[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 created https://github.com/llvm/llvm-project/pull/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


>From 7aa7a526bc6876cfe9b3c68e09e3e5f84befed03 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 +-
 .../test/CodeGenCXX/nocomdat-local-symbol.cpp | 38 +++++++++++++++++++
 2 files changed, 39 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/CodeGenCXX/nocomdat-local-symbol.cpp

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 05879cd486a8c98..82002b8d8e4d4f1 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/nocomdat-local-symbol.cpp b/clang/test/CodeGenCXX/nocomdat-local-symbol.cpp
new file mode 100644
index 000000000000000..3beb7d2197dd597
--- /dev/null
+++ b/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" }
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