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Fix PR2002. Suppose n is the initial value for the induction

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variable (with step 1) and m is its final value. Then, the correct trip 
count is SMAX(m,n)-n. Previously, we used SMAX(0,m-n), but m-n may 
overflow and can't in general be interpreted as signed.

Patch by Nick Lewycky.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47007 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Wojciech Matyjewicz 2008-02-12 15:09:36 +00:00
parent b6c36e4058
commit 7b5b76835a
2 changed files with 20 additions and 6 deletions
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10
lib/Analysis/ScalarEvolution.cpp
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@ -2527,19 +2527,17 @@ HowManyLessThans(SCEV *LHS, SCEV *RHS, const Loop *L, bool isSigned) {
if (AddRec->isAffine()) {
// The number of iterations for "{n,+,1} < m", is m-n. However, we don't
// know that m is >= n on input to the loop. If it is, the condition
// returns true zero times. To handle both cases, we return SMAX(0, m-n).
// returns true zero times. To handle both cases, we return SMAX(m, n)-n.
// FORNOW: We only support unit strides.
SCEVHandle One = SE.getIntegerSCEV(1, RHS->getType());
if (AddRec->getOperand(1) != One)
return UnknownValue;
SCEVHandle Iters = SE.getMinusSCEV(RHS, AddRec->getOperand(0));
SCEVHandle Start = AddRec->getOperand(0);
SCEVHandle End = isSigned ? SE.getSMaxExpr(RHS, Start) : (SCEVHandle)RHS;
if (isSigned)
return SE.getSMaxExpr(SE.getIntegerSCEV(0, RHS->getType()), Iters);
else
return Iters;
return SE.getMinusSCEV(End, Start);
}
return UnknownValue;

16
test/Analysis/ScalarEvolution/2008-02-12-SMAXTripCount.ll Normal file
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@ -0,0 +1,16 @@
; RUN: llvm-as < %s | opt -scalar-evolution -analyze | grep {Loop loop: ( 100 + ( -100 smax %n)) iterations!}
; PR2002
define void @foo(i8 %n) {
entry:
br label %loop
loop:
%i = phi i8 [ -100, %entry ], [ %i.inc, %next ]
%cond = icmp slt i8 %i, %n
br i1 %cond, label %next, label %return
next:
%i.inc = add i8 %i, 1
br label %loop
return:
ret void
}