1 files changed, 40 insertions, 1 deletions
diff --git a/clang/lib/CodeGen/CGDeclCXX.cpp b/clang/lib/CodeGen/CGDeclCXX.cpp
index d8768bee2cd..98d9e364d9c 100644
--- a/clang/lib/CodeGen/CGDeclCXX.cpp
+++ b/clang/lib/CodeGen/CGDeclCXX.cpp
@@ -18,6 +18,7 @@
 #include "clang/Frontend/CodeGenOptions.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/MDBuilder.h"
 #include "llvm/Support/Path.h"
 
 using namespace clang;
@@ -259,6 +260,43 @@ void CodeGenFunction::EmitCXXGuardedInit(const VarDecl &D,
   CGM.getCXXABI().EmitGuardedInit(*this, D, DeclPtr, PerformInit);
 }
 
+void CodeGenFunction::EmitCXXGuardedInitBranch(llvm::Value *NeedsInit,
+                                               llvm::BasicBlock *InitBlock,
+                                               llvm::BasicBlock *NoInitBlock,
+                                               GuardKind Kind,
+                                               const VarDecl *D) {
+  assert((Kind == GuardKind::TlsGuard || D) && "no guarded variable");
+
+  // A guess at how many times we will enter the initialization of a
+  // variable, depending on the kind of variable.
+  static const uint64_t InitsPerTLSVar = 1024;
+  static const uint64_t InitsPerLocalVar = 1024 * 1024;
+
+  llvm::MDNode *Weights;
+  if (Kind == GuardKind::VariableGuard && !D->isLocalVarDecl()) {
+    // For non-local variables, don't apply any weighting for now. Due to our
+    // use of COMDATs, we expect there to be at most one initialization of the
+    // variable per DSO, but we have no way to know how many DSOs will try to
+    // initialize the variable.
+    Weights = nullptr;
+  } else {
+    uint64_t NumInits;
+    // FIXME: For the TLS case, collect and use profiling information to
+    // determine a more accurate brach weight.
+    if (Kind == GuardKind::TlsGuard || D->getTLSKind())
+      NumInits = InitsPerTLSVar;
+    else
+      NumInits = InitsPerLocalVar;
+
+    // The probability of us entering the initializer is
+    //   1 / (total number of times we attempt to initialize the variable).
+    llvm::MDBuilder MDHelper(CGM.getLLVMContext());
+    Weights = MDHelper.createBranchWeights(1, NumInits - 1);
+  }
+
+  Builder.CreateCondBr(NeedsInit, InitBlock, NoInitBlock, Weights);
+}
+
 llvm::Function *CodeGenModule::CreateGlobalInitOrDestructFunction(
     llvm::FunctionType *FTy, const Twine &Name, const CGFunctionInfo &FI,
     SourceLocation Loc, bool TLS) {
@@ -539,7 +577,8 @@ CodeGenFunction::GenerateCXXGlobalInitFunc(llvm::Function *Fn,
                                                  "guard.uninitialized");
       llvm::BasicBlock *InitBlock = createBasicBlock("init");
       ExitBlock = createBasicBlock("exit");
-      Builder.CreateCondBr(Uninit, InitBlock, ExitBlock);
+      EmitCXXGuardedInitBranch(Uninit, InitBlock, ExitBlock,
+                               GuardKind::TlsGuard, nullptr);
       EmitBlock(InitBlock);
       // Mark as initialized before initializing anything else. If the
       // initializers use previously-initialized thread_local vars, that's