[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

2 files changed, 10 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index 2def0b54c6d..d68dd140a80 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -928,9 +928,10 @@ bool JumpThreading::ProcessImpliedCondition(BasicBlock *BB) {
     if (!PBI || !PBI->isConditional() || PBI->getSuccessor(0) != CurrentBB)
       return false;
 
-    if (isImpliedCondition(PBI->getCondition(), Cond, DL)) {
-      BI->getSuccessor(1)->removePredecessor(BB);
-      BranchInst::Create(BI->getSuccessor(0), BI);
+    bool ImpliedTrue;
+    if (isImpliedCondition(PBI->getCondition(), Cond, ImpliedTrue, DL)) {
+      BI->getSuccessor(ImpliedTrue ? 1 : 0)->removePredecessor(BB);
+      BranchInst::Create(BI->getSuccessor(ImpliedTrue ? 0 : 1), BI);
       BI->eraseFromParent();
       return true;
     }
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.
   }