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Step #4 in improving trip count analysis: HowFarToZero can analyze
NUW AddRec's much more aggressively. We now get a trip count for @test2 in nsw.ll llvm-svn: 123138
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@ -4821,8 +4821,9 @@ ScalarEvolution::HowFarToZero(const SCEV *V, const Loop *L) {
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#endif
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// Pick the smallest positive root value.
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if (ConstantInt *CB =
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dyn_cast<ConstantInt>(ConstantExpr::getICmp(ICmpInst::ICMP_ULT,
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R1->getValue(), R2->getValue()))) {
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dyn_cast<ConstantInt>(ConstantExpr::getICmp(CmpInst::ICMP_ULT,
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R1->getValue(),
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R2->getValue()))) {
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if (CB->getZExtValue() == false)
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std::swap(R1, R2); // R1 is the minimum root now.
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@ -4856,6 +4857,14 @@ ScalarEvolution::HowFarToZero(const SCEV *V, const Loop *L) {
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const SCEV *Start = getSCEVAtScope(AddRec->getStart(), L->getParentLoop());
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const SCEV *Step = getSCEVAtScope(AddRec->getOperand(1), L->getParentLoop());
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// If the AddRec is NUW, then (in an unsigned sense) it cannot be counting up
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// to wrap to 0, it must be counting down to equal 0. Also, while counting
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// down, it cannot "miss" 0 (which would cause it to wrap), regardless of what
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// the stride is. As such, NUW addrec's will always become zero in
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// "start / -stride" steps, and we know that the division is exact.
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if (AddRec->hasNoUnsignedWrap())
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return getUDivExpr(Start, getNegativeSCEV(Step));
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// For now we handle only constant steps.
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const SCEVConstant *StepC = dyn_cast<SCEVConstant>(Step);
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if (StepC == 0)
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