[OpenMP] Do not always emit unused extern variables

Currently, the precense of the OpenMP target declare metadata requires
that we always codegen a global declaration. This is undesirable in the
case that we could defer or omit this declaration as is common with
unused extern variables. This is important as it allows us, in the
runtime, to rely on static linking semantics to omit unused symbols so
they are not included when the user links it in.

This patch changes the check for always emitting these variables.
Because of this we also need to extend this logic to the generation of
the offloading entries. This has the result of derring the offload entry
generation to the canonical definitoin. So we are effectively assuming
whoever owns the storage for this variable will perform that operation.
This makes an exception for `link` attributes as those require their own
special handling.

Let me know if this is sound in the implementation, I do not have the
largest view of the standards here.

Fixes: #64133

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D156368

clang/lib/CodeGen/CGOpenMPRuntime.cpp

@@ -10166,6 +10166,13 @@ void CGOpenMPRuntime::registerTargetGlobalVariable(const VarDecl *VD,
 
   std::optional<OMPDeclareTargetDeclAttr::MapTypeTy> Res =
       OMPDeclareTargetDeclAttr::isDeclareTargetDeclaration(VD);
+
+  // If this is an 'extern' declaration we defer to the canonical definition and
+  // do not emit an offloading entry.
+  if (Res && *Res != OMPDeclareTargetDeclAttr::MT_Link &&
+      VD->hasExternalStorage())
+    return;
+
   if (!Res) {
     if (CGM.getLangOpts().OpenMPIsTargetDevice) {
       // Register non-target variables being emitted in device code (debug info

clang/lib/CodeGen/CodeGenModule.cpp

@@ -3605,6 +3605,13 @@ void CodeGenModule::EmitGlobal(GlobalDecl GD) {
         // Emit declaration of the must-be-emitted declare target variable.
         if (std::optional<OMPDeclareTargetDeclAttr::MapTypeTy> Res =
                 OMPDeclareTargetDeclAttr::isDeclareTargetDeclaration(VD)) {
+
+          // If this variable has external storage and doesn't require special
+          // link handling we defer to its canonical definition.
+          if (VD->hasExternalStorage() &&
+              Res != OMPDeclareTargetDeclAttr::MT_Link)
+            return;
+
           bool UnifiedMemoryEnabled =
               getOpenMPRuntime().hasRequiresUnifiedSharedMemory();
           if ((*Res == OMPDeclareTargetDeclAttr::MT_To ||

clang/test/OpenMP/declare_target_codegen.cpp

@@ -29,7 +29,6 @@
 // CHECK-DAG: @flag = protected global i8 undef,
 // CHECK-DAG: @dx = {{protected | }}global i32 0,
 // CHECK-DAG: @dy = {{protected | }}global i32 0,
-// CHECK-DAG: @aaa = external global i32,
 // CHECK-DAG: @bbb = {{protected | }}global i32 0,
 // CHECK-DAG: weak constant %struct.__tgt_offload_entry { ptr @bbb,
 // CHECK-DAG: @ccc = external global i32,
@@ -80,7 +79,7 @@ extern int bbb;
 extern int aaa;
 int bbb = 0;
 extern int ccc;
-int ddd = 0;
+int ddd = ccc;
 #pragma omp end declare target
 
 #pragma omp declare target
@@ -260,8 +259,6 @@ struct TTT {
 
 // CHECK-NOT: define {{.*}}{{baz1|baz4|maini1|Base|virtual_}}
 
-// CHECK-DAG: !{i32 1, !"aaa", i32 0, i32 {{[0-9]+}}}
-// CHECK-DAG: !{i32 1, !"ccc", i32 0, i32 {{[0-9]+}}}
 // CHECK-DAG: !{{{.+}}virtual_foo
 
 #ifdef OMP5

openmp/libomptarget/test/offloading/extern.c

@@ -0,0 +1,27 @@
+// RUN: %libomptarget-compile-generic -DVAR -c -o %t.o
+// RUN: %libomptarget-compile-generic %t.o && %libomptarget-run-generic | %fcheck-generic
+
+#ifdef VAR
+int x = 1;
+#else
+#include <stdio.h>
+#include <assert.h>
+extern int x;
+
+int main() {
+  int value = 0;
+#pragma omp target map(from : value)
+  value = x;
+  assert(value == 1);
+
+  x = 999;
+#pragma omp target update to(x)
+
+#pragma omp target map(from : value)
+  value = x;
+  assert(value == 999);
+
+  // CHECK: PASS
+  printf ("PASS\n");
+}
+#endif