[clang] clang: add unnamed_addr function attribute (PR #92499)

[YAMAMOTO Takashi via cfe-commits]

https://github.com/yamt created https://github.com/llvm/llvm-project/pull/92499

It simply applies the LLVM attribute with the same name to the function.

Sometimes, a programmer knows that function pointer uniqueness doesn't really matter for some of their functions. In that case, this attribute opens a possibility of certain optimizations like mergefunc with aliases. It's especially useful for code generators.

>From 1c1bedf655de10d2e4f0122975db1950f2203a96 Mon Sep 17 00:00:00 2001
From: YAMAMOTO Takashi <yamamoto at midokura.com>
Date: Fri, 17 May 2024 14:47:06 +0900
Subject: [PATCH] clang: add unnamed_addr function attribute

It simply applies the LLVM attribute with the same name to the function.

Sometimes, a programmer knows that function pointer uniqueness doesn't
really matter for some of their functions. In that case, this attribute
opens a possibility of certain optimizations like mergefunc with aliases.
It's especially useful for code generators.
---
 clang/include/clang/Basic/Attr.td   | 7 +++++++
 clang/lib/CodeGen/CodeGenModule.cpp | 4 ++++
 2 files changed, 11 insertions(+)

diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index 52552ba488560..3ee7d43d339f1 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -1944,6 +1944,13 @@ def ReturnsTwice : InheritableAttr {
   let SimpleHandler = 1;
 }
 
+def UnnamedAddr : InheritableAttr {
+  let Spellings = [Clang<"unnamed_addr">];
+  let Subjects = SubjectList<[Function]>;
+  let Documentation = [Undocumented];
+  let SimpleHandler = 1;
+}
+
 def DisableTailCalls : InheritableAttr {
   let Spellings = [Clang<"disable_tail_calls">];
   let Subjects = SubjectList<[Function, ObjCMethod]>;
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 489c08a4d4819..ac9f082a1049b 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -2506,6 +2506,10 @@ void CodeGenModule::SetLLVMFunctionAttributesForDefinition(const Decl *D,
       B.addAttribute(llvm::Attribute::MinSize);
   }
 
+  if (D->hasAttr<UnnamedAddrAttr>()) {
+    F->setUnnamedAddr(llvm::GlobalValue::UnnamedAddr::Global);
+  }
+
   F->addFnAttrs(B);
 
   unsigned alignment = D->getMaxAlignment() / Context.getCharWidth();