D36604: PR34148: Do not assume we can use a copy relocation for an `external_weak` global

An `external_weak` global may be intended to resolve as a null pointer if it's
not defined, so it doesn't make sense to use a copy relocation for it.

Differential Revision: https://reviews.llvm.org/D36604

llvm-svn: 310773

llvm/lib/Target/TargetMachine.cpp

@@ -154,8 +154,9 @@ bool TargetMachine::shouldAssumeDSOLocal(const Module &M,
       return true;
 
     bool IsTLS = GV && GV->isThreadLocal();
-    bool IsAccessViaCopyRelocs =
-        Options.MCOptions.MCPIECopyRelocations && GV && isa<GlobalVariable>(GV);
+    bool IsAccessViaCopyRelocs = Options.MCOptions.MCPIECopyRelocations && GV &&
+                                 isa<GlobalVariable>(GV) &&
+                                 !GV->hasExternalWeakLinkage();
     Triple::ArchType Arch = TT.getArch();
     bool IsPPC =
         Arch == Triple::ppc || Arch == Triple::ppc64 || Arch == Triple::ppc64le;

llvm/test/CodeGen/X86/global-access-pie-copyrelocs.ll

@@ -63,6 +63,20 @@ entry:
   ret i32 %0
 }
 
+; ExternalWeak Linkage
+@e = extern_weak global i32, align 4
+
+define i32* @my_access_global_d() #0 {
+; X32-LABEL: my_access_global_d:
+; X32:       addl $_GLOBAL_OFFSET_TABLE_{{.*}}, %eax
+; X32:       movl e@GOT(%eax), %eax
+; X64-LABEL: my_access_global_d:
+; X64:       movq e@GOTPCREL(%rip), %rax
+
+entry:
+  ret i32* @e
+}
+
 ; External Linkage, only declaration, store a value.
 
 define i32 @my_access_global_store_d() #0 {