[ubsan] Emit branch weight metadata to hint towards common case.

Results in better block placement that helps close the
performance gap when making ubsan checks recoverable.

llvm-svn: 170263

1 files changed, 9 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index d74ae889907..0be483a9071 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -1973,7 +1973,15 @@ void CodeGenFunction::EmitCheck(llvm::Value *Checked, StringRef CheckName,
   llvm::BasicBlock *Cont = createBasicBlock("cont");
 
   llvm::BasicBlock *Handler = createBasicBlock("handler." + CheckName);
-  Builder.CreateCondBr(Checked, Cont, Handler);
+
+  llvm::Instruction *Branch = Builder.CreateCondBr(Checked, Cont, Handler);
+
+  // Give hint that we very much don't expect to execute the handler
+  // Value chosen to match UR_NONTAKEN_WEIGHT, see BranchProbabilityInfo.cpp
+  llvm::MDBuilder MDHelper(getLLVMContext());
+  llvm::MDNode *Node = MDHelper.createBranchWeights((1U << 20) - 1, 1);
+  Branch->setMetadata(llvm::LLVMContext::MD_prof, Node);
+
   EmitBlock(Handler);
 
   llvm::Constant *Info = llvm::ConstantStruct::getAnon(StaticArgs);