[Mesa-dev] [PATCH 7/9] i965/vec4: avoid dependency control around Align1 instructions

[Connor Abbott]

On Thu, Nov 19, 2015 at 6:40 AM, Matt Turner <mattst88 at gmail.com> wrote:
> On Thu, Nov 19, 2015 at 2:05 AM, Iago Toral Quiroga <itoral at igalia.com> wrote:
>> From: Connor Abbott <connor.w.abbott at intel.com>
>>
>> It appears that not only math instructions, but also MOV_BYTES or
>> any instruction that uses Align1 mode cannot be in the middle
>> of a dependency control sequence or the GPU will hang (at least on my
>> BDW). This fixes GPU hangs in some fp64 tests.
>
> I'm pretty surprised by this assessment. Doubtful even.
>
>> Reviewed-by: Iago Toral Quiroga <itoral at igalia.com>
>> ---
>>  src/mesa/drivers/dri/i965/brw_vec4.cpp | 17 ++++++++++++-----
>>  1 file changed, 12 insertions(+), 5 deletions(-)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_vec4.cpp b/src/mesa/drivers/dri/i965/brw_vec4.cpp
>> index 3bcd5cb..bc0a33b 100644
>> --- a/src/mesa/drivers/dri/i965/brw_vec4.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_vec4.cpp
>> @@ -838,6 +838,17 @@ vec4_visitor::is_dep_ctrl_unsafe(const vec4_instruction *inst)
>>     }
>>
>>     /*
>> +    * Instructions that use Align1 mode cause the GPU to hang when inserted
>> +    * between a NoDDClr and NoDDChk in Align16 mode. Discovered empirically.
>> +    */
>> +
>> +   if (inst->opcode == VEC4_OPCODE_PACK_BYTES ||
>> +       inst->opcode == VEC4_OPCODE_MOV_BYTES ||
>
> PACK_BYTES sets depctrl itself in the generator, and at the time I
> added it I made a test that did
>
>   vec4 foo = vec4(packUnorm4x8(...),
>                   packUnorm4x8(...),
>                   packUnorm4x8(...),
>                   packUnorm4x8(...))
>
> and confirmed that it set depctrl properly on the whole sequence.
> There could of course be bugs somewhere, but the "hardware doesn't
> work if you mix align1 and align16 depctrl" seems unlikely.
>
> Do you know of a test that this affects?

This only affects FP64 tests, since there we use an align1 mov to do
double-to-float and float-to-double. However, I tried commenting out
emit_nir_code() and just doing essentially:

emit(MOV(...))->force_writemask_all = true;
emit(VEC4_OPCODE_PACK_BYTES, ...);
emit(MOV(...))->force_writemask_all = true;

and on my BDW it hanged. In case it's not clear: this isn't about
setting depctrl on the instruction itself, it just can't be inside of
a depctrl sequence (which we were already disallowing for math
instructions anyways).