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Remove getMinusSCEVForExitTest(). 

This function performed acrobatics to prove no-self-wrap, which we now
have for free.

llvm-svn: 127643
This commit is contained in:
Andrew Trick 2011-03-15 01:16:14 +00:00
parent 7a2873dfbe
commit a34f1b1f10
1 changed files with 3 additions and 106 deletions
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109
llvm/lib/Analysis/ScalarEvolution.cpp
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@ -2577,10 +2577,10 @@ const SCEV *ScalarEvolution::getNotSCEV(const SCEV *V) {
}
/// getMinusSCEV - Return LHS-RHS. Minus is represented in SCEV as A+B*-1.
///
/// FIXME: prohibit FlagNUW here, as soon as getMinusSCEVForExitTest goes.
const SCEV *ScalarEvolution::getMinusSCEV(const SCEV *LHS, const SCEV *RHS,
SCEV::NoWrapFlags Flags) {
assert(!maskFlags(Flags, SCEV::FlagNUW) && "subtraction does not have NUW");
// Fast path: X - X --> 0.
if (LHS == RHS)
return getConstant(LHS->getType(), 0);
@ -4080,106 +4080,6 @@ ScalarEvolution::ComputeBackedgeTakenCountFromExitCond(const Loop *L,
return ComputeBackedgeTakenCountExhaustively(L, ExitCond, !L->contains(TBB));
}
static const SCEVAddRecExpr *
isSimpleUnwrappingAddRec(const SCEV *S, const Loop *L) {
const SCEVAddRecExpr *SA = dyn_cast<SCEVAddRecExpr>(S);
// The SCEV must be an addrec of this loop.
if (!SA || SA->getLoop() != L || !SA->isAffine())
return 0;
// The SCEV must be known to not wrap in some way to be interesting.
if (!SA->getNoWrapFlags(SCEV::FlagNW))
return 0;
// The stride must be a constant so that we know if it is striding up or down.
if (!isa<SCEVConstant>(SA->getOperand(1)))
return 0;
return SA;
}
/// getMinusSCEVForExitTest - When considering an exit test for a loop with a
/// "x != y" exit test, we turn this into a computation that evaluates x-y != 0,
/// and this function returns the expression to use for x-y. We know and take
/// advantage of the fact that this subtraction is only being used in a
/// comparison by zero context.
///
/// FIXME: this can be completely removed once AddRec FlagNWs are propagated.
static const SCEV *getMinusSCEVForExitTest(const SCEV *LHS, const SCEV *RHS,
const Loop *L, ScalarEvolution &SE) {
// If either LHS or RHS is an AddRec SCEV (of this loop) that is known to not
// self-wrap, then we know that the value will either become the other one
// (and thus the loop terminates), that the loop will terminate through some
// other exit condition first, or that the loop has undefined behavior. This
// information is useful when the addrec has a stride that is != 1 or -1,
// because it means we can't "miss" the exit value.
//
// In any of these three cases, it is safe to turn the exit condition into a
// "counting down" AddRec (to zero) by subtracting the two inputs as normal,
// but since we know that the "end cannot be missed" we can force the
// resulting AddRec to be a NUW addrec. Since it is counting down, this means
// that the AddRec *cannot* pass zero.
// See if LHS and RHS are addrec's we can handle.
const SCEVAddRecExpr *LHSA = isSimpleUnwrappingAddRec(LHS, L);
const SCEVAddRecExpr *RHSA = isSimpleUnwrappingAddRec(RHS, L);
// If neither addrec is interesting, just return a minus.
if (RHSA == 0 && LHSA == 0)
return SE.getMinusSCEV(LHS, RHS);
// If only one of LHS and RHS are an AddRec of this loop, make sure it is LHS.
if (RHSA && LHSA == 0) {
// Safe because a-b === b-a for comparisons against zero.
std::swap(LHS, RHS);
std::swap(LHSA, RHSA);
}
// Handle the case when only one is advancing in a non-overflowing way.
if (RHSA == 0) {
// If RHS is loop varying, then we can't predict when LHS will cross it.
if (!SE.isLoopInvariant(RHS, L))
return SE.getMinusSCEV(LHS, RHS);
// If LHS has a positive stride, then we compute RHS-LHS, because the loop
// is counting up until it crosses RHS (which must be larger than LHS). If
// it is negative, we compute LHS-RHS because we're counting down to RHS.
const ConstantInt *Stride =
cast<SCEVConstant>(LHSA->getOperand(1))->getValue();
if (Stride->getValue().isNegative())
std::swap(LHS, RHS);
return SE.getMinusSCEV(RHS, LHS, SCEV::FlagNUW);
}
// If both LHS and RHS are interesting, we have something like:
// a+i*4 != b+i*8.
const ConstantInt *LHSStride =
cast<SCEVConstant>(LHSA->getOperand(1))->getValue();
const ConstantInt *RHSStride =
cast<SCEVConstant>(RHSA->getOperand(1))->getValue();
// If the strides are equal, then this is just a (complex) loop invariant
// comparison of a and b.
if (LHSStride == RHSStride)
return SE.getMinusSCEV(LHSA->getStart(), RHSA->getStart());
// If the signs of the strides differ, then the negative stride is counting
// down to the positive stride.
if (LHSStride->getValue().isNegative() != RHSStride->getValue().isNegative()){
if (RHSStride->getValue().isNegative())
std::swap(LHS, RHS);
} else {
// If LHS's stride is smaller than RHS's stride, then "b" must be less than
// "a" and "b" is RHS is counting up (catching up) to LHS. This is true
// whether the strides are positive or negative.
if (RHSStride->getValue().slt(LHSStride->getValue()))
std::swap(LHS, RHS);
}
return SE.getMinusSCEV(LHS, RHS, SCEV::FlagNUW);
}
/// ComputeBackedgeTakenCountFromExitCondICmp - Compute the number of times the
/// backedge of the specified loop will execute if its exit condition
/// were a conditional branch of the ICmpInst ExitCond, TBB, and FBB.
@ -4239,10 +4139,7 @@ ScalarEvolution::ComputeBackedgeTakenCountFromExitCondICmp(const Loop *L,
switch (Cond) {
case ICmpInst::ICMP_NE: { // while (X != Y)
// Convert to: while (X-Y != 0)
// FIXME: Once AddRec FlagNW are propagated, should be:
// BackedgeTakenInfo BTI = HowFarToZero(getMinusSCEV(LHS, RHS), L);
BackedgeTakenInfo BTI = HowFarToZero(getMinusSCEVForExitTest(LHS, RHS, L,
*this), L);
BackedgeTakenInfo BTI = HowFarToZero(getMinusSCEV(LHS, RHS), L);
if (BTI.hasAnyInfo()) return BTI;
break;
}