Instcombine turns -(x/INT_MIN) to x/INT_MIN

[llvmbot]

Bugzilla Link	20186
Resolution	FIXED
Resolved on	Jul 02, 2014 01:08
Version	trunk
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@majnemer,@nunoplopes,@regehr,@zygoloid

Extended Description

The transformation of -(X/C) to X/(-C) is invalid if C == INT_MIN.

Specifically, if I have

define i32 @​foo(i32 %x) #​0 {
entry:
%div = sdiv i32 %x, -2147483648
%sub = sub nsw i32 0, %div
ret i32 %sub
}

then opt -instcombine will produce

define i32 @​foo(i32 %x) #​0 {
entry:
%sub = sdiv i32 %x, -2147483648
ret i32 %sub
}

You can observe this with the following test case:

#include <stdio.h>
#include <limits.h>

int foo(int x)
{
return -(x/INT_MIN);
}

int main (void)
{
printf ("%d\n", foo(INT_MIN));
}

This will print -1 or 1, depending on the optimization level.

This appears to be the relevant code:

InstCombineAddSub.cpp:1556
// 0 - (X sdiv C) -> (X sdiv -C)
if (match(Op1, m_SDiv(m_Value(X), m_Constant(C))) &&
match(Op0, m_Zero()))
return BinaryOperator::CreateSDiv(X, ConstantExpr::getNeg(C));

[llvmbot]

assigned to @majnemer

[regehr]

I checked this against a recent LLVM on x86-64 and believe it is a real bug.

[zygoloid]

We should transform X/INT_MIN to X==INT_MIN instead.

[majnemer]

Fixed in r212164.