commit 79518b02cd3ef91a9fb5ddae381e826e80cae582
author Alex Bradbury <asb@lowrisc.org> Wed Sep 19 14:51:42 2018 +0000
committer Alex Bradbury <asb@lowrisc.org> Wed Sep 19 14:51:42 2018 +0000
tree 57897012b6770236cb7b5981f3991538b2b4eb7f
parent 023dfa5716326449662059516ddff2ed459526a1

[AtomicExpandPass]: Add a hook for custom cmpxchg expansion in IR

This involves changing the shouldExpandAtomicCmpXchgInIR interface, but I have 
updated the in-tree backends using this hook (ARM, AArch64, Hexagon) so they 
will see no functional change. Previously this hook returned bool, but it now 
returns AtomicExpansionKind.

This hook allows targets to select how a given cmpxchg is to be expanded. 
D48131 uses this to expand part-word cmpxchg to a target-specific intrinsic.

See my associated RFC for more info on the motivation for this change 
<http://lists.llvm.org/pipermail/llvm-dev/2018-June/123993.html>.

Differential Revision: https://reviews.llvm.org/D48130

llvm-svn: 342550

llvm/lib/CodeGen/AtomicExpandPass.cpp

diff --git a/llvm/lib/CodeGen/AtomicExpandPass.cpp b/llvm/lib/CodeGen/AtomicExpandPass.cpp
index 3a283ca..c1b9879 100644
--- a/llvm/lib/CodeGen/AtomicExpandPass.cpp
+++ b/llvm/lib/CodeGen/AtomicExpandPass.cpp
@@ -98,6 +98,7 @@
         AtomicOrdering MemOpOrder,
         function_ref<Value *(IRBuilder<> &, Value *)> PerformOp,
         CreateCmpXchgInstFun CreateCmpXchg);
+    bool tryExpandAtomicCmpXchg(AtomicCmpXchgInst *CI);
 
     bool expandAtomicCmpXchg(AtomicCmpXchgInst *CI);
     bool isIdempotentRMW(AtomicRMWInst *RMWI);
@@ -260,7 +261,9 @@
                           isAcquireOrStronger(RMWI->getOrdering()))) {
         FenceOrdering = RMWI->getOrdering();
         RMWI->setOrdering(AtomicOrdering::Monotonic);
-      } else if (CASI && !TLI->shouldExpandAtomicCmpXchgInIR(CASI) &&
+      } else if (CASI &&
+                 TLI->shouldExpandAtomicCmpXchgInIR(CASI) ==
+                     TargetLoweringBase::AtomicExpansionKind::None &&
                  (isReleaseOrStronger(CASI->getSuccessOrdering()) ||
                   isAcquireOrStronger(CASI->getSuccessOrdering()))) {
         // If a compare and swap is lowered to LL/SC, we can do smarter fence
@@ -334,16 +337,7 @@
         MadeChange = true;
       }
 
-      unsigned MinCASSize = TLI->getMinCmpXchgSizeInBits() / 8;
-      unsigned ValueSize = getAtomicOpSize(CASI);
-      if (ValueSize < MinCASSize) {
-        assert(!TLI->shouldExpandAtomicCmpXchgInIR(CASI) &&
-               "MinCmpXchgSizeInBits not yet supported for LL/SC expansions.");
-        expandPartwordCmpXchg(CASI);
-      } else {
-        if (TLI->shouldExpandAtomicCmpXchgInIR(CASI))
-          MadeChange |= expandAtomicCmpXchg(CASI);
-      }
+      MadeChange |= tryExpandAtomicCmpXchg(CASI);
     }
   }
   return MadeChange;
@@ -1355,6 +1349,28 @@
   return NewLoaded;
 }
 
+bool AtomicExpand::tryExpandAtomicCmpXchg(AtomicCmpXchgInst *CI) {
+  unsigned MinCASSize = TLI->getMinCmpXchgSizeInBits() / 8;
+  unsigned ValueSize = getAtomicOpSize(CI);
+
+  switch (TLI->shouldExpandAtomicCmpXchgInIR(CI)) {
+  default:
+    llvm_unreachable("Unhandled case in tryExpandAtomicCmpXchg");
+  case TargetLoweringBase::AtomicExpansionKind::None:
+    if (ValueSize < MinCASSize)
+      expandPartwordCmpXchg(CI);
+    return false;
+  case TargetLoweringBase::AtomicExpansionKind::LLSC: {
+    assert(ValueSize >= MinCASSize &&
+           "MinCmpXchgSizeInBits not yet supported for LL/SC expansions.");
+    return expandAtomicCmpXchg(CI);
+  }
+  case TargetLoweringBase::AtomicExpansionKind::MaskedIntrinsic:
+    llvm_unreachable(
+        "MaskedIntrinsic expansion of cmpxhg not yet implemented");
+  }
+}
+
 // Note: This function is exposed externally by AtomicExpandUtils.h
 bool llvm::expandAtomicRMWToCmpXchg(AtomicRMWInst *AI,
                                     CreateCmpXchgInstFun CreateCmpXchg) {