[clang] 8f025f2 - [clang] Do not emit template parameter objects as COMDATs when they have internal linkage. (#125448)

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Author: Owen Anderson
Date: 2025-02-03T23:26:22+13:00
New Revision: 8f025f2a93ee7249336115cff31ae12367f1d0d5

URL: https://github.com/llvm/llvm-project/commit/8f025f2a93ee7249336115cff31ae12367f1d0d5
DIFF: https://github.com/llvm/llvm-project/commit/8f025f2a93ee7249336115cff31ae12367f1d0d5.diff

LOG: [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

Added: 
    

Modified: 
    clang/lib/CodeGen/CodeGenModule.cpp
    clang/test/CodeGenCXX/template-param-objects.cpp

Removed: 
    


################################################################################
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..ff6acc438d137a 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"}>();