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

[Quentin Colombet]

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out screams wrong to me at a glance, and I don't think a comment would really make it better (or worse, unless it got out of date).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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