1 files changed, 15 insertions, 4 deletions
diff --git a/llvm/lib/CodeGen/AtomicExpandPass.cpp b/llvm/lib/CodeGen/AtomicExpandPass.cpp
index 53f1cfe2..d9bc042 100644
--- a/llvm/lib/CodeGen/AtomicExpandPass.cpp
+++ b/llvm/lib/CodeGen/AtomicExpandPass.cpp
@@ -38,6 +38,7 @@
 #include "llvm/IR/MDBuilder.h"
 #include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/IR/Module.h"
+#include "llvm/IR/ProfDataUtils.h"
 #include "llvm/IR/Type.h"
 #include "llvm/IR/User.h"
 #include "llvm/IR/Value.h"
@@ -1259,8 +1260,7 @@ Value *AtomicExpandImpl::insertRMWLLSCLoop(
   BasicBlock *BB = Builder.GetInsertBlock();
   Function *F = BB->getParent();
 
-  assert(AddrAlign >=
-             F->getDataLayout().getTypeStoreSize(ResultTy) &&
+  assert(AddrAlign >= F->getDataLayout().getTypeStoreSize(ResultTy) &&
          "Expected at least natural alignment at this point.");
 
   // Given: atomicrmw some_op iN* %addr, iN %incr ordering
@@ -1295,7 +1295,13 @@ Value *AtomicExpandImpl::insertRMWLLSCLoop(
       TLI->emitStoreConditional(Builder, NewVal, Addr, MemOpOrder);
   Value *TryAgain = Builder.CreateICmpNE(
       StoreSuccess, ConstantInt::get(IntegerType::get(Ctx, 32), 0), "tryagain");
-  Builder.CreateCondBr(TryAgain, LoopBB, ExitBB);
+
+  Instruction *CondBr = Builder.CreateCondBr(TryAgain, LoopBB, ExitBB);
+
+  // Atomic RMW expands to a Load-linked / Store-Conditional loop, because it is
+  // hard to predict precise branch weigths we mark the branch as "unknown"
+  // (50/50) to prevent misleading optimizations.
+  setExplicitlyUnknownBranchWeightsIfProfiled(*CondBr, DEBUG_TYPE);
 
   Builder.SetInsertPoint(ExitBB, ExitBB->begin());
   return Loaded;
@@ -1680,7 +1686,12 @@ Value *AtomicExpandImpl::insertRMWCmpXchgLoop(
 
   Loaded->addIncoming(NewLoaded, LoopBB);
 
-  Builder.CreateCondBr(Success, ExitBB, LoopBB);
+  Instruction *CondBr = Builder.CreateCondBr(Success, ExitBB, LoopBB);
+
+  // Atomic RMW expands to a cmpxchg loop, Since precise branch weights
+  // cannot be easily determined here, we mark the branch as "unknown" (50/50)
+  // to prevent misleading optimizations.
+  setExplicitlyUnknownBranchWeightsIfProfiled(*CondBr, DEBUG_TYPE);
 
   Builder.SetInsertPoint(ExitBB, ExitBB->begin());
   return NewLoaded;