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

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LinearFunctionTestReplace tries to use the *next* indvar to compare
against when possible.  However, it may be the case that the calculation
for the next indvar has NUW/NSW flags and that it may only be safely
used inside the loop.  Using it in a comparison to calculate the exit
condition could result in observing poison.

This fixes PR20680.

Differential Revision: http://reviews.llvm.org/D5174

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217102 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer
2014-09-03 23:03:18 +00:00
parent 699fd1909e
commit c0f2b8b528
6 changed files with 258 additions and 19 deletions
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24
lib/Transforms/Scalar/IndVarSimplify.cpp
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@@ -1641,15 +1641,27 @@ 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.
CmpIndVar = IndVar->getIncomingValueForBlock(L->getExitingBlock());
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.
if (ScalarEvolution::maskFlags(IncrementedIndvarSCEV->getNoWrapFlags(),
SCEV::FlagNUW | SCEV::FlagNSW) ==
SCEV::FlagAnyWrap) {
// 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;
}
}
Value *ExitCnt = genLoopLimit(IndVar, IVCount, L, Rewriter, SE);