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Teach SCEV to report a max backedge count in one interesting case in

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HowFarToZero; the case for a canonical loop.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@139126 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Nick Lewycky 2011-09-05 23:25:16 +00:00
parent 1fac6b50ea
commit 77b7352db0
2 changed files with 37 additions and 2 deletions
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8
lib/Analysis/ScalarEvolution.cpp
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@ -5184,8 +5184,12 @@ ScalarEvolution::HowFarToZero(const SCEV *V, const Loop *L) {
// Handle unitary steps, which cannot wraparound. // Handle unitary steps, which cannot wraparound.
// 1*N = -Start; -1*N = Start (mod 2^BW), so: // 1*N = -Start; -1*N = Start (mod 2^BW), so:
// N = Distance (as unsigned) // N = Distance (as unsigned)
if (StepC->getValue()->equalsInt(1) || StepC->getValue()->isAllOnesValue()) if (StepC->getValue()->equalsInt(1) || StepC->getValue()->isAllOnesValue()) {
return Distance; ConstantRange CR = getUnsignedRange(Start);
const SCEV *MaxBECount = getConstant(CountDown ? CR.getUnsignedMax()
: ~CR.getUnsignedMin());
return ExitLimit(Distance, MaxBECount);
}
// If the recurrence is known not to wraparound, unsigned divide computes the // If the recurrence is known not to wraparound, unsigned divide computes the
// back edge count. We know that the value will either become zero (and thus // back edge count. We know that the value will either become zero (and thus

31
test/Analysis/ScalarEvolution/max-trip-count.ll
View File

@ -70,3 +70,34 @@ for.end: ; preds = %for.body, %for.cond
} }
declare i32 @printf(i8*, ...) declare i32 @printf(i8*, ...)
; Before -indvars ran on this loop, SCEV solved a max loop trip count of
; 2^31-2. Afterwards, we can only solve 2^32-1.
define void @test(i8* %a, i32 %n) nounwind {
entry:
%cmp1 = icmp sgt i32 %n, 0
br i1 %cmp1, label %for.body.lr.ph, label %for.end
for.body.lr.ph: ; preds = %entry
%tmp = zext i32 %n to i64
br label %for.body
for.body: ; preds = %for.body, %for.body.lr.ph
%indvar = phi i64 [ %indvar.next, %for.body ], [ 0, %for.body.lr.ph ]
%arrayidx = getelementptr i8* %a, i64 %indvar
store i8 0, i8* %arrayidx, align 1
%indvar.next = add i64 %indvar, 1
%exitcond = icmp ne i64 %indvar.next, %tmp
br i1 %exitcond, label %for.body, label %for.cond.for.end_crit_edge
for.cond.for.end_crit_edge: ; preds = %for.body
br label %for.end
for.end: ; preds = %for.cond.for.end_crit_edge, %entry
ret void
}
; CHECK: Determining loop execution counts for: @test
; CHECK-NEXT: backedge-taken count is
; CHECK-NEXT: max backedge-taken count is -1