From 8f025f2a93ee7249336115cff31ae12367f1d0d5 Mon Sep 17 00:00:00 2001
From: Owen Anderson <resistor@mac.com>
Date: Mon, 3 Feb 2025 23:26:22 +1300
Subject: [PATCH] [clang] Do not emit template parameter objects as COMDATs
 when they have internal linkage. (#125448)

Per the ELF spec, section groups may only contain local symbols if those
symbols are only
referenced from within the section group. [1] In the case of template
parameter objects,
they can be referenced from outside the group when the type of the
object was declared
in an anonymous namespace. In that case, we can't place the object in a
COMDAT. This matches
GCC's linkage behavior on the test input.

[1]:
https://www.sco.com/developers/gabi/latest/ch4.sheader.html#section_groups
---
 clang/lib/CodeGen/CodeGenModule.cpp              |  2 +-
 clang/test/CodeGenCXX/template-param-objects.cpp | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 05879cd486a8c..82002b8d8e4d4 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -3765,7 +3765,7 @@ ConstantAddress CodeGenModule::GetAddrOfTemplateParamObject(
   auto *GV = new llvm::GlobalVariable(getModule(), Init->getType(),
                                       /*isConstant=*/true, Linkage, Init, Name);
   setGVProperties(GV, TPO);
-  if (supportsCOMDAT())
+  if (supportsCOMDAT() && Linkage == llvm::GlobalValue::LinkOnceODRLinkage)
     GV->setComdat(TheModule.getOrInsertComdat(GV->getName()));
   Emitter.finalize(GV);
 
diff --git a/clang/test/CodeGenCXX/template-param-objects.cpp b/clang/test/CodeGenCXX/template-param-objects.cpp
index 11ebd21521e83..ff6acc438d137 100644
--- a/clang/test/CodeGenCXX/template-param-objects.cpp
+++ b/clang/test/CodeGenCXX/template-param-objects.cpp
@@ -5,6 +5,9 @@ struct S { char buf[32]; };
 template<S s> constexpr const char *begin() { return s.buf; }
 template<S s> constexpr const char *end() { return s.buf + __builtin_strlen(s.buf); }
 
+namespace { struct T { char buf[32]; }; }
+template<T t> constexpr const char* begin_anon() { return t.buf; }
+
 // ITANIUM: [[HELLO:@_ZTAXtl1StlA32_cLc104ELc101ELc108ELc108ELc111ELc32ELc119ELc111ELc114ELc108ELc100EEEE]]
 // MSABI: [[HELLO:@"[?][?]__N2US@@3D0GI@@0GF@@0GM@@0GM@@0GP@@0CA@@0HH@@0GP@@0HC@@0GM@@0GE@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@@@@"]]
 // ITANIUM-SAME: = linkonce_odr constant { <{ [11 x i8], [21 x i8] }> } { <{ [11 x i8], [21 x i8] }> <{ [11 x i8] c"hello world", [21 x i8] zeroinitializer }> }, comdat
@@ -19,3 +22,10 @@ const char *p = begin<S{"hello world"}>();
 // MSABI: @"?q@@3PEBDEB"
 // CHECK-SAME: global ptr getelementptr (i8, ptr [[HELLO]], i64 11)
 const char *q = end<S{"hello world"}>();
+
+
+// CHECK: internal constant { <{ [10 x i8], [22 x i8] }> } { <{ [10 x i8], [22 x i8] }> <{ [10 x i8] c"hello anon", [22 x i8] zeroinitializer }> }
+// CHECK-NOT: comdat
+// ITANIUM: @r
+// MSABI: @"?r@@3PEBDEB"
+const char *r = begin_anon<T{"hello anon"}>();