[IndVarSimplify] Eliminate zext of a signed IV when the IV is known to be non-negative

Patch by Li Huang

Differential Revision: https://reviews.llvm.org/D18867

llvm-svn: 278269

1 files changed, 7 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
index 4f6dbd9..4cb2d0d 100644
--- a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -36,6 +36,7 @@
 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/Constants.h"
@@ -1281,7 +1282,8 @@ Instruction *WidenIV::widenIVUse(NarrowIVDefUse DU, SCEVExpander &Rewriter) {
     }
   }
   // Our raison d'etre! Eliminate sign and zero extension.
-  if (IsSigned ? isa<SExtInst>(DU.NarrowUse) : isa<ZExtInst>(DU.NarrowUse)) {
+  if ((isa<SExtInst>(DU.NarrowUse) && (IsSigned || DU.NeverNegative)) ||
+      (isa<ZExtInst>(DU.NarrowUse) && (!IsSigned || DU.NeverNegative))) {
     Value *NewDef = DU.WideDef;
     if (DU.NarrowUse->getType() != WideType) {
       unsigned CastWidth = SE->getTypeSizeInBits(DU.NarrowUse->getType());
@@ -1370,9 +1372,12 @@ Instruction *WidenIV::widenIVUse(NarrowIVDefUse DU, SCEVExpander &Rewriter) {
 ///
 void WidenIV::pushNarrowIVUsers(Instruction *NarrowDef, Instruction *WideDef) {
   const SCEV *NarrowSCEV = SE->getSCEV(NarrowDef);
+  // isKnownPredicate is enough for most cases but still need isKnownNonNegative 
+  // here to work around conservatism in ScalarEvolution about no-wrap flags. 
   bool NeverNegative =
       SE->isKnownPredicate(ICmpInst::ICMP_SGE, NarrowSCEV,
-                           SE->getConstant(NarrowSCEV->getType(), 0));
+                           SE->getConstant(NarrowSCEV->getType(), 0)) ||
+      isKnownNonNegative(NarrowDef, NarrowDef->getModule()->getDataLayout());
   for (User *U : NarrowDef->users()) {
     Instruction *NarrowUser = cast<Instruction>(U);