Revert "Added constant propagation and better variable liveness tracking to"

This reverts commit f54b07121f81a56145fb118a2e18841fc135717d.

Reason for revert: ASAN failure

Original change's description:
> Added constant propagation and better variable liveness tracking to
> skslc. 
> 
> This allows skslc to track the values of variables with constant
> values across multiple statements and replace variable references with
> constant values where appropriate.
> 
> The improved liveness tracking allows skslc to realize that a
> variable is no longer alive if all references to it have been
> replaced. It is not yet doing much with this information; better
> dead code elimination is coming in a followup change.
> 
> BUG=skia:
> 
> Change-Id: I6bf267d478b769caf0063ac3597dc16bbe618cb4
> Reviewed-on: https://skia-review.googlesource.com/7033
> Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
> Reviewed-by: Greg Daniel <egdaniel@google.com>
> 

TBR=egdaniel@google.com,ethannicholas@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Change-Id: Id2e26bce96b27df73948f8b32d3dff2e358ae0d6
Reviewed-on: https://skia-review.googlesource.com/7274
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
diff --git a/src/sksl/SkSLCFGGenerator.cpp b/src/sksl/SkSLCFGGenerator.cpp
index 31bace9..964a8dc 100644
--- a/src/sksl/SkSLCFGGenerator.cpp
+++ b/src/sksl/SkSLCFGGenerator.cpp
@@ -54,8 +54,8 @@
         printf("Block %d\n-------\nBefore: ", (int) i);
         const char* separator = "";
         for (auto iter = fBlocks[i].fBefore.begin(); iter != fBlocks[i].fBefore.end(); iter++) {
-            printf("%s%s = %s", separator, iter->first->description().c_str(),
-                   *iter->second ? (*iter->second)->description().c_str() : "<undefined>");
+            printf("%s%s = %s", separator, iter->first->description().c_str(), 
+                   iter->second ? iter->second->description().c_str() : "<undefined>");
             separator = ", ";
         }
         printf("\nEntrances: ");
@@ -66,10 +66,7 @@
         }
         printf("\n");
         for (size_t j = 0; j < fBlocks[i].fNodes.size(); j++) {
-            BasicBlock::Node& n = fBlocks[i].fNodes[j];
-            printf("Node %d: %s\n", (int) j, n.fKind == BasicBlock::Node::kExpression_Kind
-                                                           ? (*n.fExpression)->description().c_str()
-                                                           : n.fStatement->description().c_str());
+            printf("Node %d: %s\n", (int) j, fBlocks[i].fNodes[j].fNode->description().c_str());
         }
         printf("Exits: ");
         separator = "";
@@ -81,109 +78,96 @@
     }
 }
 
-void CFGGenerator::addExpression(CFG& cfg, std::unique_ptr<Expression>* e, bool constantPropagate) {
-    ASSERT(e);
-    switch ((*e)->fKind) {
+void CFGGenerator::addExpression(CFG& cfg, const Expression* e) {
+    switch (e->fKind) {
         case Expression::kBinary_Kind: {
-            BinaryExpression* b = (BinaryExpression*) e->get();
+            const BinaryExpression* b = (const BinaryExpression*) e;
             switch (b->fOperator) {
                 case Token::LOGICALAND: // fall through
                 case Token::LOGICALOR: {
                     // this isn't as precise as it could be -- we don't bother to track that if we
                     // early exit from a logical and/or, we know which branch of an 'if' we're going
                     // to hit -- but it won't make much difference in practice.
-                    this->addExpression(cfg, &b->fLeft, constantPropagate);
+                    this->addExpression(cfg, b->fLeft.get());
                     BlockId start = cfg.fCurrent;
                     cfg.newBlock();
-                    this->addExpression(cfg, &b->fRight, constantPropagate);
+                    this->addExpression(cfg, b->fRight.get());
                     cfg.newBlock();
                     cfg.addExit(start, cfg.fCurrent);
                     break;
                 }
                 case Token::EQ: {
-                    this->addExpression(cfg, &b->fRight, constantPropagate);
-                    this->addLValue(cfg, &b->fLeft);
-                    cfg.fBlocks[cfg.fCurrent].fNodes.push_back({
-                        BasicBlock::Node::kExpression_Kind,
-                        constantPropagate,
-                        e,
-                        nullptr
+                    this->addExpression(cfg, b->fRight.get());
+                    this->addLValue(cfg, b->fLeft.get());
+                    cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ 
+                        BasicBlock::Node::kExpression_Kind, 
+                        b 
                     });
                     break;
                 }
                 default:
-                    this->addExpression(cfg, &b->fLeft, !Token::IsAssignment(b->fOperator));
-                    this->addExpression(cfg, &b->fRight, constantPropagate);
-                    cfg.fBlocks[cfg.fCurrent].fNodes.push_back({
-                        BasicBlock::Node::kExpression_Kind,
-                        constantPropagate,
-                        e,
-                        nullptr
+                    this->addExpression(cfg, b->fLeft.get());
+                    this->addExpression(cfg, b->fRight.get());
+                    cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ 
+                        BasicBlock::Node::kExpression_Kind, 
+                        b 
                     });
             }
             break;
         }
         case Expression::kConstructor_Kind: {
-            Constructor* c = (Constructor*) e->get();
-            for (auto& arg : c->fArguments) {
-                this->addExpression(cfg, &arg, constantPropagate);
+            const Constructor* c = (const Constructor*) e;
+            for (const auto& arg : c->fArguments) {
+                this->addExpression(cfg, arg.get());
             }
-            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind,
-                                                         constantPropagate, e, nullptr });
+            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind, c });
             break;
         }
         case Expression::kFunctionCall_Kind: {
-            FunctionCall* c = (FunctionCall*) e->get();
-            for (auto& arg : c->fArguments) {
-                this->addExpression(cfg, &arg, constantPropagate);
+            const FunctionCall* c = (const FunctionCall*) e;
+            for (const auto& arg : c->fArguments) {
+                this->addExpression(cfg, arg.get());
             }
-            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind,
-                                                         constantPropagate, e, nullptr });
+            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind, c });
             break;
         }
         case Expression::kFieldAccess_Kind:
-            this->addExpression(cfg, &((FieldAccess*) e->get())->fBase, constantPropagate);
-            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind,
-                                                         constantPropagate, e, nullptr });
+            this->addExpression(cfg, ((const FieldAccess*) e)->fBase.get());
+            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind, e });
             break;
         case Expression::kIndex_Kind:
-            this->addExpression(cfg, &((IndexExpression*) e->get())->fBase, constantPropagate);
-            this->addExpression(cfg, &((IndexExpression*) e->get())->fIndex, constantPropagate);
-            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind,
-                                                         constantPropagate, e, nullptr });
+            this->addExpression(cfg, ((const IndexExpression*) e)->fBase.get());
+            this->addExpression(cfg, ((const IndexExpression*) e)->fIndex.get());
+            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind, e });
             break;
         case Expression::kPrefix_Kind:
-            this->addExpression(cfg, &((PrefixExpression*) e->get())->fOperand, constantPropagate);
-            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind,
-                                                         constantPropagate, e, nullptr });
+            this->addExpression(cfg, ((const PrefixExpression*) e)->fOperand.get());
+            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind, e });
             break;
         case Expression::kPostfix_Kind:
-            this->addExpression(cfg, &((PostfixExpression*) e->get())->fOperand, constantPropagate);
-            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind,
-                                                         constantPropagate, e, nullptr });
+            this->addExpression(cfg, ((const PostfixExpression*) e)->fOperand.get());
+            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind, e });
             break;
         case Expression::kSwizzle_Kind:
-            this->addExpression(cfg, &((Swizzle*) e->get())->fBase, constantPropagate);
-            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind,
-                                                         constantPropagate, e, nullptr });
+            this->addExpression(cfg, ((const Swizzle*) e)->fBase.get());
+            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind, e });
             break;
         case Expression::kBoolLiteral_Kind:  // fall through
         case Expression::kFloatLiteral_Kind: // fall through
         case Expression::kIntLiteral_Kind:   // fall through
         case Expression::kVariableReference_Kind:
-            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind,
-                                                         constantPropagate, e, nullptr });
+            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kExpression_Kind, e });
             break;
         case Expression::kTernary_Kind: {
-            TernaryExpression* t = (TernaryExpression*) e->get();
-            this->addExpression(cfg, &t->fTest, constantPropagate);
+            const TernaryExpression* t = (const TernaryExpression*) e;
+            this->addExpression(cfg, t->fTest.get());
             BlockId start = cfg.fCurrent;
             cfg.newBlock();
-            this->addExpression(cfg, &t->fIfTrue, constantPropagate);
+            this->addExpression(cfg, t->fIfTrue.get());
             BlockId next = cfg.newBlock();
             cfg.fCurrent = start;
             cfg.newBlock();
-            this->addExpression(cfg, &t->fIfFalse, constantPropagate);
+            this->addExpression(cfg, t->fIfFalse.get());
             cfg.addExit(cfg.fCurrent, next);
             cfg.fCurrent = next;
             break;
@@ -197,17 +181,17 @@
 }
 
 // adds expressions that are evaluated as part of resolving an lvalue
-void CFGGenerator::addLValue(CFG& cfg, std::unique_ptr<Expression>* e) {
-    switch ((*e)->fKind) {
+void CFGGenerator::addLValue(CFG& cfg, const Expression* e) {
+    switch (e->fKind) {
         case Expression::kFieldAccess_Kind:
-            this->addLValue(cfg, &((FieldAccess&) **e).fBase);
+            this->addLValue(cfg, ((const FieldAccess*) e)->fBase.get());
             break;
         case Expression::kIndex_Kind:
-            this->addLValue(cfg, &((IndexExpression&) **e).fBase);
-            this->addExpression(cfg, &((IndexExpression&) **e).fIndex, true);
+            this->addLValue(cfg, ((const IndexExpression*) e)->fBase.get());
+            this->addExpression(cfg, ((const IndexExpression*) e)->fIndex.get());
             break;
         case Expression::kSwizzle_Kind:
-            this->addLValue(cfg, &((Swizzle&) **e).fBase);
+            this->addLValue(cfg, ((const Swizzle*) e)->fBase.get());
             break;
         case Expression::kVariableReference_Kind:
             break;
@@ -226,8 +210,8 @@
             }
             break;
         case Statement::kIf_Kind: {
-            IfStatement* ifs = (IfStatement*) s;
-            this->addExpression(cfg, &ifs->fTest, true);
+            const IfStatement* ifs = (const IfStatement*) s;
+            this->addExpression(cfg, ifs->fTest.get());
             BlockId start = cfg.fCurrent;
             cfg.newBlock();
             this->addStatement(cfg, ifs->fIfTrue.get());
@@ -244,54 +228,49 @@
             break;
         }
         case Statement::kExpression_Kind: {
-            this->addExpression(cfg, &((ExpressionStatement&) *s).fExpression, true);
+            this->addExpression(cfg, ((ExpressionStatement&) *s).fExpression.get());
             break;
         }
         case Statement::kVarDeclarations_Kind: {
-            VarDeclarationsStatement& decls = ((VarDeclarationsStatement&) *s);
-            for (auto& vd : decls.fDeclaration->fVars) {
+            const VarDeclarationsStatement& decls = ((VarDeclarationsStatement&) *s);
+            for (const auto& vd : decls.fDeclaration->fVars) {
                 if (vd.fValue) {
-                    this->addExpression(cfg, &vd.fValue, true);
+                    this->addExpression(cfg, vd.fValue.get());
                 }
             }
-            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kStatement_Kind, false,
-                                                         nullptr, s });
+            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kStatement_Kind, s });
             break;
         }
         case Statement::kDiscard_Kind:
-            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kStatement_Kind, false,
-                                                         nullptr, s });
+            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kStatement_Kind, s });
             cfg.fCurrent = cfg.newIsolatedBlock();
             break;
         case Statement::kReturn_Kind: {
-            ReturnStatement& r = ((ReturnStatement&) *s);
+            const ReturnStatement& r = ((ReturnStatement&) *s);
             if (r.fExpression) {
-                this->addExpression(cfg, &r.fExpression, true);
+                this->addExpression(cfg, r.fExpression.get());
             }
-            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kStatement_Kind, false,
-                                                         nullptr, s });
+            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kStatement_Kind, s });
             cfg.fCurrent = cfg.newIsolatedBlock();
             break;
         }
         case Statement::kBreak_Kind:
-            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kStatement_Kind, false,
-                                                         nullptr, s });
+            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kStatement_Kind, s });
             cfg.addExit(cfg.fCurrent, fLoopExits.top());
             cfg.fCurrent = cfg.newIsolatedBlock();
             break;
         case Statement::kContinue_Kind:
-            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kStatement_Kind, false,
-                                                         nullptr, s });
+            cfg.fBlocks[cfg.fCurrent].fNodes.push_back({ BasicBlock::Node::kStatement_Kind, s });
             cfg.addExit(cfg.fCurrent, fLoopContinues.top());
             cfg.fCurrent = cfg.newIsolatedBlock();
             break;
         case Statement::kWhile_Kind: {
-            WhileStatement* w = (WhileStatement*) s;
+            const WhileStatement* w = (const WhileStatement*) s;
             BlockId loopStart = cfg.newBlock();
             fLoopContinues.push(loopStart);
             BlockId loopExit = cfg.newIsolatedBlock();
             fLoopExits.push(loopExit);
-            this->addExpression(cfg, &w->fTest, true);
+            this->addExpression(cfg, w->fTest.get());
             BlockId test = cfg.fCurrent;
             cfg.addExit(test, loopExit);
             cfg.newBlock();
@@ -303,13 +282,13 @@
             break;
         }
         case Statement::kDo_Kind: {
-            DoStatement* d = (DoStatement*) s;
+            const DoStatement* d = (const DoStatement*) s;
             BlockId loopStart = cfg.newBlock();
             fLoopContinues.push(loopStart);
             BlockId loopExit = cfg.newIsolatedBlock();
             fLoopExits.push(loopExit);
             this->addStatement(cfg, d->fStatement.get());
-            this->addExpression(cfg, &d->fTest, true);
+            this->addExpression(cfg, d->fTest.get());
             cfg.addExit(cfg.fCurrent, loopExit);
             cfg.addExit(cfg.fCurrent, loopStart);
             fLoopContinues.pop();
@@ -318,7 +297,7 @@
             break;
         }
         case Statement::kFor_Kind: {
-            ForStatement* f = (ForStatement*) s;
+            const ForStatement* f = (const ForStatement*) s;
             if (f->fInitializer) {
                 this->addStatement(cfg, f->fInitializer.get());
             }
@@ -328,7 +307,7 @@
             BlockId loopExit = cfg.newIsolatedBlock();
             fLoopExits.push(loopExit);
             if (f->fTest) {
-                this->addExpression(cfg, &f->fTest, true);
+                this->addExpression(cfg, f->fTest.get());
                 BlockId test = cfg.fCurrent;
                 cfg.addExit(test, loopExit);
             }
@@ -337,9 +316,9 @@
             cfg.addExit(cfg.fCurrent, next);
             cfg.fCurrent = next;
             if (f->fNext) {
-                this->addExpression(cfg, &f->fNext, true);
+                this->addExpression(cfg, f->fNext.get());
             }
-            cfg.addExit(cfg.fCurrent, loopStart);
+            cfg.addExit(next, loopStart);
             fLoopContinues.pop();
             fLoopExits.pop();
             cfg.fCurrent = loopExit;
diff --git a/src/sksl/SkSLCFGGenerator.h b/src/sksl/SkSLCFGGenerator.h
index 337fdfa..c378501 100644
--- a/src/sksl/SkSLCFGGenerator.h
+++ b/src/sksl/SkSLCFGGenerator.h
@@ -27,23 +27,14 @@
         };
 
         Kind fKind;
-        // if false, this node should not be subject to constant propagation. This happens with
-        // compound assignment (i.e. x *= 2), in which the value x is used as an rvalue for
-        // multiplication by 2 and then as an lvalue for assignment purposes. Since there is only
-        // one "x" node, replacing it with a constant would break the assignment and we suppress
-        // it. Down the road, we should handle this more elegantly by substituting a regular
-        // assignment if the target is constant (i.e. x = 1; x *= 2; should become x = 1; x = 1 * 2;
-        // and then collapse down to a simple x = 2;).
-        bool fConstantPropagation;
-        std::unique_ptr<Expression>* fExpression;
-        const Statement* fStatement;
+        const IRNode* fNode;
     };
-
+    
     std::vector<Node> fNodes;
     std::set<BlockId> fEntrances;
     std::set<BlockId> fExits;
     // variable definitions upon entering this basic block (null expression = undefined)
-    DefinitionMap fBefore;
+    std::unordered_map<const Variable*, const Expression*> fBefore;
 };
 
 struct CFG {
@@ -86,9 +77,9 @@
 private:
     void addStatement(CFG& cfg, const Statement* s);
 
-    void addExpression(CFG& cfg, std::unique_ptr<Expression>* e, bool constantPropagate);
+    void addExpression(CFG& cfg, const Expression* e);
 
-    void addLValue(CFG& cfg, std::unique_ptr<Expression>* e);
+    void addLValue(CFG& cfg, const Expression* e);
 
     std::stack<BlockId> fLoopContinues;
     std::stack<BlockId> fLoopExits;
diff --git a/src/sksl/SkSLCompiler.cpp b/src/sksl/SkSLCompiler.cpp
index 743745a..9faf836 100644
--- a/src/sksl/SkSLCompiler.cpp
+++ b/src/sksl/SkSLCompiler.cpp
@@ -156,8 +156,8 @@
 }
 
 // add the definition created by assigning to the lvalue to the definition set
-void Compiler::addDefinition(const Expression* lvalue, std::unique_ptr<Expression>* expr,
-                             DefinitionMap* definitions) {
+void Compiler::addDefinition(const Expression* lvalue, const Expression* expr,
+                           std::unordered_map<const Variable*, const Expression*>* definitions) {
     switch (lvalue->fKind) {
         case Expression::kVariableReference_Kind: {
             const Variable& var = ((VariableReference*) lvalue)->fVariable;
@@ -174,19 +174,19 @@
             // but since we pass foo as a whole it is flagged as an error) unless we perform a much
             // more complicated whole-program analysis. This is probably good enough.
             this->addDefinition(((Swizzle*) lvalue)->fBase.get(),
-                                (std::unique_ptr<Expression>*) &fContext.fDefined_Expression,
+                                fContext.fDefined_Expression.get(),
                                 definitions);
             break;
         case Expression::kIndex_Kind:
             // see comments in Swizzle
             this->addDefinition(((IndexExpression*) lvalue)->fBase.get(),
-                                (std::unique_ptr<Expression>*) &fContext.fDefined_Expression,
+                                fContext.fDefined_Expression.get(),
                                 definitions);
             break;
         case Expression::kFieldAccess_Kind:
             // see comments in Swizzle
             this->addDefinition(((FieldAccess*) lvalue)->fBase.get(),
-                                (std::unique_ptr<Expression>*) &fContext.fDefined_Expression,
+                                fContext.fDefined_Expression.get(),
                                 definitions);
             break;
         default:
@@ -197,58 +197,25 @@
 
 // add local variables defined by this node to the set
 void Compiler::addDefinitions(const BasicBlock::Node& node,
-                              DefinitionMap* definitions) {
+                              std::unordered_map<const Variable*, const Expression*>* definitions) {
     switch (node.fKind) {
         case BasicBlock::Node::kExpression_Kind: {
-            ASSERT(node.fExpression);
-            const Expression* expr = (Expression*) node.fExpression->get();
-            switch (expr->fKind) {
-                case Expression::kBinary_Kind: {
-                    BinaryExpression* b = (BinaryExpression*) expr;
-                    if (b->fOperator == Token::EQ) {
-                        this->addDefinition(b->fLeft.get(), &b->fRight, definitions);
-                    } else if (Token::IsAssignment(b->fOperator)) {
-                        this->addDefinition(
-                                       b->fLeft.get(),
-                                       (std::unique_ptr<Expression>*) &fContext.fDefined_Expression,
-                                       definitions);
-
-                    }
-                    break;
+            const Expression* expr = (Expression*) node.fNode;
+            if (expr->fKind == Expression::kBinary_Kind) {
+                const BinaryExpression* b = (BinaryExpression*) expr;
+                if (b->fOperator == Token::EQ) {
+                    this->addDefinition(b->fLeft.get(), b->fRight.get(), definitions);
                 }
-                case Expression::kPrefix_Kind: {
-                    const PrefixExpression* p = (PrefixExpression*) expr;
-                    if (p->fOperator == Token::MINUSMINUS || p->fOperator == Token::PLUSPLUS) {
-                        this->addDefinition(
-                                       p->fOperand.get(),
-                                       (std::unique_ptr<Expression>*) &fContext.fDefined_Expression,
-                                       definitions);
-                    }
-                    break;
-                }
-                case Expression::kPostfix_Kind: {
-                    const PostfixExpression* p = (PostfixExpression*) expr;
-                    if (p->fOperator == Token::MINUSMINUS || p->fOperator == Token::PLUSPLUS) {
-                        this->addDefinition(
-                                       p->fOperand.get(),
-                                       (std::unique_ptr<Expression>*) &fContext.fDefined_Expression,
-                                       definitions);
-
-                    }
-                    break;
-                }
-                default:
-                    break;
             }
             break;
         }
         case BasicBlock::Node::kStatement_Kind: {
-            const Statement* stmt = (Statement*) node.fStatement;
+            const Statement* stmt = (Statement*) node.fNode;
             if (stmt->fKind == Statement::kVarDeclarations_Kind) {
-                VarDeclarationsStatement* vd = (VarDeclarationsStatement*) stmt;
-                for (VarDeclaration& decl : vd->fDeclaration->fVars) {
+                const VarDeclarationsStatement* vd = (VarDeclarationsStatement*) stmt;
+                for (const VarDeclaration& decl : vd->fDeclaration->fVars) {
                     if (decl.fValue) {
-                        (*definitions)[decl.fVar] = &decl.fValue;
+                        (*definitions)[decl.fVar] = decl.fValue.get();
                     }
                 }
             }
@@ -261,7 +228,7 @@
     BasicBlock& block = cfg->fBlocks[blockId];
 
     // compute definitions after this block
-    DefinitionMap after = block.fBefore;
+    std::unordered_map<const Variable*, const Expression*> after = block.fBefore;
     for (const BasicBlock::Node& n : block.fNodes) {
         this->addDefinitions(n, &after);
     }
@@ -270,20 +237,19 @@
     for (BlockId exitId : block.fExits) {
         BasicBlock& exit = cfg->fBlocks[exitId];
         for (const auto& pair : after) {
-            std::unique_ptr<Expression>* e1 = pair.second;
-            auto found = exit.fBefore.find(pair.first);
-            if (found == exit.fBefore.end()) {
-                // exit has no definition for it, just copy it
-                workList->insert(exitId);
+            const Expression* e1 = pair.second;
+            if (exit.fBefore.find(pair.first) == exit.fBefore.end()) {
                 exit.fBefore[pair.first] = e1;
             } else {
-                // exit has a (possibly different) value already defined
-                std::unique_ptr<Expression>* e2 = exit.fBefore[pair.first];
+                const Expression* e2 = exit.fBefore[pair.first];
                 if (e1 != e2) {
                     // definition has changed, merge and add exit block to worklist
                     workList->insert(exitId);
-                    exit.fBefore[pair.first] =
-                                       (std::unique_ptr<Expression>*) &fContext.fDefined_Expression;
+                    if (!e1 || !e2) {
+                        exit.fBefore[pair.first] = nullptr;
+                    } else {
+                        exit.fBefore[pair.first] = fContext.fDefined_Expression.get();
+                    }
                 }
             }
         }
@@ -292,13 +258,12 @@
 
 // returns a map which maps all local variables in the function to null, indicating that their value
 // is initially unknown
-static DefinitionMap compute_start_state(const CFG& cfg) {
-    DefinitionMap result;
+static std::unordered_map<const Variable*, const Expression*> compute_start_state(const CFG& cfg) {
+    std::unordered_map<const Variable*, const Expression*> result;
     for (const auto& block : cfg.fBlocks) {
         for (const auto& node : block.fNodes) {
             if (node.fKind == BasicBlock::Node::kStatement_Kind) {
-                ASSERT(node.fStatement);
-                const Statement* s = node.fStatement;
+                const Statement* s = (Statement*) node.fNode;
                 if (s->fKind == Statement::kVarDeclarations_Kind) {
                     const VarDeclarationsStatement* vd = (const VarDeclarationsStatement*) s;
                     for (const VarDeclaration& decl : vd->fDeclaration->fVars) {
@@ -330,37 +295,19 @@
     for (size_t i = 0; i < cfg.fBlocks.size(); i++) {
         if (i != cfg.fStart && !cfg.fBlocks[i].fEntrances.size() &&
             cfg.fBlocks[i].fNodes.size()) {
-            Position p;
-            switch (cfg.fBlocks[i].fNodes[0].fKind) {
-                case BasicBlock::Node::kStatement_Kind:
-                    p = cfg.fBlocks[i].fNodes[0].fStatement->fPosition;
-                    break;
-                case BasicBlock::Node::kExpression_Kind:
-                    p = (*cfg.fBlocks[i].fNodes[0].fExpression)->fPosition;
-                    break;
-            }
-            this->error(p, SkString("unreachable"));
+            this->error(cfg.fBlocks[i].fNodes[0].fNode->fPosition, SkString("unreachable"));
         }
     }
     if (fErrorCount) {
         return;
     }
 
-    // check for undefined variables, perform constant propagation
-    for (BasicBlock& b : cfg.fBlocks) {
-        DefinitionMap definitions = b.fBefore;
-        for (BasicBlock::Node& n : b.fNodes) {
+    // check for undefined variables
+    for (const BasicBlock& b : cfg.fBlocks) {
+        std::unordered_map<const Variable*, const Expression*> definitions = b.fBefore;
+        for (const BasicBlock::Node& n : b.fNodes) {
             if (n.fKind == BasicBlock::Node::kExpression_Kind) {
-                ASSERT(n.fExpression);
-                Expression* expr = n.fExpression->get();
-                if (n.fConstantPropagation) {
-                    std::unique_ptr<Expression> optimized = expr->constantPropagate(*fIRGenerator,
-                                                                                    definitions);
-                    if (optimized) {
-                        n.fExpression->reset(optimized.release());
-                        expr = n.fExpression->get();
-                    }
-                }
+                const Expression* expr = (const Expression*) n.fNode;
                 if (expr->fKind == Expression::kVariableReference_Kind) {
                     const Variable& var = ((VariableReference*) expr)->fVariable;
                     if (var.fStorage == Variable::kLocal_Storage &&
diff --git a/src/sksl/SkSLCompiler.h b/src/sksl/SkSLCompiler.h
index fdca12d..0f893f7 100644
--- a/src/sksl/SkSLCompiler.h
+++ b/src/sksl/SkSLCompiler.h
@@ -60,10 +60,11 @@
     }
 
 private:
-    void addDefinition(const Expression* lvalue, std::unique_ptr<Expression>* expr,
-                       DefinitionMap* definitions);
+    void addDefinition(const Expression* lvalue, const Expression* expr,
+                       std::unordered_map<const Variable*, const Expression*>* definitions);
 
-    void addDefinitions(const BasicBlock::Node& node, DefinitionMap* definitions);
+    void addDefinitions(const BasicBlock::Node& node,
+                        std::unordered_map<const Variable*, const Expression*>* definitions);
 
     void scanCFG(CFG* cfg, BlockId block, std::set<BlockId>* workList);
 
diff --git a/src/sksl/SkSLIRGenerator.cpp b/src/sksl/SkSLIRGenerator.cpp
index 55d9d2c..9f06c97 100644
--- a/src/sksl/SkSLIRGenerator.cpp
+++ b/src/sksl/SkSLIRGenerator.cpp
@@ -551,11 +551,11 @@
         }
     }
     Type* type = new Type(intf.fPosition, intf.fInterfaceName, fields);
-    old->takeOwnership(type);
+    fSymbolTable->takeOwnership(type);
     SkString name = intf.fValueName.size() > 0 ? intf.fValueName : intf.fInterfaceName;
     Variable* var = new Variable(intf.fPosition, intf.fModifiers, name, *type,
                                  Variable::kGlobal_Storage);
-    old->takeOwnership(var);
+    fSymbolTable->takeOwnership(var);
     if (intf.fValueName.size()) {
         old->addWithoutOwnership(intf.fValueName, var);
     } else {
@@ -624,22 +624,19 @@
                                                                             f->fFunctions));
         }
         case Symbol::kVariable_Kind: {
-            Variable* var = (Variable*) result;
+            const Variable* var = (const Variable*) result;
+            this->markReadFrom(*var);
             if (var->fModifiers.fLayout.fBuiltin == SK_FRAGCOORD_BUILTIN &&
                 fSettings->fFlipY &&
                 (!fSettings->fCaps || !fSettings->fCaps->fragCoordConventionsExtensionString())) {
                 fInputs.fRTHeight = true;
             }
-            // default to kRead_RefKind; this will be corrected later if the variable is written to
-            return std::unique_ptr<VariableReference>(new VariableReference(
-                                                                 identifier.fPosition,
-                                                                 *var,
-                                                                 VariableReference::kRead_RefKind));
+            return std::unique_ptr<VariableReference>(new VariableReference(identifier.fPosition,
+                                                                            *var));
         }
         case Symbol::kField_Kind: {
             const Field* field = (const Field*) result;
-            VariableReference* base = new VariableReference(identifier.fPosition, field->fOwner,
-                                                            VariableReference::kRead_RefKind);
+            VariableReference* base = new VariableReference(identifier.fPosition, field->fOwner);
             return std::unique_ptr<Expression>(new FieldAccess(
                                                   std::unique_ptr<Expression>(base),
                                                   field->fFieldIndex,
@@ -693,6 +690,28 @@
     return left.kind() == Type::kVector_Kind && right.kind() == Type::kMatrix_Kind;
 }
 
+static bool is_assignment(Token::Kind op) {
+    switch (op) {
+        case Token::EQ:           // fall through
+        case Token::PLUSEQ:       // fall through
+        case Token::MINUSEQ:      // fall through
+        case Token::STAREQ:       // fall through
+        case Token::SLASHEQ:      // fall through
+        case Token::PERCENTEQ:    // fall through
+        case Token::SHLEQ:        // fall through
+        case Token::SHREQ:        // fall through
+        case Token::BITWISEOREQ:  // fall through
+        case Token::BITWISEXOREQ: // fall through
+        case Token::BITWISEANDEQ: // fall through
+        case Token::LOGICALOREQ:  // fall through
+        case Token::LOGICALXOREQ: // fall through
+        case Token::LOGICALANDEQ:
+            return true;
+        default:
+            return false;
+    }
+}
+
 /**
  * Determines the operand and result types of a binary expression. Returns true if the expression is
  * legal, false otherwise. If false, the values of the out parameters are undefined.
@@ -823,9 +842,14 @@
     return false;
 }
 
+/**
+ * If both operands are compile-time constants and can be folded, returns an expression representing
+ * the folded value. Otherwise, returns null. Note that unlike most other functions here, null does
+ * not represent a compilation error.
+ */
 std::unique_ptr<Expression> IRGenerator::constantFold(const Expression& left,
                                                       Token::Kind op,
-                                                      const Expression& right) const {
+                                                      const Expression& right) {
     // Note that we expressly do not worry about precision and overflow here -- we use the maximum
     // precision to calculate the results and hope the result makes sense. The plan is to move the
     // Skia caps into SkSL, so we have access to all of them including the precisions of the various
@@ -919,16 +943,15 @@
     const Type* rightType;
     const Type* resultType;
     if (!determine_binary_type(fContext, expression.fOperator, left->fType, right->fType, &leftType,
-                               &rightType, &resultType,
-                               !Token::IsAssignment(expression.fOperator))) {
+                               &rightType, &resultType, !is_assignment(expression.fOperator))) {
         fErrors.error(expression.fPosition, "type mismatch: '" +
                                             Token::OperatorName(expression.fOperator) +
                                             "' cannot operate on '" + left->fType.fName +
                                             "', '" + right->fType.fName + "'");
         return nullptr;
     }
-    if (Token::IsAssignment(expression.fOperator)) {
-        this->markWrittenTo(*left, expression.fOperator != Token::EQ);
+    if (is_assignment(expression.fOperator)) {
+        this->markWrittenTo(*left);
     }
     left = this->coerce(std::move(left), *leftType);
     right = this->coerce(std::move(right), *rightType);
@@ -1028,7 +1051,7 @@
             return nullptr;
         }
         if (arguments[i] && (function.fParameters[i]->fModifiers.fFlags & Modifiers::kOut_Flag)) {
-            this->markWrittenTo(*arguments[i], true);
+            this->markWrittenTo(*arguments[i]);
         }
     }
     return std::unique_ptr<FunctionCall>(new FunctionCall(position, *returnType, function,
@@ -1238,7 +1261,7 @@
                               "' cannot operate on '" + base->fType.description() + "'");
                 return nullptr;
             }
-            this->markWrittenTo(*base, true);
+            this->markWrittenTo(*base);
             break;
         case Token::MINUSMINUS:
             if (!base->fType.isNumber()) {
@@ -1247,7 +1270,7 @@
                               "' cannot operate on '" + base->fType.description() + "'");
                 return nullptr;
             }
-            this->markWrittenTo(*base, true);
+            this->markWrittenTo(*base);
             break;
         case Token::LOGICALNOT:
             if (base->fType != *fContext.fBool_Type) {
@@ -1441,7 +1464,7 @@
                               "'++' cannot operate on '" + base->fType.description() + "'");
                 return nullptr;
             }
-            this->markWrittenTo(*base, true);
+            this->markWrittenTo(*base);
             return std::unique_ptr<Expression>(new PostfixExpression(std::move(base),
                                                                      Token::PLUSPLUS));
         case ASTSuffix::kPostDecrement_Kind:
@@ -1450,7 +1473,7 @@
                               "'--' cannot operate on '" + base->fType.description() + "'");
                 return nullptr;
             }
-            this->markWrittenTo(*base, true);
+            this->markWrittenTo(*base);
             return std::unique_ptr<Expression>(new PostfixExpression(std::move(base),
                                                                      Token::MINUSMINUS));
         default:
@@ -1473,6 +1496,10 @@
     }
 }
 
+void IRGenerator::markReadFrom(const Variable& var) {
+    var.fIsReadFrom = true;
+}
+
 static bool has_duplicates(const Swizzle& swizzle) {
     int bits = 0;
     for (int idx : swizzle.fComponents) {
@@ -1486,7 +1513,7 @@
     return false;
 }
 
-void IRGenerator::markWrittenTo(const Expression& expr, bool readWrite) {
+void IRGenerator::markWrittenTo(const Expression& expr) {
     switch (expr.fKind) {
         case Expression::kVariableReference_Kind: {
             const Variable& var = ((VariableReference&) expr).fVariable;
@@ -1494,22 +1521,21 @@
                 fErrors.error(expr.fPosition,
                               "cannot modify immutable variable '" + var.fName + "'");
             }
-            ((VariableReference&) expr).setRefKind(readWrite ? VariableReference::kReadWrite_RefKind
-                                                             : VariableReference::kWrite_RefKind);
+            var.fIsWrittenTo = true;
             break;
         }
         case Expression::kFieldAccess_Kind:
-            this->markWrittenTo(*((FieldAccess&) expr).fBase, readWrite);
+            this->markWrittenTo(*((FieldAccess&) expr).fBase);
             break;
         case Expression::kSwizzle_Kind:
             if (has_duplicates((Swizzle&) expr)) {
                 fErrors.error(expr.fPosition,
                               "cannot write to the same swizzle field more than once");
             }
-            this->markWrittenTo(*((Swizzle&) expr).fBase, readWrite);
+            this->markWrittenTo(*((Swizzle&) expr).fBase);
             break;
         case Expression::kIndex_Kind:
-            this->markWrittenTo(*((IndexExpression&) expr).fBase, readWrite);
+            this->markWrittenTo(*((IndexExpression&) expr).fBase);
             break;
         default:
             fErrors.error(expr.fPosition, "cannot assign to '" + expr.description() + "'");
diff --git a/src/sksl/SkSLIRGenerator.h b/src/sksl/SkSLIRGenerator.h
index 2ffcb0d..13b20fb 100644
--- a/src/sksl/SkSLIRGenerator.h
+++ b/src/sksl/SkSLIRGenerator.h
@@ -88,16 +88,7 @@
     std::unique_ptr<ModifiersDeclaration> convertModifiersDeclaration(
                                                                   const ASTModifiersDeclaration& m);
 
-    /**
-     * If both operands are compile-time constants and can be folded, returns an expression
-     * representing the folded value. Otherwise, returns null. Note that unlike most other functions
-     * here, null does not represent a compilation error.
-     */
-    std::unique_ptr<Expression> constantFold(const Expression& left,
-                                             Token::Kind op,
-                                             const Expression& right) const;
     Program::Inputs fInputs;
-    const Context& fContext;
 
 private:
     /**
@@ -133,6 +124,11 @@
     std::unique_ptr<Statement> convertDiscard(const ASTDiscardStatement& d);
     std::unique_ptr<Statement> convertDo(const ASTDoStatement& d);
     std::unique_ptr<Expression> convertBinaryExpression(const ASTBinaryExpression& expression);
+    // Returns null if it cannot fold the expression. Note that unlike most other functions here, a
+    // null return does not represent a compilation error.
+    std::unique_ptr<Expression> constantFold(const Expression& left,
+                                             Token::Kind op,
+                                             const Expression& right);
     std::unique_ptr<Extension> convertExtension(const ASTExtension& e);
     std::unique_ptr<Statement> convertExpressionStatement(const ASTExpressionStatement& s);
     std::unique_ptr<Statement> convertFor(const ASTForStatement& f);
@@ -155,8 +151,10 @@
     std::unique_ptr<Statement> convertWhile(const ASTWhileStatement& w);
 
     void checkValid(const Expression& expr);
-    void markWrittenTo(const Expression& expr, bool readWrite);
+    void markReadFrom(const Variable& var);
+    void markWrittenTo(const Expression& expr);
 
+    const Context& fContext;
     const FunctionDeclaration* fCurrentFunction;
     const Program::Settings* fSettings;
     std::unordered_map<SkString, CapValue> fCapsMap;
diff --git a/src/sksl/SkSLSPIRVCodeGenerator.cpp b/src/sksl/SkSLSPIRVCodeGenerator.cpp
index d43e4c4..8afd136 100644
--- a/src/sksl/SkSLSPIRVCodeGenerator.cpp
+++ b/src/sksl/SkSLSPIRVCodeGenerator.cpp
@@ -2540,7 +2540,7 @@
             kind != Program::kFragment_Kind) {
             continue;
         }
-        if (!var->fReadCount && !var->fWriteCount &&
+        if (!var->fIsReadFrom && !var->fIsWrittenTo &&
                 !(var->fModifiers.fFlags & (Modifiers::kIn_Flag |
                                             Modifiers::kOut_Flag |
                                             Modifiers::kUniform_Flag))) {
diff --git a/src/sksl/SkSLToken.h b/src/sksl/SkSLToken.h
index 197781f..5c8c2bd 100644
--- a/src/sksl/SkSLToken.h
+++ b/src/sksl/SkSLToken.h
@@ -160,28 +160,6 @@
     , fKind(kind)
     , fText(std::move(text)) {}
 
-    static bool IsAssignment(Token::Kind op) {
-        switch (op) {
-            case Token::EQ:           // fall through
-            case Token::PLUSEQ:       // fall through
-            case Token::MINUSEQ:      // fall through
-            case Token::STAREQ:       // fall through
-            case Token::SLASHEQ:      // fall through
-            case Token::PERCENTEQ:    // fall through
-            case Token::SHLEQ:        // fall through
-            case Token::SHREQ:        // fall through
-            case Token::BITWISEOREQ:  // fall through
-            case Token::BITWISEXOREQ: // fall through
-            case Token::BITWISEANDEQ: // fall through
-            case Token::LOGICALOREQ:  // fall through
-            case Token::LOGICALXOREQ: // fall through
-            case Token::LOGICALANDEQ:
-                return true;
-            default:
-                return false;
-        }
-    }
-
     Position fPosition;
     Kind fKind;
     // will be the empty string unless the token has variable text content (identifiers, numeric
diff --git a/src/sksl/ir/SkSLBinaryExpression.h b/src/sksl/ir/SkSLBinaryExpression.h
index de85e48..132513e 100644
--- a/src/sksl/ir/SkSLBinaryExpression.h
+++ b/src/sksl/ir/SkSLBinaryExpression.h
@@ -4,19 +4,17 @@
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
-
+ 
 #ifndef SKSL_BINARYEXPRESSION
 #define SKSL_BINARYEXPRESSION
 
 #include "SkSLExpression.h"
-#include "SkSLExpression.h"
-#include "../SkSLIRGenerator.h"
 #include "../SkSLToken.h"
 
 namespace SkSL {
 
 /**
- * A binary operation.
+ * A binary operation. 
  */
 struct BinaryExpression : public Expression {
     BinaryExpression(Position position, std::unique_ptr<Expression> left, Token::Kind op,
@@ -26,22 +24,14 @@
     , fOperator(op)
     , fRight(std::move(right)) {}
 
-    virtual std::unique_ptr<Expression> constantPropagate(
-                                                        const IRGenerator& irGenerator,
-                                                        const DefinitionMap& definitions) override {
-        return irGenerator.constantFold(*fLeft,
-                                        fOperator,
-                                        *fRight);
-    }
-
     virtual SkString description() const override {
         return "(" + fLeft->description() + " " + Token::OperatorName(fOperator) + " " +
                fRight->description() + ")";
     }
 
-    std::unique_ptr<Expression> fLeft;
+    const std::unique_ptr<Expression> fLeft;
     const Token::Kind fOperator;
-    std::unique_ptr<Expression> fRight;
+    const std::unique_ptr<Expression> fRight;
 
     typedef Expression INHERITED;
 };
diff --git a/src/sksl/ir/SkSLBlock.h b/src/sksl/ir/SkSLBlock.h
index 17970fd..f975d16 100644
--- a/src/sksl/ir/SkSLBlock.h
+++ b/src/sksl/ir/SkSLBlock.h
@@ -20,8 +20,8 @@
     Block(Position position, std::vector<std::unique_ptr<Statement>> statements,
           const std::shared_ptr<SymbolTable> symbols)
     : INHERITED(position, kBlock_Kind)
-    , fSymbols(std::move(symbols))
-    , fStatements(std::move(statements)) {}
+    , fStatements(std::move(statements))
+    , fSymbols(std::move(symbols)) {}
 
     SkString description() const override {
         SkString result("{");
@@ -33,10 +33,8 @@
         return result;
     }
 
-    // it's important to keep fStatements defined after (and thus destroyed before) fSymbols,
-    // because destroying statements can modify reference counts in symbols
-    const std::shared_ptr<SymbolTable> fSymbols;
     const std::vector<std::unique_ptr<Statement>> fStatements;
+    const std::shared_ptr<SymbolTable> fSymbols;
 
     typedef Statement INHERITED;
 };
diff --git a/src/sksl/ir/SkSLConstructor.h b/src/sksl/ir/SkSLConstructor.h
index 691bea1..63c692b 100644
--- a/src/sksl/ir/SkSLConstructor.h
+++ b/src/sksl/ir/SkSLConstructor.h
@@ -9,9 +9,6 @@
 #define SKSL_CONSTRUCTOR
 
 #include "SkSLExpression.h"
-#include "SkSLFloatLiteral.h"
-#include "SkSLIntLiteral.h"
-#include "SkSLIRGenerator.h"
 
 namespace SkSL {
 
@@ -24,20 +21,6 @@
     : INHERITED(position, kConstructor_Kind, type)
     , fArguments(std::move(arguments)) {}
 
-    virtual std::unique_ptr<Expression> constantPropagate(
-                                                        const IRGenerator& irGenerator,
-                                                        const DefinitionMap& definitions) override {
-        if (fArguments.size() == 1 && fArguments[0]->fKind == Expression::kIntLiteral_Kind &&
-            // promote float(1) to 1.0
-            fType == *irGenerator.fContext.fFloat_Type) {
-            int64_t intValue = ((IntLiteral&) *fArguments[0]).fValue;
-            return std::unique_ptr<Expression>(new FloatLiteral(irGenerator.fContext,
-                                                                fPosition,
-                                                                intValue));
-        }
-        return nullptr;
-    }
-
     SkString description() const override {
         SkString result = fType.description() + "(";
         SkString separator;
@@ -59,7 +42,7 @@
         return true;
     }
 
-    std::vector<std::unique_ptr<Expression>> fArguments;
+    const std::vector<std::unique_ptr<Expression>> fArguments;
 
     typedef Expression INHERITED;
 };
diff --git a/src/sksl/ir/SkSLDoStatement.h b/src/sksl/ir/SkSLDoStatement.h
index e26d3dc..78c0a1b 100644
--- a/src/sksl/ir/SkSLDoStatement.h
+++ b/src/sksl/ir/SkSLDoStatement.h
@@ -28,7 +28,7 @@
     }
 
     const std::unique_ptr<Statement> fStatement;
-    std::unique_ptr<Expression> fTest;
+    const std::unique_ptr<Expression> fTest;
 
     typedef Statement INHERITED;
 };
diff --git a/src/sksl/ir/SkSLExpression.h b/src/sksl/ir/SkSLExpression.h
index f87d810..b4ed37c 100644
--- a/src/sksl/ir/SkSLExpression.h
+++ b/src/sksl/ir/SkSLExpression.h
@@ -4,24 +4,17 @@
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
-
+ 
 #ifndef SKSL_EXPRESSION
 #define SKSL_EXPRESSION
 
+#include "SkSLIRNode.h"
 #include "SkSLType.h"
-#include "SkSLVariable.h"
-
-#include <unordered_map>
 
 namespace SkSL {
 
-struct Expression;
-class IRGenerator;
-
-typedef std::unordered_map<const Variable*, std::unique_ptr<Expression>*> DefinitionMap;
-
 /**
- * Abstract supertype of all expressions.
+ * Abstract supertype of all expressions. 
  */
 struct Expression : public IRNode {
     enum Kind {
@@ -52,18 +45,6 @@
         return false;
     }
 
-    /**
-     * Given a map of known constant variable values, substitute them in for references to those
-     * variables occurring in this expression and its subexpressions.  Similar simplifications, such
-     * as folding a constant binary expression down to a single value, may also be performed.
-     * Returns a new expression which replaces this expression, or null if no replacements were
-     * made. If a new expression is returned, this expression is no longer valid.
-     */
-    virtual std::unique_ptr<Expression> constantPropagate(const IRGenerator& irGenerator,
-                                                          const DefinitionMap& definitions) {
-        return nullptr;
-    }
-
     const Kind fKind;
     const Type& fType;
 
diff --git a/src/sksl/ir/SkSLExpressionStatement.h b/src/sksl/ir/SkSLExpressionStatement.h
index 088b1c9..677c647 100644
--- a/src/sksl/ir/SkSLExpressionStatement.h
+++ b/src/sksl/ir/SkSLExpressionStatement.h
@@ -25,7 +25,7 @@
         return fExpression->description() + ";";
     }
 
-    std::unique_ptr<Expression> fExpression;
+    const std::unique_ptr<Expression> fExpression;
 
     typedef Statement INHERITED;
 };
diff --git a/src/sksl/ir/SkSLFieldAccess.h b/src/sksl/ir/SkSLFieldAccess.h
index de26a3f..fb727e0 100644
--- a/src/sksl/ir/SkSLFieldAccess.h
+++ b/src/sksl/ir/SkSLFieldAccess.h
@@ -35,7 +35,7 @@
         return fBase->description() + "." + fBase->fType.fields()[fFieldIndex].fName;
     }
 
-    std::unique_ptr<Expression> fBase;
+    const std::unique_ptr<Expression> fBase;
     const int fFieldIndex;
     const OwnerKind fOwnerKind;
 
diff --git a/src/sksl/ir/SkSLForStatement.h b/src/sksl/ir/SkSLForStatement.h
index 6f03e2b..ff03d0d 100644
--- a/src/sksl/ir/SkSLForStatement.h
+++ b/src/sksl/ir/SkSLForStatement.h
@@ -46,8 +46,8 @@
     }
 
     const std::unique_ptr<Statement> fInitializer;
-    std::unique_ptr<Expression> fTest;
-    std::unique_ptr<Expression> fNext;
+    const std::unique_ptr<Expression> fTest;
+    const std::unique_ptr<Expression> fNext;
     const std::unique_ptr<Statement> fStatement;
     const std::shared_ptr<SymbolTable> fSymbols;
 
diff --git a/src/sksl/ir/SkSLFunctionCall.h b/src/sksl/ir/SkSLFunctionCall.h
index 1838076..971af36 100644
--- a/src/sksl/ir/SkSLFunctionCall.h
+++ b/src/sksl/ir/SkSLFunctionCall.h
@@ -36,7 +36,7 @@
     }
 
     const FunctionDeclaration& fFunction;
-    std::vector<std::unique_ptr<Expression>> fArguments;
+    const std::vector<std::unique_ptr<Expression>> fArguments;
 
     typedef Expression INHERITED;
 };
diff --git a/src/sksl/ir/SkSLIfStatement.h b/src/sksl/ir/SkSLIfStatement.h
index 8667e93..f8beded 100644
--- a/src/sksl/ir/SkSLIfStatement.h
+++ b/src/sksl/ir/SkSLIfStatement.h
@@ -32,7 +32,7 @@
         return result;
     }
 
-    std::unique_ptr<Expression> fTest;
+    const std::unique_ptr<Expression> fTest;
     const std::unique_ptr<Statement> fIfTrue;
     const std::unique_ptr<Statement> fIfFalse;
 
diff --git a/src/sksl/ir/SkSLIndexExpression.h b/src/sksl/ir/SkSLIndexExpression.h
index d255c7d..079dde5 100644
--- a/src/sksl/ir/SkSLIndexExpression.h
+++ b/src/sksl/ir/SkSLIndexExpression.h
@@ -55,8 +55,8 @@
         return fBase->description() + "[" + fIndex->description() + "]";
     }
 
-    std::unique_ptr<Expression> fBase;
-    std::unique_ptr<Expression> fIndex;
+    const std::unique_ptr<Expression> fBase;
+    const std::unique_ptr<Expression> fIndex;
 
     typedef Expression INHERITED;
 };
diff --git a/src/sksl/ir/SkSLPostfixExpression.h b/src/sksl/ir/SkSLPostfixExpression.h
index 6c9fafe..01671b5 100644
--- a/src/sksl/ir/SkSLPostfixExpression.h
+++ b/src/sksl/ir/SkSLPostfixExpression.h
@@ -25,7 +25,7 @@
         return fOperand->description() + Token::OperatorName(fOperator);
     }
 
-    std::unique_ptr<Expression> fOperand;
+    const std::unique_ptr<Expression> fOperand;
     const Token::Kind fOperator;
 
     typedef Expression INHERITED;
diff --git a/src/sksl/ir/SkSLPrefixExpression.h b/src/sksl/ir/SkSLPrefixExpression.h
index b7db99a..790c5ab 100644
--- a/src/sksl/ir/SkSLPrefixExpression.h
+++ b/src/sksl/ir/SkSLPrefixExpression.h
@@ -25,7 +25,7 @@
         return Token::OperatorName(fOperator) + fOperand->description();
     }
 
-    std::unique_ptr<Expression> fOperand;
+    const std::unique_ptr<Expression> fOperand;
     const Token::Kind fOperator;
 
     typedef Expression INHERITED;
diff --git a/src/sksl/ir/SkSLProgram.h b/src/sksl/ir/SkSLProgram.h
index 6a73be6..ac49d6d 100644
--- a/src/sksl/ir/SkSLProgram.h
+++ b/src/sksl/ir/SkSLProgram.h
@@ -59,8 +59,8 @@
     , fSettings(settings)
     , fDefaultPrecision(defaultPrecision)
     , fContext(context)
-    , fSymbols(symbols)
     , fElements(std::move(elements))
+    , fSymbols(symbols)
     , fInputs(inputs) {}
 
     Kind fKind;
@@ -68,10 +68,8 @@
     // FIXME handle different types; currently it assumes this is for floats
     Modifiers::Flag fDefaultPrecision;
     Context* fContext;
-    // it's important to keep fElements defined after (and thus destroyed before) fSymbols,
-    // because destroying elements can modify reference counts in symbols
-    std::shared_ptr<SymbolTable> fSymbols;
     std::vector<std::unique_ptr<ProgramElement>> fElements;
+    std::shared_ptr<SymbolTable> fSymbols;
     Inputs fInputs;
 };
 
diff --git a/src/sksl/ir/SkSLReturnStatement.h b/src/sksl/ir/SkSLReturnStatement.h
index dc5ec9a..c83b450 100644
--- a/src/sksl/ir/SkSLReturnStatement.h
+++ b/src/sksl/ir/SkSLReturnStatement.h
@@ -32,7 +32,7 @@
         }
     }
 
-    std::unique_ptr<Expression> fExpression;
+    const std::unique_ptr<Expression> fExpression;
 
     typedef Statement INHERITED;
 };
diff --git a/src/sksl/ir/SkSLSwizzle.h b/src/sksl/ir/SkSLSwizzle.h
index 8ad9001..c9397ae 100644
--- a/src/sksl/ir/SkSLSwizzle.h
+++ b/src/sksl/ir/SkSLSwizzle.h
@@ -76,7 +76,7 @@
         return result;
     }
 
-    std::unique_ptr<Expression> fBase;
+    const std::unique_ptr<Expression> fBase;
     const std::vector<int> fComponents;
 
     typedef Expression INHERITED;
diff --git a/src/sksl/ir/SkSLTernaryExpression.h b/src/sksl/ir/SkSLTernaryExpression.h
index 0275004..4a35253 100644
--- a/src/sksl/ir/SkSLTernaryExpression.h
+++ b/src/sksl/ir/SkSLTernaryExpression.h
@@ -31,9 +31,9 @@
                fIfFalse->description() + ")";
     }
 
-    std::unique_ptr<Expression> fTest;
-    std::unique_ptr<Expression> fIfTrue;
-    std::unique_ptr<Expression> fIfFalse;
+    const std::unique_ptr<Expression> fTest;
+    const std::unique_ptr<Expression> fIfTrue;
+    const std::unique_ptr<Expression> fIfFalse;
 
     typedef Expression INHERITED;
 };
diff --git a/src/sksl/ir/SkSLVarDeclarations.h b/src/sksl/ir/SkSLVarDeclarations.h
index 490259a..295c0b6 100644
--- a/src/sksl/ir/SkSLVarDeclarations.h
+++ b/src/sksl/ir/SkSLVarDeclarations.h
@@ -72,7 +72,7 @@
     }
 
     const Type& fBaseType;
-    std::vector<VarDeclaration> fVars;
+    const std::vector<VarDeclaration> fVars;
 
     typedef ProgramElement INHERITED;
 };
diff --git a/src/sksl/ir/SkSLVarDeclarationsStatement.h b/src/sksl/ir/SkSLVarDeclarationsStatement.h
index 66b570f..7a29656 100644
--- a/src/sksl/ir/SkSLVarDeclarationsStatement.h
+++ b/src/sksl/ir/SkSLVarDeclarationsStatement.h
@@ -18,14 +18,14 @@
  */
 struct VarDeclarationsStatement : public Statement {
     VarDeclarationsStatement(std::unique_ptr<VarDeclarations> decl)
-    : INHERITED(decl->fPosition, kVarDeclarations_Kind)
+    : INHERITED(decl->fPosition, kVarDeclarations_Kind) 
     , fDeclaration(std::move(decl)) {}
 
     SkString description() const override {
         return fDeclaration->description();
     }
 
-    std::shared_ptr<VarDeclarations> fDeclaration;
+    const std::shared_ptr<VarDeclarations> fDeclaration;
 
     typedef Statement INHERITED;
 };
diff --git a/src/sksl/ir/SkSLVariable.h b/src/sksl/ir/SkSLVariable.h
index 2c3391d..39b8482 100644
--- a/src/sksl/ir/SkSLVariable.h
+++ b/src/sksl/ir/SkSLVariable.h
@@ -33,8 +33,8 @@
     , fModifiers(modifiers)
     , fType(type)
     , fStorage(storage)
-    , fReadCount(0)
-    , fWriteCount(0) {}
+    , fIsReadFrom(false)
+    , fIsWrittenTo(false) {}
 
     virtual SkString description() const override {
         return fModifiers.description() + fType.fName + " " + fName;
@@ -44,12 +44,8 @@
     const Type& fType;
     const Storage fStorage;
 
-    // Tracks how many sites read from the variable. If this is zero for a non-out variable (or
-    // becomes zero during optimization), the variable is dead and may be eliminated.
-    mutable int fReadCount;
-    // Tracks how many sites write to the variable. If this is zero, the variable is dead and may be
-    // eliminated.
-    mutable int fWriteCount;
+    mutable bool fIsReadFrom;
+    mutable bool fIsWrittenTo;
 
     typedef Symbol INHERITED;
 };
diff --git a/src/sksl/ir/SkSLVariableReference.h b/src/sksl/ir/SkSLVariableReference.h
index fecb04e..c6a2ea0 100644
--- a/src/sksl/ir/SkSLVariableReference.h
+++ b/src/sksl/ir/SkSLVariableReference.h
@@ -20,83 +20,16 @@
  * there is only one Variable 'x', but two VariableReferences to it.
  */
 struct VariableReference : public Expression {
-    enum RefKind {
-        kRead_RefKind,
-        kWrite_RefKind,
-        kReadWrite_RefKind
-    };
-
-    VariableReference(Position position, const Variable& variable, RefKind refKind = kRead_RefKind)
+    VariableReference(Position position, const Variable& variable)
     : INHERITED(position, kVariableReference_Kind, variable.fType)
-    , fVariable(variable)
-    , fRefKind(refKind) {
-        if (refKind != kRead_RefKind) {
-            fVariable.fWriteCount++;
-        }
-        if (refKind != kWrite_RefKind) {
-            fVariable.fReadCount++;
-        }
-    }
-
-    virtual ~VariableReference() override {
-        if (fRefKind != kWrite_RefKind) {
-            fVariable.fReadCount--;
-        }
-    }
-
-    RefKind refKind() {
-        return fRefKind;
-    }
-
-    void setRefKind(RefKind refKind) {
-        if (fRefKind != kRead_RefKind) {
-            fVariable.fWriteCount--;
-        }
-        if (fRefKind != kWrite_RefKind) {
-            fVariable.fReadCount--;
-        }
-        if (refKind != kRead_RefKind) {
-            fVariable.fWriteCount++;
-        }
-        if (refKind != kWrite_RefKind) {
-            fVariable.fReadCount++;
-        }
-        fRefKind = refKind;
-    }
+    , fVariable(variable) {}
 
     SkString description() const override {
         return fVariable.fName;
     }
 
-    virtual std::unique_ptr<Expression> constantPropagate(
-                                                        const IRGenerator& irGenerator,
-                                                        const DefinitionMap& definitions) override {
-        auto exprIter = definitions.find(&fVariable);
-        if (exprIter != definitions.end() && exprIter->second) {
-            const Expression* expr = exprIter->second->get();
-            switch (expr->fKind) {
-                case Expression::kIntLiteral_Kind:
-                    return std::unique_ptr<Expression>(new IntLiteral(
-                                                                     irGenerator.fContext,
-                                                                     Position(),
-                                                                     ((IntLiteral*) expr)->fValue));
-                case Expression::kFloatLiteral_Kind:
-                    return std::unique_ptr<Expression>(new FloatLiteral(
-                                                                   irGenerator.fContext,
-                                                                   Position(),
-                                                                   ((FloatLiteral*) expr)->fValue));
-                default:
-                    break;
-            }
-        }
-        return nullptr;
-    }
-
     const Variable& fVariable;
 
-private:
-    RefKind fRefKind;
-
     typedef Expression INHERITED;
 };
 
diff --git a/src/sksl/ir/SkSLWhileStatement.h b/src/sksl/ir/SkSLWhileStatement.h
index a741a04..7c6a290 100644
--- a/src/sksl/ir/SkSLWhileStatement.h
+++ b/src/sksl/ir/SkSLWhileStatement.h
@@ -27,7 +27,7 @@
         return "while (" + fTest->description() + ") " + fStatement->description();
     }
 
-    std::unique_ptr<Expression> fTest;
+    const std::unique_ptr<Expression> fTest;
     const std::unique_ptr<Statement> fStatement;
 
     typedef Statement INHERITED;
diff --git a/tests/SkSLGLSLTest.cpp b/tests/SkSLGLSLTest.cpp
index 1501dc5..12ac4d1 100644
--- a/tests/SkSLGLSLTest.cpp
+++ b/tests/SkSLGLSLTest.cpp
@@ -61,7 +61,7 @@
          "while (i < 10) sk_FragColor *= 0.5;"
          "do { sk_FragColor += 0.01; } while (sk_FragColor.x < 0.75);"
          "for (int i = 0; i < 10; i++) {"
-         "if (i % 2 == 1) break; else continue;"
+         "if (i % 0 == 1) break; else continue;"
          "}"
          "return;"
          "}",
@@ -75,12 +75,12 @@
          "        discard;\n"
          "    }\n"
          "    int i = 0;\n"
-         "    while (true) sk_FragColor *= 0.5;\n"
+         "    while (i < 10) sk_FragColor *= 0.5;\n"
          "    do {\n"
          "        sk_FragColor += 0.01;\n"
          "    } while (sk_FragColor.x < 0.75);\n"
          "    for (int i = 0;i < 10; i++) {\n"
-         "        if (i % 2 == 1) break; else continue;\n"
+         "        if (i % 0 == 1) break; else continue;\n"
          "    }\n"
          "    return;\n"
          "}\n");
@@ -106,8 +106,8 @@
          "}\n"
          "void main() {\n"
          "    float x = 10.0;\n"
-         "    bar(10.0);\n"
-         "    sk_FragColor = vec4(10.0);\n"
+         "    bar(x);\n"
+         "    sk_FragColor = vec4(x);\n"
          "}\n");
 }
 
@@ -116,7 +116,7 @@
          "void main() {"
          "float x = 1, y = 2;"
          "int z = 3;"
-         "x = x - x + y * z * x * (y - z);"
+         "x = x + y * z * x * (y - z);"
          "y = x / y / z;"
          "z = (z / 2 % 3 << 4) >> 2 << 1;"
          "bool b = (x > 4) == x < 2 || 2 >= sqrt(2) && y <= z;"
@@ -139,10 +139,10 @@
          "void main() {\n"
          "    float x = 1.0, y = 2.0;\n"
          "    int z = 3;\n"
-         "    x = -6.0;\n"
-         "    y = -1.0;\n"
-         "    z = 8;\n"
-         "    bool b = false == true || 2.0 >= sqrt(2.0) && true;\n"
+         "    x = x + ((y * float(z)) * x) * (y - float(z));\n"
+         "    y = (x / y) / float(z);\n"
+         "    z = (((z / 2) % 3 << 4) >> 2) << 1;\n"
+         "    bool b = x > 4.0 == x < 2.0 || 2.0 >= sqrt(2.0) && y <= float(z);\n"
          "    x += 12.0;\n"
          "    x -= 12.0;\n"
          "    x *= (y /= float(z = 10));\n"
@@ -287,7 +287,7 @@
          "out vec4 sk_FragColor;\n"
          "void main() {\n"
          "    float x = -5.0;\n"
-         "    x = min(abs(-5.0), 6.0);\n"
+         "    x = min(abs(x), 6.0);\n"
          "}\n");
 
     test(r,
@@ -302,7 +302,7 @@
          "    float minAbsHackVar0;\n"
          "    float minAbsHackVar1;\n"
          "    float x = -5.0;\n"
-         "    x = ((minAbsHackVar0 = abs(-5.0)) < (minAbsHackVar1 = 6.0) ? minAbsHackVar0 : "
+         "    x = ((minAbsHackVar0 = abs(x)) < (minAbsHackVar1 = 6.0) ? minAbsHackVar0 : "
                                                                                 "minAbsHackVar1);\n"
          "}\n");
 }