[InstCombine] Determine the result of a select based on a dominating condition.

Differential Revision: http://reviews.llvm.org/D19550

llvm-svn: 268104

2 files changed, 22 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index feddbafa0d5..4097322a2b9 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -3941,7 +3941,10 @@ Optional<bool> llvm::isImpliedCondition(Value *LHS, Value *RHS,
                                         unsigned Depth, AssumptionCache *AC,
                                         const Instruction *CxtI,
                                         const DominatorTree *DT) {
-  assert(LHS->getType() == RHS->getType() && "mismatched type");
+  // A mismatch occurs when we compare a scalar cmp to a vector cmp, for example.
+  if (LHS->getType() != RHS->getType())
+    return None;
+
   Type *OpTy = LHS->getType();
   assert(OpTy->getScalarType()->isIntegerTy(1));
 
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 8595996c29c..ecf94dc70a7 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1213,5 +1213,23 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
     }
   }
 
+  // See if we can determine the result of this select based on a dominating
+  // condition.
+  BasicBlock *Parent = SI.getParent();
+  if (BasicBlock *Dom = Parent->getSinglePredecessor()) {
+    auto *PBI = dyn_cast_or_null<BranchInst>(Dom->getTerminator());
+    if (PBI && PBI->isConditional() &&
+        PBI->getSuccessor(0) != PBI->getSuccessor(1) &&
+        (PBI->getSuccessor(0) == Parent || PBI->getSuccessor(1) == Parent)) {
+      bool CondIsFalse = PBI->getSuccessor(1) == Parent;
+      Optional<bool> Implication = isImpliedCondition(
+        PBI->getCondition(), SI.getCondition(), DL, CondIsFalse);
+      if (Implication) {
+        Value *V = *Implication ? TrueVal : FalseVal;
+        return replaceInstUsesWith(SI, V);
+      }
+    }
+  }
+
   return nullptr;
 }