[Mesa-dev] [PATCH 2/2] intel/compiler: Use null destination register for memory fence messages

[Francisco Jerez]

Anuj Phogat <anuj.phogat at gmail.com> writes:

> On Wed, Feb 7, 2018 at 11:14 AM, Kenneth Graunke <kenneth at whitecape.org>
> wrote:
>
>> On Tuesday, February 6, 2018 5:09:10 PM PST Anuj Phogat wrote:
>> > From Message Descriptor section in gfxspecs:
>> >  "Memory fence messages without Commit Enable set do not return
>> >   anything to the thread (response length is 0 and destination
>> >   register is null)."
>> >
>> > It fixes a piglit GPU hang in simulation environment.
>> > Piglit test: arb_shader_image_load_store-shader-mem-barrier
>> >
>> > Signed-off-by: Anuj Phogat <anuj.phogat at gmail.com>
>> > Cc: mesa-stable at lists.freedesktop.org
>> > ---
>> >  src/intel/compiler/brw_eu_emit.c | 8 +++++++-
>> >  1 file changed, 7 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/src/intel/compiler/brw_eu_emit.c
>> b/src/intel/compiler/brw_eu_emit.c
>> > index 1fb9aab51c..c66b813af8 100644
>> > --- a/src/intel/compiler/brw_eu_emit.c
>> > +++ b/src/intel/compiler/brw_eu_emit.c
>> > @@ -3290,7 +3290,13 @@ brw_memory_fence(struct brw_codegen *p,
>> >      */
>> >     insn = next_insn(p, BRW_OPCODE_SEND);
>> >     dst = retype(dst, BRW_REGISTER_TYPE_UW);
>> > -   brw_set_dest(p, insn, dst);
>> > +
>> > +  /* From Message Descriptor section in gfxspecs:
>> > +        "Memory fence messages without Commit Enable set do not return
>> > +         anything to the thread (response length is 0 and destination
>> > +         register is null).
>> > +   */
>> > +   brw_set_dest(p, insn, retype(vec1(brw_null_reg()),
>> BRW_REGISTER_TYPE_UW));
>> >     brw_set_src0(p, insn, dst);
>> >     brw_set_memory_fence_message(p, insn, GEN7_SFID_DATAPORT_DATA_CACHE,
>> >                                  commit_enable);
>> >
>>
>> This seems wrong - it looks like you're doing this for messages with
>> commit enable set, which do return things...
>>
>> ​Right. I've to check for commit_enable to use null or non-null destination
> here. I'll send out a v2.
> ​
>

You can also just drop the patch, unless you want to change the
front-end in addition to stop allocating a destination for memory fences
on HSW-SKL in order to save a small amount of register pressure?

>
>> Cc'ing Curro.
>>
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