IRLinker: fix double scheduling of mapping a global value because of an alias

[mehdi_amini]

Summary

This test was hitting an assertion in the value mapper because
the IRLinker was trying to map two times @A while materializing
the initializer for @C.

Fix http://llvm.org/PR27850

[• rafael]

LGTM, thanks!

lib/Linker/IRMover.cpp

if (auto *F = dyn_cast<Function>(New)) {
     if (V->hasInitializer() || V->hasAppendingLinkage())
       return;
   } else {
     auto *A = cast<GlobalAlias>(New);
     if (A->getAliasee())
       return;
   }
 
+  // When linking a global for an alias, it will always be linked. However we
+  // need to check if it was not already scheduled to satify a reference from a
+  // regular global value initializer. We know if it has been schedule if the
+  // "New" GlobalValue that is mapped here for the alias is the same as the one
+  // already mapped. If there is an entry in the ValueMap but the value is
+  // different, it means that the value already had a definition in the
+  // destination module (linkonce for instance), but we need a new definition
+  // for the alias ("New" will be different.
+  if (ForAlias && ValueMap.lookup(Old) == New)
+    return;
+
   if (ForAlias || shouldLink(New, *Old))
     linkGlobalValueBody(*New, *Old);
 }
 
 /// Loop through the global variables in the src module and merge them into the
 /// dest module.
 GlobalVariable *IRLinker::copyGlobalVariableProto(const GlobalVariable *SGVar) {
   // No linking to be performed or linking from the source: simply create an

test/Linker/alias-3.ll

+; RUN: llvm-link %s -S -o - | FileCheck %s
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.9"
+
+; CHECK-DAG: @A = internal constant i8 1
+; CHECK-DAG: @B = alias i8, i8* @A
+; CHECK-DAG: @C = global [2 x i8*] [i8* @A, i8* @B]
+
+@A = internal constant i8 1
+@B = alias i8, i8* @A
+@C = global [2 x i8*] [i8* @A, i8* @B]
+
+