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Reapply r140979 with fix! We never did get a testcase, but careful review of the 

logic by David Meyer revealed this bug.

llvm-svn: 140992
This commit is contained in:
Nick Lewycky 2011-10-03 07:10:45 +00:00
parent 0038e0632c
commit 3155552461
2 changed files with 43 additions and 4 deletions
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19
llvm/lib/Analysis/ScalarEvolution.cpp
View File

@ -5119,7 +5119,7 @@ SolveQuadraticEquation(const SCEVAddRecExpr *AddRec, ScalarEvolution &SE) {
// Compute the two solutions for the quadratic formula.
// The divisions must be performed as signed divisions.
APInt NegB(-B);
APInt TwoA( A << 1 );
APInt TwoA(A << 1);
if (TwoA.isMinValue()) {
const SCEV *CNC = SE.getCouldNotCompute();
return std::make_pair(CNC, CNC);
@ -5134,7 +5134,7 @@ SolveQuadraticEquation(const SCEVAddRecExpr *AddRec, ScalarEvolution &SE) {
return std::make_pair(SE.getConstant(Solution1),
SE.getConstant(Solution2));
} // end APIntOps namespace
} // end APIntOps namespace
}
/// HowFarToZero - Return the number of times a backedge comparing the specified
@ -5228,8 +5228,19 @@ ScalarEvolution::HowFarToZero(const SCEV *V, const Loop *L) {
// Handle unitary steps, which cannot wraparound.
// 1*N = -Start; -1*N = Start (mod 2^BW), so:
// N = Distance (as unsigned)
if (StepC->getValue()->equalsInt(1) || StepC->getValue()->isAllOnesValue())
return Distance;
if (StepC->getValue()->equalsInt(1) || StepC->getValue()->isAllOnesValue()) {
ConstantRange CR = getUnsignedRange(Start);
const SCEV *MaxBECount;
if (!CountDown && CR.getUnsignedMin().isMinValue())
// When counting up, the worst starting value is 1, not 0.
MaxBECount = CR.getUnsignedMax().isMinValue()
? getConstant(APInt::getMinValue(CR.getBitWidth()))
: getConstant(APInt::getMaxValue(CR.getBitWidth()));
else
MaxBECount = getConstant(CountDown ? CR.getUnsignedMax()
: -CR.getUnsignedMin());
return ExitLimit(Distance, MaxBECount);
}
// 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

28
llvm/test/Analysis/ScalarEvolution/max-trip-count.ll
View File

@ -70,3 +70,31 @@ for.end: ; preds = %for.body, %for.cond
}
declare i32 @printf(i8*, ...)
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