[Clang][CodeGen] Fix linkage and visibility of template parameter objects

[alexander-shaposhnikov]

Summary

This diff fixes linkage and visibility of template parameter objects.
The associated GitHub issue: https://github.com/llvm/llvm-project/issues/51571#

Test plan:
1/ ninja check-all
2/ bootstrapped Clang passes tests

[rsmith]

Looks good. If you want to look at the visibility side of this separately I think that's fine (I only called it out here because we often consider them at the same time).

clang/lib/CodeGen/CodeGenModule.cpp

3241–3246

I think we should be taking the visibility into account in some way too. For example, if the type of the template parameter object involves a type with hidden visibility, then the template parameter object itself should have hidden visibility.

Can we call setGVProperties(GV, TPO); here? It looks like that would do the right thing for visibility.

clang/lib/CodeGen/CodeGenModule.cpp

ConstantAddress CodeGenModule::GetAddrOfTemplateParamObject(
   llvm::Constant *Init = Emitter.emitForInitializer(
         TPO->getValue(), TPO->getType().getAddressSpace(), TPO->getType());
 
   if (!Init) {
     ErrorUnsupported(TPO, "template parameter object");
     return ConstantAddress::invalid();
   }
 
-  auto *GV = new llvm::GlobalVariable(
-      getModule(), Init->getType(),
-      /*isConstant=*/true, llvm::GlobalValue::LinkOnceODRLinkage, Init, Name);
+  llvm::GlobalValue::LinkageTypes Linkage =
+      isExternallyVisible(TPO->getLinkageAndVisibility().getLinkage())
+          ? llvm::GlobalValue::LinkOnceODRLinkage
+          : llvm::GlobalValue::InternalLinkage;
+  auto *GV = new llvm::GlobalVariable(getModule(), Init->getType(),
+                                      /*isConstant=*/true, Linkage, Init, Name);
   if (supportsCOMDAT())
     GV->setComdat(TheModule.getOrInsertComdat(GV->getName()));
   Emitter.finalize(GV);
 
   return ConstantAddress(GV, GV->getValueType(), Alignment);
 }
 
 ConstantAddress CodeGenModule::GetWeakRefReference(const ValueDecl *VD) {

clang/test/CodeGenCXX/template-param-objects-linkage.cpp

+// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++20 %s -emit-llvm -o - | FileCheck %s
+
+struct S { char buf[32]; };
+template<S s> constexpr const char* f() { return s.buf; }
+const char* fbuf = f<S{"a"}>();
+// CHECK: @_ZTAXtl1StlA32_cLc97EEEE = linkonce_odr constant { <{ i8, [31 x i8] }> }
+
+namespace {
+  struct UN { char buf[64]; };
+}
+template <UN un> constexpr const char* g() { return un.buf; }
+const char* gbuf = g<UN{"b"}>();
+// CHECK: @_ZTAXtlN12_GLOBAL__N_12UNEtlA64_cLc98EEEE = internal constant { <{ i8, [63 x i8] }> }
+
+struct Foo { int *i; };
+int m = 0;
+namespace { int n; }
+
+template <Foo foo>
+const int* h() { return foo.i; }
+
+const int* hm = h<Foo{&m}>();
+// CHECK: @_ZTAXtl3FooadL_Z1mEEE = linkonce_odr constant %struct.Foo { ptr @m }
+
+const int* hn = h<Foo{&n}>();
+// CHECK: @_ZTAXtl3FooadL_ZN12_GLOBAL__N_11nEEEE = internal constant %struct.Foo { ptr @_ZN12_GLOBAL__N_11nE }