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When SCEV can determine the loop test is X < X, set ExactBECount=0.

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When ExactBECount is a constant, use it for MaxBECount.
When MaxBECount cannot be computed, replace it with ExactBECount.
Fixes PR9424.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127342 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Andrew Trick 2011-03-09 17:29:58 +00:00
parent dd54ffda26
commit e62289b98f
2 changed files with 51 additions and 1 deletions
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18
lib/Analysis/ScalarEvolution.cpp
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@ -5646,6 +5646,13 @@ const SCEV *ScalarEvolution::getBECount(const SCEV *Start,
"This code doesn't handle negative strides yet!"); "This code doesn't handle negative strides yet!");
const Type *Ty = Start->getType(); const Type *Ty = Start->getType();
// When Start == End, we have an exact BECount == 0. Short-circuit this case
// here because SCEV may not be able to determine that the unsigned division
// after rounding is zero.
if (Start == End)
return getConstant(Ty, 0);
const SCEV *NegOne = getConstant(Ty, (uint64_t)-1); const SCEV *NegOne = getConstant(Ty, (uint64_t)-1);
const SCEV *Diff = getMinusSCEV(End, Start); const SCEV *Diff = getMinusSCEV(End, Start);
const SCEV *RoundUp = getAddExpr(Step, NegOne); const SCEV *RoundUp = getAddExpr(Step, NegOne);
@ -5768,7 +5775,16 @@ ScalarEvolution::HowManyLessThans(const SCEV *LHS, const SCEV *RHS,
// The maximum backedge count is similar, except using the minimum start // The maximum backedge count is similar, except using the minimum start
// value and the maximum end value. // value and the maximum end value.
const SCEV *MaxBECount = getBECount(MinStart, MaxEnd, Step, NoWrap); // If we already have an exact constant BECount, use it instead.
const SCEV *MaxBECount = isa<SCEVConstant>(BECount) ? BECount
: getBECount(MinStart, MaxEnd, Step, NoWrap);
// If the stride is nonconstant, and NoWrap == true, then
// getBECount(MinStart, MaxEnd) may not compute. This would result in an
// exact BECount and invalid MaxBECount, which should be avoided to catch
// more optimization opportunities.
if (isa<SCEVCouldNotCompute>(MaxBECount))
MaxBECount = BECount;
return BackedgeTakenInfo(BECount, MaxBECount); return BackedgeTakenInfo(BECount, MaxBECount);
} }

34
test/Analysis/ScalarEvolution/2011-03-09-ExactNoMaxBECount.ll Normal file
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@ -0,0 +1,34 @@
; RUN: opt -indvars %s
; PR9424: Attempt to use a SCEVCouldNotCompute object!
; The inner loop computes the Step and Start of the outer loop.
; Call that Vexit. The outer End value is max(2,Vexit), because
; the condition "icmp %4 < 2" does not guard the outer loop.
; SCEV knows that Vexit has range [2,4], so End == Vexit == Start.
; Now we have ExactBECount == 0. However, MinStart == 2 and MaxEnd == 4.
; Since the stride is variable and may wrap, we cannot compute
; MaxBECount. SCEV should override MaxBECount with ExactBECount.
define void @bar() nounwind {
entry:
%. = select i1 undef, i32 2, i32 1
br label %"5.preheader"
"4": ; preds = %"5.preheader", %"4"
%0 = phi i32 [ 0, %"5.preheader" ], [ %1, %"4" ]
%1 = add nsw i32 %0, 1
%2 = icmp sgt i32 %., %1
br i1 %2, label %"4", label %"9"
"9": ; preds = %"4"
%3 = add i32 %6, 1
%4 = add i32 %3, %1
%5 = icmp slt i32 %4, 2
br i1 %5, label %"5.preheader", label %return
"5.preheader": ; preds = %"9", %entry
%6 = phi i32 [ 0, %entry ], [ %4, %"9" ]
br label %"4"
return: ; preds = %"9"
ret void
}