[clang][CodeGen] Emit `llvm.tbaa.errno` metadata during module creation

Let Clang emit `llvm.tbaa.errno` metadata in order to let LLVM
carry out optimizations around errno-writing libcalls to, as
long as it is proved the involved memory location does not alias
`errno`.

Previous discussion: https://discourse.llvm.org/t/rfc-modelling-errno-memory-effects/82972.

1 files changed, 12 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index c5eb14e..1085f45 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -83,6 +83,7 @@ static llvm::cl::opt<bool> LimitedCoverage(
     llvm::cl::desc("Emit limited coverage mapping information (experimental)"));
 
 static const char AnnotationSection[] = "llvm.metadata";
+static constexpr auto ErrnoTBAAMDName = "llvm.errno.tbaa";
 
 static CGCXXABI *createCXXABI(CodeGenModule &CGM) {
   switch (CGM.getContext().getCXXABIKind()) {
@@ -1583,6 +1584,17 @@ void CodeGenModule::Release() {
       }
     }
   }
+
+  // Emit `!llvm.errno.tbaa`, a module-level metadata that specifies the TBAA
+  // for an int access. This allows LLVM to reason about what memory can be
+  // accessed by certain library calls that only touch errno.
+  if (TBAA) {
+    TBAAAccessInfo TBAAInfo = getTBAAAccessInfo(Context.IntTy);
+    if (llvm::MDNode *IntegerNode = getTBAAAccessTagInfo(TBAAInfo)) {
+      auto *ErrnoTBAAMD = TheModule.getOrInsertNamedMetadata(ErrnoTBAAMDName);
+      ErrnoTBAAMD->addOperand(IntegerNode);
+    }
+  }
 }
 
 void CodeGenModule::EmitOpenCLMetadata() {