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Fix for PR2607: SCEV miscomputing the loop count for loops with an 

SGT exit condition.  Essentially, the correct way to flip an inequality 
in 2's complement is the not operator, not the negation operator.  
That said, the difference only affects cases involving INT_MIN.

Also, enhance the pre-test search logic to be a bit smarter about 
inequalities flipped with a not operator, so it can eliminate the smax 
from the iteration count for simple loops.

llvm-svn: 54184
This commit is contained in:
Eli Friedman 2008-07-30 00:04:08 +00:00
parent 8cf5045012
commit 5ae90441c4
2 changed files with 34 additions and 3 deletions
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10
llvm/lib/Analysis/ScalarEvolution.cpp
View File

@ -1976,8 +1976,8 @@ SCEVHandle ScalarEvolutionsImpl::ComputeIterationCount(const Loop *L) {
break;
}
case ICmpInst::ICMP_SGT: {
SCEVHandle TC = HowManyLessThans(SE.getNegativeSCEV(LHS),
SE.getNegativeSCEV(RHS), L, true);
SCEVHandle TC = HowManyLessThans(SE.getNotSCEV(LHS),
SE.getNotSCEV(RHS), L, true);
if (!isa<SCEVCouldNotCompute>(TC)) return TC;
break;
}
@ -2724,7 +2724,11 @@ bool ScalarEvolutionsImpl::executesAtLeastOnce(const Loop *L, bool isSigned,
if (!PreCondLHS->getType()->isInteger()) return false;
return LHS == getSCEV(PreCondLHS) && RHS == getSCEV(PreCondRHS);
SCEVHandle PreCondLHSSCEV = getSCEV(PreCondLHS);
SCEVHandle PreCondRHSSCEV = getSCEV(PreCondRHS);
return (LHS == PreCondLHSSCEV && RHS == PreCondRHSSCEV) ||
(LHS == SE.getNotSCEV(PreCondRHSSCEV) &&
RHS == SE.getNotSCEV(PreCondLHSSCEV));
}
/// HowManyLessThans - Return the number of times a backedge containing the

27
llvm/test/Analysis/ScalarEvolution/2008-07-29-SGTTripCount.ll Normal file
View File

@ -0,0 +1,27 @@
; RUN: llvm-as < %s | opt -analyze -scalar-evolution -disable-output \
; RUN: -scalar-evolution-max-iterations=0 | \
; RUN: grep -F "( -1 + ( -1 * %j)) iterations"
; PR2607
define i32 @_Z1aj(i32 %j) nounwind {
entry:
icmp sgt i32 0, %j ; <i1>:0 [#uses=1]
br i1 %0, label %bb.preheader, label %return
bb.preheader: ; preds = %entry
br label %bb
bb: ; preds = %bb, %bb.preheader
%i.01 = phi i32 [ %1, %bb ], [ 0, %bb.preheader ] ; <i32> [#uses=1]
add i32 %i.01, -1 ; <i32>:1 [#uses=3]
icmp sgt i32 %1, %j ; <i1>:2 [#uses=1]
br i1 %2, label %bb, label %return.loopexit
return.loopexit: ; preds = %bb
br label %return
return: ; preds = %return.loopexit, %entry
%i.0.lcssa = phi i32 [ 0, %entry ], [ %1, %return.loopexit ] ; <i32> [#uses=1]
ret i32 %i.0.lcssa
}