[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

Author: resistor

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);
 

clang/test/CodeGenCXX/nocomdat-local-symbol.cpp

@@ -0,0 +1,38 @@
+// RUN: %clang_cc1 -emit-llvm -std=c++20 -triple x86_64-unknown-linux-gnu %s -o - | FileCheck %s
+
+// COMDAT groups in ELF objects are not permitted to contain local symbols. While template parameter
+// objects are normally emitted in COMDATs, we shouldn't do so if they would have internal linkage.
+
+extern "C" int printf(...);
+typedef __typeof__(sizeof(0)) size_t;
+
+namespace {
+template<size_t N>
+struct DebugContext
+{
+    char value[N];
+    constexpr DebugContext(const char (&str)[N]) {
+        for (size_t i = 0; i < N; ++i) {
+            value[i] = str[i];
+        }
+    }
+};
+}
+
+template<DebugContext Context>
+struct ConditionalDebug
+{
+    public:
+    static void log() {
+        printf("%s", Context.value);
+    }
+};
+
+using Debug = ConditionalDebug<"compartment A">;
+
+void foo() {
+	Debug::log();
+}
+
+// CHECK-NOT: $_ZTAXtlN12_GLOBAL__N_112DebugContextILm14EEEtlA14_cLc99ELc111ELc109ELc112ELc97ELc114ELc116ELc109ELc101ELc110ELc116ELc32ELc65EEEE = comdat any
+// CHECK: @_ZTAXtlN12_GLOBAL__N_112DebugContextILm14EEEtlA14_cLc99ELc111ELc109ELc112ELc97ELc114ELc116ELc109ELc101ELc110ELc116ELc32ELc65EEEE = internal constant %"struct.(anonymous namespace)::DebugContext" { [14 x i8] c"compartment A\00" }