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InstSimplify: The upper bound of X / C was missing a rounding step

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Summary:
When calculating the upper bound of X / -8589934592, we would perform
the following calculation: Floor[INT_MAX / 8589934592]

However, flooring the result would make us wrongly come to the
conclusion that 1073741824 was not in the set of possible values.
Instead, use the ceiling of the result.

Reviewers: nicholas

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D4502

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212976 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer 2014-07-14 19:49:57 +00:00
parent 497ba6c273
commit 7ceba3a1b0
2 changed files with 20 additions and 1 deletions
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10
lib/Analysis/InstructionSimplify.cpp
View File

@ -1964,7 +1964,15 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
APInt Val = CI2->getValue().abs();
if (!Val.isMinValue()) {
Lower = IntMin.sdiv(Val);
Upper = IntMax.sdiv(Val) + 1;
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;
assert(Upper != Lower && "Upper part of range has wrapped!");
}
} else if (match(LHS, m_LShr(m_Value(), m_ConstantInt(CI2)))) {
// 'lshr x, CI2' produces [0, UINT_MAX >> CI2].

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

@ -913,3 +913,14 @@ define i1 @icmp_sdiv_int_min(i32 %a) {
; CHECK-NEXT: [[CMP:%.*]] = icmp ne i32 [[DIV]], -1073741824
; CHECK-NEXT: ret i1 [[CMP]]
}
define i1 @icmp_sdiv_pr20288(i64 %a) {
%div = sdiv i64 %a, -8589934592
%cmp = icmp ne i64 %div, 1073741824
ret i1 %cmp
; CHECK-LABEL: @icmp_sdiv_pr20288
; CHECK-NEXT: [[DIV:%.*]] = sdiv i64 %a, -8589934592
; CHECK-NEXT: [[CMP:%.*]] = icmp ne i64 [[DIV]], 1073741824
; CHECK-NEXT: ret i1 [[CMP]]
}