[KeyInstr][Clang] Ret atom (#134652)

This patch is part of a stack that teaches Clang to generate Key Instructions
metadata for C and C++.

When returning a value, stores to the `retval` allocas and branches to `return`
block are put in the same atom group. They are both rank 1, which could in
theory introduce an extra step in some optimized code. This low risk currently
feels an acceptable for keeping the code a bit simpler (as opposed to adding
scaffolding to make the store rank 2).

In the case of a single return (no control flow) the return instruction inherits
the atom group of the branch to the return block when the blocks get folded
togather.

RFC:
https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

The feature is only functional in LLVM if LLVM is built with CMake flag
LLVM_EXPERIMENTAL_KEY_INSTRUCTIONs. Eventually that flag will be removed.

1 files changed, 3 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index 0388c67..2ac7e9d 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -444,8 +444,10 @@ void CodeGenFunction::FinishFunction(SourceLocation EndLoc) {
 
   // Reset the debug location to that of the simple 'return' expression, if any
   // rather than that of the end of the function's scope '}'.
+  uint64_t RetKeyInstructionsAtomGroup = Loc ? Loc->getAtomGroup() : 0;
   ApplyDebugLocation AL(*this, Loc);
-  EmitFunctionEpilog(*CurFnInfo, EmitRetDbgLoc, EndLoc);
+  EmitFunctionEpilog(*CurFnInfo, EmitRetDbgLoc, EndLoc,
+                     RetKeyInstructionsAtomGroup);
   EmitEndEHSpec(CurCodeDecl);
 
   assert(EHStack.empty() &&