[clang] [llvm] [Clang][AIX] Add -mloadtime-comment-vars flag to preserve identifying variables (PR #187986)

[Aaron Ballman via llvm-commits]

================
@@ -15460,6 +15461,108 @@ void Sema::CheckThreadLocalForLargeAlignment(VarDecl *VD) {
   }
 }
 
+/// Process a variable definition whose mangled name may be listed in
+/// '-mloadtime-comment-vars=': attach an implicit attribute to supported
+/// string variables so CodeGen preserves them as loadtime identifying
+/// strings, and warn when a named variable cannot be preserved.
+static void processForLoadTimeCommentVar(Sema &S, VarDecl *VD) {
+  // Declarations that cannot be name-matched are silently skipped: an
+  // automatic variable has no symbol of its own, and neither does a template
+  // pattern (only its specializations do, and those are processed
+  // separately). Only definitions are considered.
+  if (VD->hasLocalStorage())
+    return;
+  if (VD->isTemplated())
+    return;
+  if (VD->isThisDeclarationADefinition(S.Context) != VarDecl::Definition)
+    return;
+
+  // Only plain `char` pointers/arrays with an initializer are supported; a
+  // matched variable of any other form (int, struct, wide or explicitly
+  // signed/unsigned character types, no initializer, ...) is silently
+  // ignored.
+  QualType Ty = VD->getType();
+  const PointerType *PT = Ty->getAsCanonical<PointerType>();
+  const ArrayType *AT = PT ? nullptr : S.Context.getAsArrayType(Ty);
+  QualType Pointee = PT   ? PT->getPointeeType()
+                     : AT ? AT->getElementType()
+                          : QualType();
+  if (Pointee.isNull() ||
+      !S.Context.hasSameUnqualifiedType(Pointee, S.Context.CharTy) ||
+      !VD->hasInit())
+    return;
+
+  // Names are matched against the mangled name, as it appears in the object
+  // file. For plain C file-scope variables this is the source identifier; for
+  // C++ variables it is the mangled symbol.
+  if (!S.getLangOpts().isLoadTimeCommentVar(
+          ASTNameGenerator(S.Context).getName(VD)))
----------------
AaronBallman wrote:

This causes a dynamic allocation and deallocation on every call and I think we call this for every declaration in the TU. I wonder what kind of compile time hit this will have. It might make sense to early return if no load time comment option is passed so the only people paying the price are the ones using the feature.

https://github.com/llvm/llvm-project/pull/187986