[Mesa-dev] [PATCH 2/2] i965/skl: Disable SIMD16 when 3-source instructions are used

[Neil Roberts]

Ilia Mirkin <imirkin at alum.mit.edu> writes:

> On Wed, Mar 4, 2015 at 9:33 AM, Neil Roberts <neil at linux.intel.com> wrote:
>> Stepping C0 of Skylake fails when using SIMD16 with 3-source
>> instructions (such as MAD). This patch just makes it disable SIMD16 in
>> that case.
>>
>> This implements WaDisableSIMD16On3SrcInstr and fixes ~190 Piglit
>> tests.
>
> Just curious -- is this faster than lowering MAD into MUL + ADD but
> staying in SIMD16? (Should be fairly easy to implement as a post-RA
> fixup I'd think, although not totally familiar with how the i965
> compiler works.)

I haven't tested it but I think you're right that it would probably be
faster to lower the instruction. However that sounds like more work and
it's only relevant for a limited set of hardware so it doesn't seem
worth doing. I think there are also other 3-source instructions and it
might be more difficult to lower those (although I'm not sure).

Regards,
- Neil