[llvm] r214011 - [x86] Fix PR20355 (for real). There are many layers to this bug.

[Sanjay Patel]

Hi all -
There's another magic constant division codegen bug in:
http://llvm.org/bugs/show_bug.cgi?id=20494


On Mon, Aug 4, 2014 at 11:48 AM, Quentin Colombet <qcolombet at apple.com>
wrote:

> Ping?
>
> Benjamin, any idea on how we can write a test for the lowering of the low
> part of the MUL_LOHI on x86?
>
> Thanks,
> -Quentin
>
> On Jul 29, 2014, at 9:22 AM, Quentin Colombet <qcolombet at apple.com> wrote:
>
>
> On Jul 28, 2014, at 2:21 PM, Quentin Colombet <qcolombet at apple.com> wrote:
>
>
> On Jul 28, 2014, at 12:58 PM, Chandler Carruth <chandlerc at gmail.com>
> wrote:
>
>
> On Mon, Jul 28, 2014 at 12:46 PM, Quentin Colombet <qcolombet at apple.com>
> wrote:
>
>> I think we still have a bug in the current implementation.
>> See my inline comments.
>>
>
> Seems likely. Can you craft a test case to demonstrate it?
>
>
> I will see if I can come up with something.
>
>
> I haven’t found anything yet. The case I know that was using this path
> (div -> mul) just uses the high part of the result. Thus, the problem does
> not show up.
>
> CC’ed Benjamin, he may have an idea on how to test that.
>
> Cheers,
> -Quentin
>
>
>
>
>>
>> Also, I think it would be nice to update the comments on the masks value
>> instead of removing them.
>> When I reversed engineered this code, it took me ages to figure out what
>> were these magic values.
>>
>
> I'm not sure that's going to help. The problem with comments is that they
> can get out of sync. I also don't think that little endian offsets are
> surprising or need lots of clarification here? The 1 instead of 0 thing you
> point out screams wrong to me at a glance, and I don't think a comment
> would really make it better (or worse, unless it got out of date).
>
>
> Fair enough!
>
> -Quentin
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