add an early out to ProcessBranchOnXOR to speed it up,

handle the case when we can infer an input to the xor
from all inputs that agree, instead of going into an
infinite loop.  Another part of PR6199


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@94321 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/JumpThreading.cpp

@@ -1123,6 +1123,11 @@ bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
       isa<ConstantInt>(BO->getOperand(1)))
     return false;
   
+  // If the first instruction in BB isn't a phi, we won't be able to infer
+  // anything special about any particular predecessor.
+  if (!isa<PHINode>(BB->front()))
+    return false;
+  
   // If we have a xor as the branch input to this block, and we know that the
   // LHS or RHS of the xor in any predecessor is true/false, then we can clone
   // the condition into the predecessor and fix that value to true, saving some
@@ -1180,6 +1185,26 @@ bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
     BlocksToFoldInto.push_back(XorOpValues[i].second);
   }
   
+  // If we inferred a value for all of the predecessors, then duplication won't
+  // help us.  However, we can just replace the LHS or RHS with the constant.
+  if (BlocksToFoldInto.size() ==
+      cast<PHINode>(BB->front()).getNumIncomingValues()) {
+    if (SplitVal == 0) {
+      // If all preds provide undef, just nuke the xor, because it is undef too.
+      BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
+      BO->eraseFromParent();
+    } else if (SplitVal->isZero()) {
+      // If all preds provide 0, replace the xor with the other input.
+      BO->replaceAllUsesWith(BO->getOperand(isLHS));
+      BO->eraseFromParent();
+    } else {
+      // If all preds provide 1, set the computed value to 1.
+      BO->setOperand(!isLHS, SplitVal);
+    }
+    
+    return true;
+  }
+  
   // Try to duplicate BB into PredBB.
   return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
 }