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

[Matt Turner]

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?