[llvm] r214628 - [x86] Largely complete the use of PSHUFB in the new vector shuffle

[Chandler Carruth]

On Thu, Aug 14, 2014 at 4:07 AM, Chandler Carruth <chandlerc at gmail.com>
wrote:

> On Thu, Aug 14, 2014 at 3:45 AM, Patrik Hägglund H <
> patrik.h.hagglund at ericsson.com> wrote:
>
>> Hi Chandler,
>>
>> Ping!
>>
>
> Sorry, I got caught up in other bug chasing activities and forgot about
> this one. I'll try to get to it if you don't sooner....
>
>
>>
>> I don't know anything about how this code works. Anyhow, I took a quick
>> look at this. It seems like the call to getTargetShuffleMask (in
>> combineX86ShufflesRecursively) returns a Mask with 'undef' (-1) values.
>> However, the code following the call seems to assume that no elements are
>> 'undef'.
>>
>
> That seems... very, very strange. If it isn't a bug in
> getTargetShuffleMask (How does it know they are *undef*???) then I think
> the code following it just needs to be taught to deal with undef masks.
>

Oh, this isn't undef stuff at all.

X86 target shuffles sometimes will force-zero a lane, and we decode that to
a '-1' entry in the *target* shuffle mask. Not to be confused with a '-1'
in the shuffle masks used everywhere else. =/

Anyways, the subsequent code looks *completely and utterly wrong*, both in
the face of this and other issues. I'll rewrite it shortly.
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