[HIP] Make sure, unused hip-pinned-shadow global var is kept within device code

Summary:
hip-pinned-shadow global var should remain in the final code object irrespective
of whether it is used or not within the code. Add it to used list, so that it
will not get eliminated when it is unused.

Reviewers: yaxunl, tra, hliao

Reviewed By: yaxunl

Subscribers: hliao, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75402

Author: hsmahesha

clang/lib/CodeGen/CodeGenModule.cpp

@@ -1916,9 +1916,9 @@ void CodeGenModule::SetFunctionAttributes(GlobalDecl GD, llvm::Function *F,
   }
 }
 
-void CodeGenModule::addUsedGlobal(llvm::GlobalValue *GV) {
-  assert(!GV->isDeclaration() &&
-         "Only globals with definition can force usage.");
+void CodeGenModule::addUsedGlobal(llvm::GlobalValue *GV, bool SkipCheck) {
+  assert(SkipCheck || (!GV->isDeclaration() &&
+                       "Only globals with definition can force usage."));
   LLVMUsed.emplace_back(GV);
 }
 
@@ -4071,9 +4071,17 @@ void CodeGenModule::EmitGlobalVarDefinition(const VarDecl *D,
     }
   }
 
-  if (!IsHIPPinnedShadowVar)
+  // HIPPinnedShadowVar should remain in the final code object irrespective of
+  // whether it is used or not within the code. Add it to used list, so that
+  // it will not get eliminated when it is unused. Also, it is an extern var
+  // within device code, and it should *not* get initialized within device code.
+  if (IsHIPPinnedShadowVar)
+    addUsedGlobal(GV, /*SkipCheck=*/true);
+  else
     GV->setInitializer(Init);
-  if (emitter) emitter->finalize(GV);
+
+  if (emitter)
+    emitter->finalize(GV);
 
   // If it is safe to mark the global 'constant', do so now.
   GV->setConstant(!NeedsGlobalCtor && !NeedsGlobalDtor &&

clang/lib/CodeGen/CodeGenModule.h

@@ -1037,7 +1037,7 @@ class CodeGenModule : public CodeGenTypeCache {
   void MaybeHandleStaticInExternC(const SomeDecl *D, llvm::GlobalValue *GV);
 
   /// Add a global to a list to be added to the llvm.used metadata.
-  void addUsedGlobal(llvm::GlobalValue *GV);
+  void addUsedGlobal(llvm::GlobalValue *GV, bool SkipCheck = false);
 
   /// Add a global to a list to be added to the llvm.compiler.used metadata.
   void addCompilerUsedGlobal(llvm::GlobalValue *GV);

clang/test/CodeGenCUDA/hip-pinned-shadow.cu

@@ -4,6 +4,8 @@
 // RUN:     -emit-llvm -o - -x hip %s | FileCheck -check-prefixes=HIPDEV %s
 // RUN: %clang_cc1 -triple x86_64 -std=c++11 \
 // RUN:     -emit-llvm -o - -x hip %s | FileCheck -check-prefixes=HIPHOST %s
+// RUN: %clang_cc1 -triple amdgcn -fcuda-is-device -std=c++11 -fvisibility hidden -fapply-global-visibility-to-externs \
+// RUN:     -O3 -emit-llvm -o - -x hip %s | FileCheck -check-prefixes=HIPDEVUNSED %s
 
 struct textureReference {
   int a;
@@ -21,3 +23,5 @@ __attribute__((hip_pinned_shadow)) texture<float, 2, 1> tex;
 // HIPDEV-NOT: declare{{.*}}void @_ZN7textureIfLi2ELi1EEC1Ev
 // HIPHOST:  define{{.*}}@_ZN7textureIfLi2ELi1EEC1Ev
 // HIPHOST:  call i32 @__hipRegisterVar{{.*}}@tex{{.*}}i32 0, i32 4, i32 0, i32 0)
+// HIPDEVUNSED: @tex = external addrspace(1) global %struct.texture
+// HIPDEVUNSED-NOT: declare{{.*}}void @_ZN7textureIfLi2ELi1EEC1Ev