Revert some changes that were made to fix PR20680.

This re-lands change r230921.  r230921 was reverted because it broke a
clang test; a checkin fixing the clang test will be commited shortly.

Summary:
As far as I can tell, the real bug causing the issue was fixed in
r230533.  SCEVExpander should mark an increment operation as nuw or nsw
only if it can *prove* that the operation does not overflow.  There
shouldn't be any situation where we have to do something different
because of no-wrap flags generated by SCEVExpander.

Revert "IndVarSimplify: Allow LFTR to fire more often"

This reverts commit 1ade0f0faa98877b688e0b9da58e876052c1e04e (SVN: 222213).

Revert "IndVarSimplify: Don't let LFTR compare against a poison value"

This reverts commit c0f2b8b528d8a37b0a1522aae90af649d6357eb5 (SVN: 217102).

Reviewers: majnemer, atrick, spatel

Differential Revision: http://reviews.llvm.org/D7979

llvm-svn: 231018

1 files changed, 6 insertions, 42 deletions

diff --git a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
index f99ebbc453f..462c2b6f90d 100644
--- a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -1705,51 +1705,15 @@ LinearFunctionTestReplace(Loop *L,
   // compare against the post-incremented value, otherwise we must compare
   // against the preincremented value.
   if (L->getExitingBlock() == L->getLoopLatch()) {
+    // 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().
+    IVCount = SE->getAddExpr(BackedgeTakenCount,
+                             SE->getConstant(BackedgeTakenCount->getType(), 1));
     // The BackedgeTaken expression contains the number of times that the
     // backedge branches to the loop header.  This is one less than the
     // number of times the loop executes, so use the incremented indvar.
-    llvm::Value *IncrementedIndvar =
-        IndVar->getIncomingValueForBlock(L->getExitingBlock());
-    const auto *IncrementedIndvarSCEV =
-        cast<SCEVAddRecExpr>(SE->getSCEV(IncrementedIndvar));
-    // It is unsafe to use the incremented indvar if it has a wrapping flag, we
-    // don't want to compare against a poison value.  Check the SCEV that
-    // corresponds to the incremented indvar, the SCEVExpander will only insert
-    // flags in the IR if the SCEV originally had wrapping flags.
-    // 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(),
-                                    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().
-      IVCount =
-          SE->getAddExpr(BackedgeTakenCount,
-                         SE->getConstant(BackedgeTakenCount->getType(), 1));
-      CmpIndVar = IncrementedIndvar;
-    }
+    CmpIndVar = IndVar->getIncomingValueForBlock(L->getExitingBlock());
   }
 
   Value *ExitCnt = genLoopLimit(IndVar, IVCount, L, Rewriter, SE);