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InstSimplify: Correct sdiv x / -1

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Determining the bounds of x/ -1 would start off with us dividing it by
INT_MIN.  Suffice to say, this would not work very well.

Instead, handle it upfront by checking for -1 and mapping it to the
range: [INT_MIN + 1, INT_MAX.  This means that the result of our
division can be any value other than INT_MIN.

llvm-svn: 212981
This commit is contained in:
David Majnemer 2014-07-14 20:38:45 +00:00
parent 21e980408c
commit 6e615bab35
2 changed files with 24 additions and 11 deletions
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24
lib/Analysis/InstructionSimplify.cpp
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@ -1958,20 +1958,22 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
Lower = (-Upper) + 1;
}
} else if (match(LHS, m_SDiv(m_Value(), m_ConstantInt(CI2)))) {
// 'sdiv x, CI2' produces [INT_MIN / CI2, INT_MAX / CI2].
APInt IntMin = APInt::getSignedMinValue(Width);
APInt IntMax = APInt::getSignedMaxValue(Width);
APInt Val = CI2->getValue().abs();
if (!Val.isMinValue()) {
APInt Val = CI2->getValue();
if (Val.isAllOnesValue()) {
// 'sdiv x, -1' produces [INT_MIN + 1, INT_MAX]
// where CI2 != -1 and CI2 != 0 and CI2 != 1
Lower = IntMin + 1;
Upper = IntMax + 1;
} else if (Val.countLeadingZeros() < Width - 1) {
// 'sdiv x, CI2' produces [INT_MIN / CI2, INT_MAX / CI2]
// where CI2 != -1 and CI2 != 0 and CI2 != 1
Lower = IntMin.sdiv(Val);
APInt Rem;
APInt::sdivrem(IntMax, Val, Upper, Rem);
// We may need to round the result of the INT_MAX / CI2 calculation up
// if we see that the division was not exact.
if (Rem.isMinValue())
Upper = Upper + 1;
else
Upper = Upper + 2;
Upper = IntMax.sdiv(Val);
if (Lower.sgt(Upper))
std::swap(Lower, Upper);
Upper = Upper + 1;
assert(Upper != Lower && "Upper part of range has wrapped!");
}
} else if (match(LHS, m_LShr(m_Value(), m_ConstantInt(CI2)))) {

11
test/Transforms/InstSimplify/compare.ll
View File

@ -924,3 +924,14 @@ define i1 @icmp_sdiv_pr20288(i64 %a) {
; CHECK-NEXT: [[CMP:%.*]] = icmp ne i64 [[DIV]], 1073741824
; CHECK-NEXT: ret i1 [[CMP]]
}
define i1 @icmp_sdiv_neg1(i64 %a) {
%div = sdiv i64 %call, -1
%cmp = icmp ne i64 %div, 1073741824
ret i1 %cmp
; CHECK-LABEL: @icmp_sdiv_neg1
; CHECK-NEXT: [[DIV:%.*]] = sdiv i64 %a, -1
; CHECK-NEXT: [[CMP:%.*]] = icmp ne i64 [[DIV]], 1073741824
; CHECK-NEXT: ret i1 [[CMP]]
}