::ShortOpcodeName(static_cast<WasmOpcode>(*pc));
+  }
+
+  Value Pop(int index, LocalType expected) {
+    Value val = Pop();
+    if (val.type != expected) {
+      if (val.type != kAstEnd) {
+        error(pc_, val.pc, "%s[%d] expected type %s, found %s of type %s",
+              SafeOpcodeNameAt(pc_), index, WasmOpcodes::TypeName(expected),
+              SafeOpcodeNameAt(val.pc), WasmOpcodes::TypeName(val.type));
       }
     }
+    return val;
+  }
 
-    BUILD(Return, retcount, buffer);
+  Value Pop() {
+    size_t limit = control_.empty() ? 0 : control_.back().stack_depth;
+    if (stack_.size() <= limit) {
+      Value val = {pc_, nullptr, kAstStmt};
+      error(pc_, pc_, "%s found empty stack", SafeOpcodeNameAt(pc_));
+      return val;
+    }
+    Value val = stack_.back();
+    stack_.pop_back();
+    return val;
+  }
+
+  Value PopUpTo(int stack_depth) {
+    if (stack_depth == stack_.size()) {
+      Value val = {pc_, nullptr, kAstStmt};
+      return val;
+    } else {
+      DCHECK_LE(stack_depth, static_cast<int>(stack_.size()));
+      Value val = Pop();
+      stack_.resize(stack_depth);
+      return val;
+    }
   }
 
   int baserel(const byte* ptr) {
@@ -937,437 +1185,65 @@
 
   int startrel(const byte* ptr) { return static_cast<int>(ptr - start_); }
 
-  void Reduce(Production* p) {
-    WasmOpcode opcode = p->opcode();
-    TRACE("-----reduce module+%-6d %s func+%d: 0x%02x %s\n", baserel(p->pc()),
-          indentation(), startrel(p->pc()), opcode,
-          WasmOpcodes::OpcodeName(opcode));
-    FunctionSig* sig = WasmOpcodes::Signature(opcode);
-    if (sig) {
-      // A simple expression with a fixed signature.
-      TypeCheckLast(p, sig->GetParam(p->index - 1));
-      if (p->done() && build()) {
-        if (sig->parameter_count() == 2) {
-          p->tree->node = builder_->Binop(opcode, p->tree->children[0]->node,
-                                          p->tree->children[1]->node);
-        } else if (sig->parameter_count() == 1) {
-          p->tree->node = builder_->Unop(opcode, p->tree->children[0]->node);
-        } else {
-          UNREACHABLE();
-        }
-      }
-      return;
-    }
-
-    switch (opcode) {
-      case kExprBlock: {
-        if (p->done()) {
-          Block* last = &blocks_.back();
-          DCHECK_EQ(stack_.size() - 1, last->stack_depth);
-          // fallthrough with the last expression.
-          ReduceBreakToExprBlock(p, last);
-          SetEnv("block:end", last->ssa_env);
-          blocks_.pop_back();
-        }
-        break;
-      }
-      case kExprLoop: {
-        if (p->done()) {
-          // Pop the continue environment.
-          blocks_.pop_back();
-          // Get the break environment.
-          Block* last = &blocks_.back();
-          DCHECK_EQ(stack_.size() - 1, last->stack_depth);
-          // fallthrough with the last expression.
-          ReduceBreakToExprBlock(p, last);
-          SetEnv("loop:end", last->ssa_env);
-          blocks_.pop_back();
-        }
-        break;
-      }
-      case kExprIf: {
-        if (p->index == 1) {
-          // Condition done. Split environment for true branch.
-          TypeCheckLast(p, kAstI32);
-          SsaEnv* false_env = ssa_env_;
-          SsaEnv* true_env = Split(ssa_env_);
-          ifs_.push_back({nullptr, false_env, nullptr});
-          BUILD(Branch, p->last()->node, &true_env->control,
-                &false_env->control);
-          SetEnv("if:true", true_env);
-        } else if (p->index == 2) {
-          // True block done. Merge true and false environments.
-          IfEnv* env = &ifs_.back();
-          SsaEnv* merge = env->merge_env;
-          if (merge->go()) {
-            merge->state = SsaEnv::kReached;
-            Goto(ssa_env_, merge);
-          }
-          SetEnv("if:merge", merge);
-          ifs_.pop_back();
-        }
-        break;
-      }
-      case kExprIfElse: {
-        if (p->index == 1) {
-          // Condition done. Split environment for true and false branches.
-          TypeCheckLast(p, kAstI32);
-          SsaEnv* merge_env = ssa_env_;
-          TFNode* if_true = nullptr;
-          TFNode* if_false = nullptr;
-          BUILD(Branch, p->last()->node, &if_true, &if_false);
-          SsaEnv* false_env = Split(ssa_env_);
-          SsaEnv* true_env = Steal(ssa_env_);
-          false_env->control = if_false;
-          true_env->control = if_true;
-          ifs_.push_back({false_env, merge_env, nullptr});
-          SetEnv("if_else:true", true_env);
-        } else if (p->index == 2) {
-          // True expr done.
-          IfEnv* env = &ifs_.back();
-          MergeIntoProduction(p, env->merge_env, p->last());
-          // Switch to environment for false branch.
-          SsaEnv* false_env = ifs_.back().false_env;
-          SetEnv("if_else:false", false_env);
-        } else if (p->index == 3) {
-          // False expr done.
-          IfEnv* env = &ifs_.back();
-          MergeIntoProduction(p, env->merge_env, p->last());
-          SetEnv("if_else:merge", env->merge_env);
-          ifs_.pop_back();
-        }
-        break;
-      }
-      case kExprSelect: {
-        if (p->index == 1) {
-          // True expression done.
-          p->tree->type = p->last()->type;
-          if (p->tree->type == kAstStmt) {
-            error(p->pc(), p->tree->children[1]->pc,
-                  "select operand should be expression");
-          }
-        } else if (p->index == 2) {
-          // False expression done.
-          TypeCheckLast(p, p->tree->type);
-        } else {
-          // Condition done.
-          DCHECK(p->done());
-          TypeCheckLast(p, kAstI32);
-          if (build()) {
-            TFNode* controls[2];
-            builder_->Branch(p->tree->children[2]->node, &controls[0],
-                             &controls[1]);
-            TFNode* merge = builder_->Merge(2, controls);
-            TFNode* vals[2] = {p->tree->children[0]->node,
-                               p->tree->children[1]->node};
-            TFNode* phi = builder_->Phi(p->tree->type, 2, vals, merge);
-            p->tree->node = phi;
-            ssa_env_->control = merge;
-          }
-        }
-        break;
-      }
-      case kExprBr: {
-        BreakDepthOperand operand(this, p->pc());
-        CHECK(Validate(p->pc(), operand, blocks_));
-        ReduceBreakToExprBlock(p, operand.target);
-        break;
-      }
-      case kExprBrIf: {
-        if (p->done()) {
-          TypeCheckLast(p, kAstI32);
-          BreakDepthOperand operand(this, p->pc());
-          CHECK(Validate(p->pc(), operand, blocks_));
-          SsaEnv* fenv = ssa_env_;
-          SsaEnv* tenv = Split(fenv);
-          BUILD(Branch, p->tree->children[1]->node, &tenv->control,
-                &fenv->control);
-          ssa_env_ = tenv;
-          ReduceBreakToExprBlock(p, operand.target, p->tree->children[0]);
-          ssa_env_ = fenv;
-        }
-        break;
-      }
-      case kExprBrTable: {
-        if (p->index == 1) {
-          // Switch key finished.
-          TypeCheckLast(p, kAstI32);
-          if (failed()) break;
-
-          BranchTableOperand operand(this, p->pc());
-          DCHECK(Validate(p->pc(), operand, blocks_.size()));
-
-          // Build a switch only if it has more than just a default target.
-          bool build_switch = operand.table_count > 0;
-          TFNode* sw = nullptr;
-          if (build_switch) {
-            sw = BUILD(Switch, operand.table_count + 1, p->last()->node);
-          }
-
-          // Process the targets of the break table.
-          SsaEnv* prev = ssa_env_;
-          SsaEnv* copy = Steal(prev);
-          for (uint32_t i = 0; i < operand.table_count + 1; i++) {
-            uint32_t target = operand.read_entry(this, i);
-            SsaEnv* env = copy;
-            if (build_switch) {
-              ssa_env_ = env = Split(env);
-              env->control = i == operand.table_count ? BUILD(IfDefault, sw)
-                                                      : BUILD(IfValue, i, sw);
-            }
-            SsaEnv* tenv = blocks_[blocks_.size() - target - 1].ssa_env;
-            Goto(env, tenv);
-          }
-          ssa_env_ = prev;
-        }
-        break;
-      }
-      case kExprReturn: {
-        TypeCheckLast(p, sig_->GetReturn(p->index - 1));
-        if (p->done()) {
-          if (build()) {
-            int count = p->tree->count;
-            TFNode** buffer = builder_->Buffer(count);
-            for (int i = 0; i < count; i++) {
-              buffer[i] = p->tree->children[i]->node;
-            }
-            BUILD(Return, count, buffer);
-          }
-          ssa_env_->Kill(SsaEnv::kControlEnd);
-        }
-        break;
-      }
-      case kExprSetLocal: {
-        LocalIndexOperand operand(this, p->pc());
-        CHECK(Validate(p->pc(), operand));
-        Tree* val = p->last();
-        if (operand.type == val->type) {
-          if (build()) ssa_env_->locals[operand.index] = val->node;
-          p->tree->node = val->node;
-        } else {
-          error(p->pc(), val->pc, "Typecheck failed in SetLocal");
-        }
-        break;
-      }
-      case kExprStoreGlobal: {
-        GlobalIndexOperand operand(this, p->pc());
-        CHECK(Validate(p->pc(), operand));
-        Tree* val = p->last();
-        if (operand.type == val->type) {
-          BUILD(StoreGlobal, operand.index, val->node);
-          p->tree->node = val->node;
-        } else {
-          error(p->pc(), val->pc, "Typecheck failed in StoreGlobal");
-        }
-        break;
-      }
-
-      case kExprI32LoadMem8S:
-        return ReduceLoadMem(p, kAstI32, MachineType::Int8());
-      case kExprI32LoadMem8U:
-        return ReduceLoadMem(p, kAstI32, MachineType::Uint8());
-      case kExprI32LoadMem16S:
-        return ReduceLoadMem(p, kAstI32, MachineType::Int16());
-      case kExprI32LoadMem16U:
-        return ReduceLoadMem(p, kAstI32, MachineType::Uint16());
-      case kExprI32LoadMem:
-        return ReduceLoadMem(p, kAstI32, MachineType::Int32());
-
-      case kExprI64LoadMem8S:
-        return ReduceLoadMem(p, kAstI64, MachineType::Int8());
-      case kExprI64LoadMem8U:
-        return ReduceLoadMem(p, kAstI64, MachineType::Uint8());
-      case kExprI64LoadMem16S:
-        return ReduceLoadMem(p, kAstI64, MachineType::Int16());
-      case kExprI64LoadMem16U:
-        return ReduceLoadMem(p, kAstI64, MachineType::Uint16());
-      case kExprI64LoadMem32S:
-        return ReduceLoadMem(p, kAstI64, MachineType::Int32());
-      case kExprI64LoadMem32U:
-        return ReduceLoadMem(p, kAstI64, MachineType::Uint32());
-      case kExprI64LoadMem:
-        return ReduceLoadMem(p, kAstI64, MachineType::Int64());
-
-      case kExprF32LoadMem:
-        return ReduceLoadMem(p, kAstF32, MachineType::Float32());
-
-      case kExprF64LoadMem:
-        return ReduceLoadMem(p, kAstF64, MachineType::Float64());
-
-      case kExprI32StoreMem8:
-        return ReduceStoreMem(p, kAstI32, MachineType::Int8());
-      case kExprI32StoreMem16:
-        return ReduceStoreMem(p, kAstI32, MachineType::Int16());
-      case kExprI32StoreMem:
-        return ReduceStoreMem(p, kAstI32, MachineType::Int32());
-
-      case kExprI64StoreMem8:
-        return ReduceStoreMem(p, kAstI64, MachineType::Int8());
-      case kExprI64StoreMem16:
-        return ReduceStoreMem(p, kAstI64, MachineType::Int16());
-      case kExprI64StoreMem32:
-        return ReduceStoreMem(p, kAstI64, MachineType::Int32());
-      case kExprI64StoreMem:
-        return ReduceStoreMem(p, kAstI64, MachineType::Int64());
-
-      case kExprF32StoreMem:
-        return ReduceStoreMem(p, kAstF32, MachineType::Float32());
-
-      case kExprF64StoreMem:
-        return ReduceStoreMem(p, kAstF64, MachineType::Float64());
-
-      case kExprGrowMemory:
-        TypeCheckLast(p, kAstI32);
-        // TODO(titzer): build node for GrowMemory
-        p->tree->node = BUILD(Int32Constant, 0);
-        return;
-
-      case kExprCallFunction: {
-        FunctionIndexOperand operand(this, p->pc());
-        CHECK(Validate(p->pc(), operand));
-        if (p->index > 0) {
-          TypeCheckLast(p, operand.sig->GetParam(p->index - 1));
-        }
-        if (p->done() && build()) {
-          uint32_t count = p->tree->count + 1;
-          TFNode** buffer = builder_->Buffer(count);
-          buffer[0] = nullptr;  // reserved for code object.
-          for (uint32_t i = 1; i < count; i++) {
-            buffer[i] = p->tree->children[i - 1]->node;
-          }
-          p->tree->node = builder_->CallDirect(operand.index, buffer);
-        }
-        break;
-      }
-      case kExprCallIndirect: {
-        SignatureIndexOperand operand(this, p->pc());
-        CHECK(Validate(p->pc(), operand));
-        if (p->index == 1) {
-          TypeCheckLast(p, kAstI32);
-        } else {
-          TypeCheckLast(p, operand.sig->GetParam(p->index - 2));
-        }
-        if (p->done() && build()) {
-          uint32_t count = p->tree->count;
-          TFNode** buffer = builder_->Buffer(count);
-          for (uint32_t i = 0; i < count; i++) {
-            buffer[i] = p->tree->children[i]->node;
-          }
-          p->tree->node = builder_->CallIndirect(operand.index, buffer);
-        }
-        break;
-      }
-      case kExprCallImport: {
-        ImportIndexOperand operand(this, p->pc());
-        CHECK(Validate(p->pc(), operand));
-        if (p->index > 0) {
-          TypeCheckLast(p, operand.sig->GetParam(p->index - 1));
-        }
-        if (p->done() && build()) {
-          uint32_t count = p->tree->count + 1;
-          TFNode** buffer = builder_->Buffer(count);
-          buffer[0] = nullptr;  // reserved for code object.
-          for (uint32_t i = 1; i < count; i++) {
-            buffer[i] = p->tree->children[i - 1]->node;
-          }
-          p->tree->node = builder_->CallImport(operand.index, buffer);
-        }
-        break;
-      }
-      default:
-        break;
-    }
-  }
-
-  void ReduceBreakToExprBlock(Production* p, Block* block) {
-    ReduceBreakToExprBlock(p, block, p->tree->count > 0 ? p->last() : nullptr);
-  }
-
-  void ReduceBreakToExprBlock(Production* p, Block* block, Tree* val) {
-    if (block->stack_depth < 0) {
+  void BreakTo(Control* block, Value& val) {
+    if (block->is_loop) {
       // This is the inner loop block, which does not have a value.
-      Goto(ssa_env_, block->ssa_env);
+      Goto(ssa_env_, block->end_env);
     } else {
       // Merge the value into the production for the block.
-      Production* bp = &stack_[block->stack_depth];
-      MergeIntoProduction(bp, block->ssa_env, val);
+      MergeInto(block->end_env, &block->node, &block->type, val);
     }
   }
 
-  void MergeIntoProduction(Production* p, SsaEnv* target, Tree* expr) {
+  void MergeInto(SsaEnv* target, TFNode** node, LocalType* type, Value& val) {
     if (!ssa_env_->go()) return;
+    DCHECK_NE(kAstEnd, val.type);
 
     bool first = target->state == SsaEnv::kUnreachable;
     Goto(ssa_env_, target);
-    if (expr == nullptr || expr->type == kAstEnd) return;
 
     if (first) {
       // first merge to this environment; set the type and the node.
-      p->tree->type = expr->type;
-      p->tree->node = expr->node;
-    } else {
+      *type = val.type;
+      *node = val.node;
+    } else if (val.type == *type && val.type != kAstStmt) {
       // merge with the existing value for this block.
-      LocalType type = p->tree->type;
-      if (expr->type != type) {
-        type = kAstStmt;
-        p->tree->type = kAstStmt;
-        p->tree->node = nullptr;
-      } else if (type != kAstStmt) {
-        p->tree->node = CreateOrMergeIntoPhi(type, target->control,
-                                             p->tree->node, expr->node);
-      }
-    }
-  }
-
-  void ReduceLoadMem(Production* p, LocalType type, MachineType mem_type) {
-    DCHECK_EQ(1, p->index);
-    TypeCheckLast(p, kAstI32);  // index
-    if (build()) {
-      MemoryAccessOperand operand(this, p->pc());
-      p->tree->node =
-          builder_->LoadMem(type, mem_type, p->last()->node, operand.offset);
-    }
-  }
-
-  void ReduceStoreMem(Production* p, LocalType type, MachineType mem_type) {
-    if (p->index == 1) {
-      TypeCheckLast(p, kAstI32);  // index
+      *node = CreateOrMergeIntoPhi(*type, target->control, *node, val.node);
     } else {
-      DCHECK_EQ(2, p->index);
-      TypeCheckLast(p, type);
-      if (build()) {
-        MemoryAccessOperand operand(this, p->pc());
-        TFNode* val = p->tree->children[1]->node;
-        builder_->StoreMem(mem_type, p->tree->children[0]->node, operand.offset,
-                           val);
-        p->tree->node = val;
-      }
-    }
-  }
-
-  void TypeCheckLast(Production* p, LocalType expected) {
-    LocalType result = p->last()->type;
-    if (result == expected) return;
-    if (result == kAstEnd) return;
-    if (expected != kAstStmt) {
-      error(p->pc(), p->last()->pc,
-            "%s[%d] expected type %s, found %s of type %s",
-            WasmOpcodes::OpcodeName(p->opcode()), p->index - 1,
-            WasmOpcodes::TypeName(expected),
-            WasmOpcodes::OpcodeName(p->last()->opcode()),
-            WasmOpcodes::TypeName(p->last()->type));
+      // types don't match, or block is already a stmt.
+      *type = kAstStmt;
+      *node = nullptr;
     }
   }
 
   void SetEnv(const char* reason, SsaEnv* env) {
 #if DEBUG
-    TRACE("  env = %p, block depth = %d, reason = %s", static_cast<void*>(env),
-          static_cast<int>(blocks_.size()), reason);
-    if (FLAG_trace_wasm_decoder && env && env->control) {
-      TRACE(", control = ");
-      compiler::WasmGraphBuilder::PrintDebugName(env->control);
+    if (FLAG_trace_wasm_decoder) {
+      char state = 'X';
+      if (env) {
+        switch (env->state) {
+          case SsaEnv::kReached:
+            state = 'R';
+            break;
+          case SsaEnv::kUnreachable:
+            state = 'U';
+            break;
+          case SsaEnv::kMerged:
+            state = 'M';
+            break;
+          case SsaEnv::kControlEnd:
+            state = 'E';
+            break;
+        }
+      }
+      PrintF("  env = %p, state = %c, reason = %s", static_cast<void*>(env),
+             state, reason);
+      if (env && env->control) {
+        PrintF(", control = ");
+        compiler::WasmGraphBuilder::PrintDebugName(env->control);
+      }
+      PrintF("\n");
     }
-    TRACE("\n");
 #endif
     ssa_env_ = env;
     if (builder_) {
@@ -1417,7 +1293,7 @@
         builder_->AppendToMerge(merge, from->control);
         // Merge effects.
         if (builder_->IsPhiWithMerge(to->effect, merge)) {
-          builder_->AppendToPhi(merge, to->effect, from->effect);
+          builder_->AppendToPhi(to->effect, from->effect);
         } else if (to->effect != from->effect) {
           uint32_t count = builder_->InputCount(merge);
           TFNode** effects = builder_->Buffer(count);
@@ -1432,7 +1308,7 @@
           TFNode* tnode = to->locals[i];
           TFNode* fnode = from->locals[i];
           if (builder_->IsPhiWithMerge(tnode, merge)) {
-            builder_->AppendToPhi(merge, tnode, fnode);
+            builder_->AppendToPhi(tnode, fnode);
           } else if (tnode != fnode) {
             uint32_t count = builder_->InputCount(merge);
             TFNode** vals = builder_->Buffer(count);
@@ -1455,7 +1331,7 @@
   TFNode* CreateOrMergeIntoPhi(LocalType type, TFNode* merge, TFNode* tnode,
                                TFNode* fnode) {
     if (builder_->IsPhiWithMerge(tnode, merge)) {
-      builder_->AppendToPhi(merge, tnode, fnode);
+      builder_->AppendToPhi(tnode, fnode);
     } else if (tnode != fnode) {
       uint32_t count = builder_->InputCount(merge);
       TFNode** vals = builder_->Buffer(count);
@@ -1501,8 +1377,6 @@
     size_t size = sizeof(TFNode*) * EnvironmentCount();
     result->control = from->control;
     result->effect = from->effect;
-    result->state = from->state == SsaEnv::kUnreachable ? SsaEnv::kUnreachable
-                                                        : SsaEnv::kReached;
 
     if (from->go()) {
       result->state = SsaEnv::kReached;
@@ -1549,89 +1423,54 @@
   virtual void onFirstError() {
     limit_ = start_;     // Terminate decoding loop.
     builder_ = nullptr;  // Don't build any more nodes.
-#if DEBUG
-    PrintStackForDebugging();
-#endif
+    TRACE(" !%s\n", error_msg_.get());
   }
-
-#if DEBUG
-  void PrintStackForDebugging() { PrintProduction(0); }
-
-  void PrintProduction(size_t depth) {
-    if (depth >= stack_.size()) return;
-    Production* p = &stack_[depth];
-    for (size_t d = 0; d < depth; d++) PrintF("  ");
-
-    PrintF("@%d %s [%d]\n", static_cast<int>(p->tree->pc - start_),
-           WasmOpcodes::OpcodeName(p->opcode()), p->tree->count);
-    for (int i = 0; i < p->index; i++) {
-      Tree* child = p->tree->children[i];
-      for (size_t d = 0; d <= depth; d++) PrintF("  ");
-      PrintF("@%d %s [%d]", static_cast<int>(child->pc - start_),
-             WasmOpcodes::OpcodeName(child->opcode()), child->count);
-      if (child->node) {
-        PrintF(" => TF");
-        compiler::WasmGraphBuilder::PrintDebugName(child->node);
-      }
-      PrintF("\n");
-    }
-    PrintProduction(depth + 1);
-  }
-#endif
-
   BitVector* AnalyzeLoopAssignment(const byte* pc) {
     if (pc >= limit_) return nullptr;
     if (*pc != kExprLoop) return nullptr;
 
     BitVector* assigned =
-        new (zone_) BitVector(static_cast<int>(total_locals_), zone_);
-    // Keep a stack to model the nesting of expressions.
-    std::vector<int> arity_stack;
-    arity_stack.push_back(OpcodeArity(pc));
-    pc += OpcodeLength(pc);
-
+        new (zone_) BitVector(static_cast<int>(local_type_vec_.size()), zone_);
+    int depth = 0;
     // Iteratively process all AST nodes nested inside the loop.
     while (pc < limit_) {
       WasmOpcode opcode = static_cast<WasmOpcode>(*pc);
-      int arity = 0;
       int length = 1;
-      int assigned_index = -1;
-      if (opcode == kExprSetLocal) {
-        LocalIndexOperand operand(this, pc);
-        if (assigned->length() > 0 &&
-            static_cast<int>(operand.index) < assigned->length()) {
-          // Unverified code might have an out-of-bounds index.
-          // Ignore out-of-bounds indices, as the main verification will fail.
-          assigned->Add(operand.index);
-          assigned_index = operand.index;
+      switch (opcode) {
+        case kExprLoop:
+        case kExprIf:
+        case kExprBlock:
+          depth++;
+          DCHECK_EQ(1, OpcodeLength(pc));
+          break;
+        case kExprSetLocal: {
+          LocalIndexOperand operand(this, pc);
+          if (assigned->length() > 0 &&
+              static_cast<int>(operand.index) < assigned->length()) {
+            // Unverified code might have an out-of-bounds index.
+            assigned->Add(operand.index);
+          }
+          length = 1 + operand.length;
+          break;
         }
-        arity = 1;
-        length = 1 + operand.length;
-      } else {
-        arity = OpcodeArity(pc);
-        length = OpcodeLength(pc);
+        case kExprEnd:
+          depth--;
+          break;
+        default:
+          length = OpcodeLength(pc);
+          break;
       }
-
-      TRACE("loop-assign module+%-6d %s func+%d: 0x%02x %s", baserel(pc),
-            indentation(), startrel(pc), opcode,
-            WasmOpcodes::OpcodeName(opcode));
-
-      if (assigned_index >= 0) {
-        TRACE(" (assigned local #%d)\n", assigned_index);
-      } else {
-        TRACE("\n");
-      }
-
+      if (depth <= 0) break;
       pc += length;
-      arity_stack.push_back(arity);
-      while (arity_stack.back() == 0) {
-        arity_stack.pop_back();
-        if (arity_stack.empty()) return assigned;  // reached end of loop
-        arity_stack.back()--;
-      }
     }
     return assigned;
   }
+
+  inline wasm::WasmCodePosition position() {
+    int offset = static_cast<int>(pc_ - start_);
+    DCHECK_EQ(pc_ - start_, offset);  // overflows cannot happen
+    return offset;
+  }
 };
 
 bool DecodeLocalDecls(AstLocalDecls& decls, const byte* start,
@@ -1647,19 +1486,18 @@
                           FunctionBody& body) {
   Zone zone(allocator);
   SR_WasmDecoder decoder(&zone, nullptr, body);
-  TreeResult result = decoder.Decode();
-  return result;
+  decoder.Decode();
+  return decoder.toResult<Tree*>(nullptr);
 }
 
 TreeResult BuildTFGraph(base::AccountingAllocator* allocator,
                         TFBuilder* builder, FunctionBody& body) {
   Zone zone(allocator);
   SR_WasmDecoder decoder(&zone, builder, body);
-  TreeResult result = decoder.Decode();
-  return result;
+  decoder.Decode();
+  return decoder.toResult<Tree*>(nullptr);
 }
 
-
 std::ostream& operator<<(std::ostream& os, const Tree& tree) {
   if (tree.pc == nullptr) {
     os << "null";
@@ -1675,28 +1513,22 @@
   return os;
 }
 
-
-ReadUnsignedLEB128ErrorCode ReadUnsignedLEB128Operand(const byte* pc,
-                                                      const byte* limit,
-                                                      int* length,
-                                                      uint32_t* result) {
-  Decoder decoder(pc, limit);
-  *result = decoder.checked_read_u32v(pc, 0, length);
-  if (decoder.ok()) return kNoError;
-  return (limit - pc) > 1 ? kInvalidLEB128 : kMissingLEB128;
-}
-
 int OpcodeLength(const byte* pc, const byte* end) {
   WasmDecoder decoder(nullptr, nullptr, pc, end);
   return decoder.OpcodeLength(pc);
 }
 
-int OpcodeArity(ModuleEnv* module, FunctionSig* sig, const byte* pc,
-                const byte* end) {
-  WasmDecoder decoder(module, sig, pc, end);
+int OpcodeArity(const byte* pc, const byte* end) {
+  WasmDecoder decoder(nullptr, nullptr, pc, end);
   return decoder.OpcodeArity(pc);
 }
 
+void PrintAstForDebugging(const byte* start, const byte* end) {
+  FunctionBody body = {nullptr, nullptr, start, start, end};
+  base::AccountingAllocator allocator;
+  PrintAst(&allocator, body);
+}
+
 void PrintAst(base::AccountingAllocator* allocator, FunctionBody& body) {
   Zone zone(allocator);
   SR_WasmDecoder decoder(&zone, nullptr, body);
@@ -1713,7 +1545,7 @@
   decoder.DecodeLocalDecls(decls);
   const byte* pc = decoder.pc();
   if (body.start != decoder.pc()) {
-    printf("// locals:");
+    os << "// locals: ";
     for (auto p : decls.local_types) {
       LocalType type = p.first;
       uint32_t count = p.second;
@@ -1724,64 +1556,90 @@
     for (const byte* locals = body.start; locals < pc; locals++) {
       printf(" 0x%02x,", *locals);
     }
-    printf("\n");
+    os << std::endl;
   }
 
-  printf("// body: \n");
-  std::vector<int> arity_stack;
+  os << "// body: \n";
+  int control_depth = 0;
   while (pc < body.end) {
-    int arity = decoder.OpcodeArity(pc);
     size_t length = decoder.OpcodeLength(pc);
 
-    for (auto arity : arity_stack) {
-      printf("  ");
-      USE(arity);
-    }
-
     WasmOpcode opcode = static_cast<WasmOpcode>(*pc);
+    if (opcode == kExprElse) control_depth--;
+
+    for (int i = 0; i < control_depth && i < 32; i++) printf("  ");
     printf("k%s,", WasmOpcodes::OpcodeName(opcode));
 
     for (size_t i = 1; i < length; i++) {
       printf(" 0x%02x,", pc[i]);
     }
 
-    if (body.module) {
-      switch (opcode) {
-        case kExprCallIndirect: {
-          SignatureIndexOperand operand(&decoder, pc);
-          if (decoder.Validate(pc, operand)) {
-            os << " // sig #" << operand.index << ": " << *operand.sig;
-          }
-          break;
-        }
-        case kExprCallImport: {
-          ImportIndexOperand operand(&decoder, pc);
-          if (decoder.Validate(pc, operand)) {
-            os << " // import #" << operand.index << ": " << *operand.sig;
-          }
-          break;
-        }
-        case kExprCallFunction: {
-          FunctionIndexOperand operand(&decoder, pc);
-          if (decoder.Validate(pc, operand)) {
-            os << " // function #" << operand.index << ": " << *operand.sig;
-          }
-          break;
-        }
-        default:
-          break;
+    switch (opcode) {
+      case kExprIf:
+      case kExprElse:
+      case kExprLoop:
+      case kExprBlock:
+        os << "   // @" << static_cast<int>(pc - body.start);
+        control_depth++;
+        break;
+      case kExprEnd:
+        os << "   // @" << static_cast<int>(pc - body.start);
+        control_depth--;
+        break;
+      case kExprBr: {
+        BreakDepthOperand operand(&decoder, pc);
+        os << "   // arity=" << operand.arity << " depth=" << operand.depth;
+        break;
       }
-    }
+      case kExprBrIf: {
+        BreakDepthOperand operand(&decoder, pc);
+        os << "   // arity=" << operand.arity << " depth" << operand.depth;
+        break;
+      }
+      case kExprBrTable: {
+        BranchTableOperand operand(&decoder, pc);
+        os << "   // arity=" << operand.arity
+           << " entries=" << operand.table_count;
+        break;
+      }
+      case kExprCallIndirect: {
+        CallIndirectOperand operand(&decoder, pc);
+        if (decoder.Validate(pc, operand)) {
+          os << "   // sig #" << operand.index << ": " << *operand.sig;
+        } else {
+          os << " // arity=" << operand.arity << " sig #" << operand.index;
+        }
+        break;
+      }
+      case kExprCallImport: {
+        CallImportOperand operand(&decoder, pc);
+        if (decoder.Validate(pc, operand)) {
+          os << "   // import #" << operand.index << ": " << *operand.sig;
+        } else {
+          os << " // arity=" << operand.arity << " import #" << operand.index;
+        }
+        break;
+      }
+      case kExprCallFunction: {
+        CallFunctionOperand operand(&decoder, pc);
+        if (decoder.Validate(pc, operand)) {
+          os << "   // function #" << operand.index << ": " << *operand.sig;
+        } else {
+          os << " // arity=" << operand.arity << " function #" << operand.index;
+        }
+        break;
+      }
+      case kExprReturn: {
+        ReturnArityOperand operand(&decoder, pc);
+        os << "   // arity=" << operand.arity;
+        break;
+      }
+      default:
+        break;
+      }
 
     pc += length;
-    printf("\n");
-
-    arity_stack.push_back(arity);
-    while (arity_stack.back() == 0) {
-      arity_stack.pop_back();
-      if (arity_stack.empty()) break;
-      arity_stack.back()--;
-    }
+    os << std::endl;
   }
 }