IndVarSimplify: Allow LFTR to fire more often

I added a pessimization in r217102 to prevent miscompiles when the
incremented induction variable was used in a comparison; it would be
poison.

Try to use the incremented induction variable more often when we can be
sure that the increment won't end in poison.

Differential Revision: http://reviews.llvm.org/D6222

llvm-svn: 222213

1 files changed, 22 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
index bf90912ebd0..fc6d1837060 100644
--- a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -1719,8 +1719,29 @@ LinearFunctionTestReplace(Loop *L,
     // FIXME: In theory, SCEV could drop flags even though they exist in IR.
     // A more robust solution would involve getting a new expression for
     // CmpIndVar by applying non-NSW/NUW AddExprs.
+    auto WrappingFlags =
+        ScalarEvolution::setFlags(SCEV::FlagNUW, SCEV::FlagNSW);
+    const SCEV *IVInit = IncrementedIndvarSCEV->getStart();
+    if (SE->getTypeSizeInBits(IVInit->getType()) >
+        SE->getTypeSizeInBits(IVCount->getType()))
+      IVInit = SE->getTruncateExpr(IVInit, IVCount->getType());
+    unsigned BitWidth = SE->getTypeSizeInBits(IVCount->getType());
+    Type *WideTy = IntegerType::get(SE->getContext(), BitWidth + 1);
+    // Check if InitIV + BECount+1 requires sign/zero extension.
+    // If not, clear the corresponding flag from WrappingFlags because it is not
+    // necessary for those flags in the IncrementedIndvarSCEV expression.
+    if (SE->getSignExtendExpr(SE->getAddExpr(IVInit, BackedgeTakenCount),
+                              WideTy) ==
+        SE->getAddExpr(SE->getSignExtendExpr(IVInit, WideTy),
+                       SE->getSignExtendExpr(BackedgeTakenCount, WideTy)))
+      WrappingFlags = ScalarEvolution::clearFlags(WrappingFlags, SCEV::FlagNSW);
+    if (SE->getZeroExtendExpr(SE->getAddExpr(IVInit, BackedgeTakenCount),
+                              WideTy) ==
+        SE->getAddExpr(SE->getZeroExtendExpr(IVInit, WideTy),
+                       SE->getZeroExtendExpr(BackedgeTakenCount, WideTy)))
+      WrappingFlags = ScalarEvolution::clearFlags(WrappingFlags, SCEV::FlagNUW);
     if (!ScalarEvolution::maskFlags(IncrementedIndvarSCEV->getNoWrapFlags(),
-                                    SCEV::FlagNUW | SCEV::FlagNSW)) {
+                                    WrappingFlags)) {
       // Add one to the "backedge-taken" count to get the trip count.
       // This addition may overflow, which is valid as long as the comparison is
       // truncated to BackedgeTakenCount->getType().