[clang] [clang] Do not emit template parameter objects as COMDATs when they have internal linkage. (PR #125448)

[Owen Anderson via cfe-commits]

https://github.com/resistor updated https://github.com/llvm/llvm-project/pull/125448

>From b3078fa4389f6be3f0b8c463ab9e79f1b4a9d23b Mon Sep 17 00:00:00 2001
From: Owen Anderson <resistor at mac.com>
Date: Mon, 3 Feb 2025 15:41:20 +1300
Subject: [PATCH] [clang] Do not emit template parameter objects as COMDATs
 when they have internal linkage.

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 | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 05879cd486a8c9..82002b8d8e4d4f 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 11ebd21521e83b..4b78e1e84f8638 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 +22,13 @@ 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"}>();
+
+
+// ITANIUM: [[HELLOANON:@_ZTAXtlN12_GLOBAL__N_11TEtlA32_cLc104ELc101ELc108ELc108ELc111ELc32ELc97ELc110ELc111ELc110EEEE]]
+// MSABI: [[HELLOANON:@"\?\?__N2UT@\?A0xFD1119C8@@3D0GI@@0GF@@0GM@@0GM@@0GP@@0CA@@0GB@@0GO@@0GP@@0GO@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@0A@@@@@"]]
+// CHECK-SAME: internal constant { <{ [10 x i8], [22 x i8] }> } { <{ [10 x i8], [22 x i8] }> <{ [10 x i8] c"hello anon", [22 x i8] zeroinitializer }> }
+// ITANIUM: @r
+// MSABI: @"?r@@3PEBDEB"
+// CHECK-SAME: global ptr [[HELLOANON]]
+// CHECK-NOT: comdat
+const char *r = begin_anon<T{"hello anon"}>();