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Don't try to analyze this "backward" case. This is overly conservative

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pending a correct solution.

llvm-svn: 61589
This commit is contained in:
Nick Lewycky 2009-01-02 18:54:17 +00:00
parent 7d77c2fac7
commit 2c01a8db3d
2 changed files with 52 additions and 0 deletions
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12
lib/Analysis/ScalarEvolution.cpp
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@ -2905,6 +2905,11 @@ bool ScalarEvolutionsImpl::executesAtLeastOnce(const Loop *L, bool isSigned,
return false; return false;
} }
static bool isNegative(SCEVHandle X) {
if (SCEVConstant *C = dyn_cast<SCEVConstant>(X))
return C->getValue()->getValue().isNegative();
}
/// potentialInfiniteLoop - Test whether the loop might jump over the exit value /// potentialInfiniteLoop - Test whether the loop might jump over the exit value
/// due to wrapping around 2^n. /// due to wrapping around 2^n.
bool ScalarEvolutionsImpl::potentialInfiniteLoop(SCEV *Stride, SCEV *RHS, bool ScalarEvolutionsImpl::potentialInfiniteLoop(SCEV *Stride, SCEV *RHS,
@ -2958,6 +2963,13 @@ HowManyLessThans(SCEV *LHS, SCEV *RHS, const Loop *L,
if (potentialInfiniteLoop(Stride, RHS, isSigned, trueWhenEqual)) if (potentialInfiniteLoop(Stride, RHS, isSigned, trueWhenEqual))
return UnknownValue; return UnknownValue;
// We don't handle this correctly at the moment. The problem is that when
// the stride is negative, we're not counting how many times 'less-than' is
// true as we approach it, we're counting how far away we are from wrapping
// around the backside.
if (isSigned && isNegative(Stride))
return UnknownValue;
// We know the LHS is of the form {n,+,s} and the RHS is some loop-invariant // We know the LHS is of the form {n,+,s} and the RHS is some loop-invariant
// m. So, we count the number of iterations in which {n,+,s} < m is true. // m. So, we count the number of iterations in which {n,+,s} < m is true.
// Note that we cannot simply return max(m-n,0)/s because it's not safe to // Note that we cannot simply return max(m-n,0)/s because it's not safe to

40
test/Analysis/ScalarEvolution/2009-01-02-SignedNegativeStride.ll Normal file
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@ -0,0 +1,40 @@
; RUN: llvm-as < %s | opt -analyze -scalar-evolution | not grep {/u -1}
; PR3275
@g_16 = external global i16 ; <i16*> [#uses=3]
@.str = external constant [4 x i8] ; <[4 x i8]*> [#uses=0]
define void @func_15() nounwind {
entry:
%0 = load i16* @g_16, align 2 ; <i16> [#uses=1]
%1 = icmp sgt i16 %0, 0 ; <i1> [#uses=1]
br i1 %1, label %bb2, label %bb.nph
bb.nph: ; preds = %entry
%g_16.promoted = load i16* @g_16 ; <i16> [#uses=1]
br label %bb
bb: ; preds = %bb1, %bb.nph
%g_16.tmp.0 = phi i16 [ %g_16.promoted, %bb.nph ], [ %2, %bb1 ] ; <i16> [#uses=1]
%2 = add i16 %g_16.tmp.0, -1 ; <i16> [#uses=3]
br label %bb1
bb1: ; preds = %bb
%3 = icmp sgt i16 %2, 0 ; <i1> [#uses=1]
br i1 %3, label %bb1.bb2_crit_edge, label %bb
bb1.bb2_crit_edge: ; preds = %bb1
store i16 %2, i16* @g_16
br label %bb2
bb2: ; preds = %bb1.bb2_crit_edge, %entry
br label %return
return: ; preds = %bb2
ret void
}
declare i32 @main() nounwind
declare i32 @printf(i8*, ...) nounwind