Re: [PATCH][RTL ifcvt] PR rtl-optimization/66940: Avoid signed overflow in noce_get_alt_condition

[Richard Biener <richard dot guenther at gmail dot com>]

n Mon, May 23, 2016 at 2:28 PM, Kyrill Tkachov
<kyrylo.tkachov@foss.arm.com> wrote:
>
> On 23/05/16 12:27, Richard Biener wrote:
>>
>> On Mon, May 23, 2016 at 1:17 PM, Kyrill Tkachov
>> <kyrylo.tkachov@foss.arm.com> wrote:
>>>
>>> Hi all,
>>>
>>> In this PR we end up hitting a signed overflow in noce_get_alt_condition
>>> when we try to
>>> increment or decrement a HOST_WIDE_INT that might be HOST_WIDE_INT_MAX or
>>> HOST_WIDE_INT_MIN.
>>>
>>> I've confirmed the overflow by adding an assert before the operation:
>>> gcc_assert (desired_val != HOST_WIDE_INT_MAX);
>>>
>>> This patch fixes those cases by catching the cases when desired_val has
>>> the
>>> extreme
>>> value and avoids the transformation that function is trying to make.
>>>
>>> Bootstrapped and tested on arm, aarch64, x86_64.
>>>
>>> I've added the testcase that I used to trigger the assert mentioned above
>>> as
>>> a compile test,
>>> though I'm not sure how much value it has...
>>>
>>> Ok for trunk?
>>
>> If this isn't also a wrong-code issue (runtime testcase?) then why not
>> perform
>> the operation in unsigned HOST_WIDE_INT instead?
>
>
> This part of the code transforms a comparison "x < CST" to "x <= CST - 1"
> and similar transformations. Fro what I understand the LT,LE,GT,GE RTL
> comparison
> operators operate on signed integers, so I'm not sure how valid it would be
> to do all this on unsigned HOST_WIDE_INT.

But then this is a wrong-code issue and you should see miscompiles
and thus can add a dg-do run testcase instead?

Richard.

> Thanks,
> Kyrill
>
>
>
>> Richard.
>>
>>> Thanks,
>>> Kyrill
>>>
>>> 2016-05-23  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>>>
>>>      PR rtl-optimization/66940
>>>      * ifcvt.c (noce_get_alt_condition): Check that incrementing or
>>>      decrementing desired_val will not overflow before performing these
>>>      operations.
>>>
>>> 2016-05-23  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>>>
>>>      PR rtl-optimization/66940
>>>      * gcc.c-torture/compile/pr66940.c: New test.
>
>