llvm-diff: Perform structural comparison on GlobalVariables, if possible

[ddcc]

Summary

Names of GlobalVariables may not be preserved depending on compilation options, so prefer a structural diff

[rjmccall]

Maybe this should depend on linkage?

[ddcc]

Which linkage types specifically? hasUniqueInitializer() checks !isExternallyInitialized() and isStrongDefinitionForLinker(), which expands to !isDeclarationForLinker() (!hasAvailableExternallyLinkage() && !isDeclaration()) and !isWeakForLinker()), which I believe limits the variable to private and internal linkages.

[rjmccall]

No, that's not true; something with External linkage would not trip any of those conditions normally. ("Externally initialized" in particular is basically unrelated to linkage.) I think you want a condition something like L->getName() == R->getName() || (L->hasLocalLinkage() && R->hasLocalLinkage()).

[ddcc]

Check linkages

[rjmccall]

Okay.

llvm/tools/llvm-diff/DifferenceEngine.cpp

void DifferenceEngine::diff(Module *L, Module *R) {
 
   for (SmallVectorImpl<std::pair<Function*,Function*> >::iterator
          I = Queue.begin(), E = Queue.end(); I != E; ++I)
     diff(I->first, I->second);
 }
 
 bool DifferenceEngine::equivalentAsOperands(GlobalValue *L, GlobalValue *R) {
   if (globalValueOracle) return (*globalValueOracle)(L, R);
+
+  if (isa<GlobalVariable>(L) && isa<GlobalVariable>(R)) {
+    GlobalVariable *GVL = cast<GlobalVariable>(L);
+    GlobalVariable *GVR = cast<GlobalVariable>(R);
+    if (GVL->hasLocalLinkage() && GVL->hasUniqueInitializer() &&
+        GVR->hasLocalLinkage() && GVR->hasUniqueInitializer())
+      return GVL->getInitializer() == GVR->getInitializer();
+  }
+
   return L->getName() == R->getName();
 }