[ValueTracking] Improve isImpliedCondition for conditions with matching operands.

This patch improves SimplifyCFG to catch cases like:

  if (a < b) {
    if (a > b) <- known to be false
      unreachable;
  }

Phabricator Revision: http://reviews.llvm.org/D18905

llvm-svn: 266767

1 files changed, 6 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 1c27bc67cfb..4976893ad51 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2722,13 +2722,17 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
 
   // If BI is reached from the true path of PBI and PBI's condition implies
   // BI's condition, we know the direction of the BI branch.
+  bool ImpliedTrue;
   if (PBI->getSuccessor(0) == BI->getParent() &&
-      isImpliedCondition(PBI->getCondition(), BI->getCondition(), DL) &&
+      isImpliedCondition(PBI->getCondition(), BI->getCondition(), ImpliedTrue,
+                         DL) &&
       PBI->getSuccessor(0) != PBI->getSuccessor(1) &&
       BB->getSinglePredecessor()) {
     // Turn this into a branch on constant.
     auto *OldCond = BI->getCondition();
-    BI->setCondition(ConstantInt::getTrue(BB->getContext()));
+    ConstantInt *CI = ImpliedTrue ? ConstantInt::getTrue(BB->getContext())
+                                  : ConstantInt::getFalse(BB->getContext());
+    BI->setCondition(CI);
     RecursivelyDeleteTriviallyDeadInstructions(OldCond);
     return true;  // Nuke the branch on constant.
   }