[Mesa-dev] [PATCH 10/16] i965/fs: Eliminate null-dst instructions without side-effects.

[Matt Turner]

On Tue, Jan 20, 2015 at 12:53 AM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> On Monday, January 19, 2015 03:31:09 PM Matt Turner wrote:
>> ---
>>  src/mesa/drivers/dri/i965/brw_fs_dead_code_eliminate.cpp | 11 +++++++++++
>>  1 file changed, 11 insertions(+)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_fs_dead_code_eliminate.cpp b/src/mesa/drivers/dri/i965/brw_fs_dead_code_eliminate.cpp
>> index 81be4de..d66808b 100644
>> --- a/src/mesa/drivers/dri/i965/brw_fs_dead_code_eliminate.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_fs_dead_code_eliminate.cpp
>> @@ -85,6 +85,17 @@ fs_visitor::dead_code_eliminate()
>>              }
>>           }
>>
>> +         if ((inst->opcode != BRW_OPCODE_IF &&
>> +              inst->opcode != BRW_OPCODE_WHILE) &&
>> +             inst->dst.is_null() &&
>> +             !inst->has_side_effects() &&
>> +             !inst->writes_flag() &&
>> +             !inst->writes_accumulator) {
>> +            inst->opcode = BRW_OPCODE_NOP;
>> +            progress = true;
>> +            continue;
>> +         }
>> +
>>           if (inst->dst.file == GRF) {
>>              if (!inst->is_partial_write()) {
>>                 int var = live_intervals->var_from_reg(inst->dst);
>>
>
> Seems like these should be handled too...
>
> - BRW_OPCODE_ELSE
> - FS_OPCODE_DISCARD_JUMP
> - FS_OPCODE_PLACEHOLDER_HALT
> - SHADER_OPCODE_SHADER_TIME_ADD

These have BAD_FILE destinations.

> - SHADER_OPCODE_GEN4_SCRATCH_READ
> - SHADER_OPCODE_GEN4_SCRATCH_WRITE
> - SHADER_OPCODE_GEN7_SCRATCH_READ

The READs have non-null destinations (they have to return the data somewhere).

And we only emit SCRATCH_* from spilling registers as part of register
allocation. We can't ever call dead code elimination after we've
assigned registers. (Not only do we not, but it couldn't work)

> Maybe some of these should be added to has_side_effects()?
>
> I'm kind of surprised you didn't see regressions in Piglit...maybe I'm
> missing something.