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Step #4 in improving trip count analysis: HowFarToZero can analyze

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NUW AddRec's much more aggressively.  We now get a trip count
for @test2 in nsw.ll


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123138 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2011-01-09 22:58:47 +00:00
parent da1f1f4950
commit 53e1d45adb
1 changed files with 11 additions and 2 deletions
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13
lib/Analysis/ScalarEvolution.cpp
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@ -4821,8 +4821,9 @@ ScalarEvolution::HowFarToZero(const SCEV *V, const Loop *L) {
#endif
// Pick the smallest positive root value.
if (ConstantInt *CB =
dyn_cast<ConstantInt>(ConstantExpr::getICmp(ICmpInst::ICMP_ULT,
R1->getValue(), R2->getValue()))) {
dyn_cast<ConstantInt>(ConstantExpr::getICmp(CmpInst::ICMP_ULT,
R1->getValue(),
R2->getValue()))) {
if (CB->getZExtValue() == false)
std::swap(R1, R2); // R1 is the minimum root now.
@ -4856,6 +4857,14 @@ ScalarEvolution::HowFarToZero(const SCEV *V, const Loop *L) {
const SCEV *Start = getSCEVAtScope(AddRec->getStart(), L->getParentLoop());
const SCEV *Step = getSCEVAtScope(AddRec->getOperand(1), L->getParentLoop());
// If the AddRec is NUW, then (in an unsigned sense) it cannot be counting up
// to wrap to 0, it must be counting down to equal 0. Also, while counting
// down, it cannot "miss" 0 (which would cause it to wrap), regardless of what
// the stride is. As such, NUW addrec's will always become zero in
// "start / -stride" steps, and we know that the division is exact.
if (AddRec->hasNoUnsignedWrap())
return getUDivExpr(Start, getNegativeSCEV(Step));
// For now we handle only constant steps.
const SCEVConstant *StepC = dyn_cast<SCEVConstant>(Step);
if (StepC == 0)