ObjCARC: Don't look at users of ConstantData

Stop looking at users of UndefValue and ConstantPointerNull in the
objective C ARC optimizers.  The other users aren't actually
interesting, since they're not pointing at a particular object.  I
imagine these calls could be optimized through -instcombine... maybe
they already are?

These early returns will be required at some point in the future, with a
WIP patch that asserts when someone accesses a use-list on ConstantData.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282338 91177308-0d34-0410-b5e6-96231b3b80d8

Author: dexonsmith

lib/Transforms/ObjCARC/ObjCARCOpts.cpp

@@ -53,6 +53,11 @@ using namespace llvm::objcarc;
 /// \brief This is similar to GetRCIdentityRoot but it stops as soon
 /// as it finds a value with multiple uses.
 static const Value *FindSingleUseIdentifiedObject(const Value *Arg) {
+  // ConstantData (like ConstantPointerNull and UndefValue) is used across
+  // modules.  It's never a single-use value.
+  if (isa<ConstantData>(Arg))
+    return nullptr;
+
   if (Arg->hasOneUse()) {
     if (const BitCastInst *BC = dyn_cast<BitCastInst>(Arg))
       return FindSingleUseIdentifiedObject(BC->getOperand(0));
@@ -644,6 +649,12 @@ void ObjCARCOpt::OptimizeAutoreleaseRVCall(Function &F,
                                            ARCInstKind &Class) {
   // Check for a return of the pointer value.
   const Value *Ptr = GetArgRCIdentityRoot(AutoreleaseRV);
+
+  // If the argument is ConstantPointerNull or UndefValue, its other users
+  // aren't actually interesting to look at.
+  if (isa<ConstantData>(Ptr))
+    return;
+
   SmallVector<const Value *, 2> Users;
   Users.push_back(Ptr);
   do {