[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

Cherry-picked from llvm/llvm-project@8f025f2

Author: resistor

clang/lib/CodeGen/CodeGenModule.cpp

@@ -3565,7 +3565,7 @@ 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);
 

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"}>();