Division by INT64_MIN

[Matthew Fernandez]

On 09/10/15 02:08, Joseph Myers wrote:
> On Thu, 8 Oct 2015, Matthew Fernandez wrote:
>
>> On 06/10/15 01:09, Joseph Myers wrote:
>>> On Mon, 5 Oct 2015, Matthew Fernandez wrote:
>>>
>>>> on x86 is emitted as a call to __divdi3. If the numerator or denominator
>>>> are
>>>> negative, __divdi3 negates them. If either of these values is INT64_MIN, I
>>>> believe this negation is undefined. Is this correct? If this is the case,
>>>> then
>>>> it seems code like "INT64_MIN / INT64_MIN" which should be perfectly legal
>>>> accidentally causes undefined behaviour via libgcc. In practice,
>>>> everything
>>>> seems to work as expected, but it seems to me that the C code of __divdi3
>>>> should not be relying on these negations working consistently.
>>>
>>> It sounds like some casts to UDWtype should be inserted so the negation
>>> takes place on an unsigned type.
>>
>> Thanks for the speedy response, Joseph. Should I open a new bug report about
>> this? I suspect there are GCC developers out there who would disagree that
>> this constitutes a bug.
>
> If you don't intend to send a patch, I suggest opening a bug report (but I
> expect it's a bug only in the source code, not in the libgcc binaries).

OK, created as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67902.

In my own implementation I just special cased all the tricky pairs of operands, which worked fine as I'm not doing any 
64-bit divisions in performance-critical sections. However, I'm hesitant to submit something like this as a patch 
because I suspect the performance degradation would be unpalatable to others.