[PATCH] Inefficient code generation for 128-bit->256-bit typecast intrinsics (BZ #15712)

[Chandler Carruth]

On Thu, Jul 18, 2013 at 6:11 PM, Craig Topper <craig.topper at gmail.com>wrote:

> Would __builtin_shufflevector(__a, __a, 0, 1, -1, -1)  work?
>

Personally, I would prefer a defined way to produce an undef input in
general... but if folks are worried about exposing such an interface, then
sure, we could just allow the shuffle builtin itself to designate an
"undef" input with goofy indices.


>
>
> On Thu, Jul 18, 2013 at 5:42 PM, Chandler Carruth <chandlerc at google.com>wrote:
>
>>
>> On Thu, Jul 18, 2013 at 5:32 PM, Katya Romanova <
>> Katya_Romanova at playstation.sony.com> wrote:
>>
>>> -  __m128d __zero = _mm_setzero_pd();
>>> -  return __builtin_shufflevector(__a, __zero, 0, 1, 2, 2);
>>> +  return (__m256d)__builtin_ia32_pd256_pd((__v2df)__a);
>>>
>>
>> I think this is the wrong approach.
>>
>> Rather than switching these to use an x86-specific builtin, instead it
>> would be better to provide some generic form to produce an undef input to a
>> shufflevector. That is a generally useful and completely target independent
>> concept.
>>
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>>
>
>
> --
> ~Craig
>
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