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IndVarSimplify: Address review comments for r217102 

No functional change intended, just some cleanups and comments added.

llvm-svn: 217115
This commit is contained in:
David Majnemer 2014-09-04 00:23:13 +00:00
parent 54e0614efd
commit 13046deef3
1 changed files with 7 additions and 4 deletions
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11
llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
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@ -1644,16 +1644,19 @@ LinearFunctionTestReplace(Loop *L,
// 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());
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) {
// 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.
if (!ScalarEvolution::maskFlags(IncrementedIndvarSCEV->getNoWrapFlags(),
SCEV::FlagNUW | SCEV::FlagNSW)) {
// 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().