Add branch weights to branches for static initializers.

The initializer for a static local variable cannot be hot, because it runs at
most once per program. That's not quite the same thing as having a low branch
probability, but under the assumption that the function is invoked many times,
modeling this as a branch probability seems reasonable.

For TLS variables, the situation is less clear, since the initialization side
of the branch can run multiple times in a program execution, but we still
expect initialization to be rare relative to non-initialization uses. It would
seem worthwhile to add a PGO counter along this path to make this estimation
more accurate in future.

For globals with guarded initialization, we don't yet apply any branch weights.
Due to our use of COMDATs, the guard will be reached exactly once per DSO, but
we have no idea how many DSOs will define the variable.

llvm-svn: 309195

1 files changed, 6 insertions, 4 deletions

diff --git a/clang/lib/CodeGen/MicrosoftCXXABI.cpp b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
index 78b510bb466..409bad72ee5 100644
--- a/clang/lib/CodeGen/MicrosoftCXXABI.cpp
+++ b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
@@ -2463,11 +2463,12 @@ void MicrosoftCXXABI::EmitGuardedInit(CodeGenFunction &CGF, const VarDecl &D,
     // Test our bit from the guard variable.
     llvm::ConstantInt *Bit = llvm::ConstantInt::get(GuardTy, 1ULL << GuardNum);
     llvm::LoadInst *LI = Builder.CreateLoad(GuardAddr);
-    llvm::Value *IsInitialized =
-        Builder.CreateICmpNE(Builder.CreateAnd(LI, Bit), Zero);
+    llvm::Value *NeedsInit =
+        Builder.CreateICmpEQ(Builder.CreateAnd(LI, Bit), Zero);
     llvm::BasicBlock *InitBlock = CGF.createBasicBlock("init");
     llvm::BasicBlock *EndBlock = CGF.createBasicBlock("init.end");
-    Builder.CreateCondBr(IsInitialized, EndBlock, InitBlock);
+    CGF.EmitCXXGuardedInitBranch(NeedsInit, InitBlock, EndBlock,
+                                 CodeGenFunction::GuardKind::VariableGuard, &D);
 
     // Set our bit in the guard variable and emit the initializer and add a global
     // destructor if appropriate.
@@ -2502,7 +2503,8 @@ void MicrosoftCXXABI::EmitGuardedInit(CodeGenFunction &CGF, const VarDecl &D,
         Builder.CreateICmpSGT(FirstGuardLoad, InitThreadEpoch);
     llvm::BasicBlock *AttemptInitBlock = CGF.createBasicBlock("init.attempt");
     llvm::BasicBlock *EndBlock = CGF.createBasicBlock("init.end");
-    Builder.CreateCondBr(IsUninitialized, AttemptInitBlock, EndBlock);
+    CGF.EmitCXXGuardedInitBranch(IsUninitialized, AttemptInitBlock, EndBlock,
+                                 CodeGenFunction::GuardKind::VariableGuard, &D);
 
     // This BasicBlock attempts to determine whether or not this thread is
     // responsible for doing the initialization.