[SYCL] Emit llvm.compiler.used for internal device globals (#5958)

This prevents early optimizations from removing device globals with
internal linkage.

Author: Fznamznon

clang/lib/CodeGen/CodeGenModule.cpp

@@ -2254,6 +2254,19 @@ void CodeGenModule::SetCommonAttributes(GlobalDecl GD, llvm::GlobalValue *GV) {
         VD->getStorageDuration() == SD_Static)
       addUsedOrCompilerUsedGlobal(GV);
   }
+
+  if (getLangOpts().SYCLIsDevice) {
+    // Add internal device_global variables to llvm.compiler.used array to
+    // prevent early optimizations from removing these variables from the
+    // module.
+    if (D && isa<VarDecl>(D)) {
+      const auto *VD = cast<VarDecl>(D);
+      const RecordDecl *RD = VD->getType()->getAsRecordDecl();
+      if (RD && RD->hasAttr<SYCLDeviceGlobalAttr>() &&
+          VD->getFormalLinkage() == InternalLinkage)
+        addUsedOrCompilerUsedGlobal(GV);
+    }
+  }
 }
 
 bool CodeGenModule::GetCPUAndFeaturesAttributes(GlobalDecl GD,

clang/test/CodeGenSYCL/device_global.cpp

@@ -91,6 +91,9 @@ void bar() {
 
 // CHECK: @llvm.global_ctors = appending global [2 x { i32, ptr, ptr addrspace(4) }] [{ i32, ptr, ptr addrspace(4) } { i32 65535, ptr @__cxx_global_var_init{{.*}}, ptr addrspace(4) addrspacecast (ptr addrspace(1) @[[TEMPL_DEV_GLOB]] to ptr addrspace(4)) }, { i32, ptr, ptr addrspace(4) } { i32 65535, ptr @_GLOBAL__sub_I_device_global.cpp, ptr addrspace(4) null }]
 // CHECK: @llvm.used = appending global [1 x ptr addrspace(4)] [ptr addrspace(4) addrspacecast (ptr addrspace(1) @[[TEMPL_DEV_GLOB]] to ptr addrspace(4))], section "llvm.metadata"
+// CHECK: @llvm.compiler.used = appending global [2 x ptr addrspace(4)]
+// CHECK-SAME: @_ZL1B
+// CHECK-SAME: @_ZN12_GLOBAL__N_19same_nameE
 
 // CHECK: attributes #[[A_ATTRS]] = { "sycl-unique-id"="_Z1A" }
 // CHECK: attributes #[[B_ATTRS]] = { "sycl-unique-id"="THE_PREFIX____ZL1B" }