[Mesa-dev] [PATCH 2/2] i965: Make DCE set null destinations on messages with side effects.

[Matt Turner]

On Tue, Jan 17, 2017 at 11:40 AM, Francisco Jerez <currojerez at riseup.net> wrote:
> Kenneth Graunke <kenneth at whitecape.org> writes:
>
>> (Co-authored by Matt Turner.)
>>
>> Image atomics, for example, return a value - but the shader may not
>> want to use it.  We assigned a useless VGRF destination.  This seemed
>> harmless, but it can actually be quite harmful.  The register allocator
>> has to assign that VGRF to a real register.  It may assign the same
>> actual GRF to the destination of an instruction that follows soon after.
>>
>> This results in a write-after-write (WAW) dependency, and stall.
>>
>> A number of "Deus Ex: Mankind Divided" shaders use image atomics, but
>> don't use the return value.  Several of these were hitting WAW stalls
>> for nearly 14,000 (poorly estimated) cycles a pop.  Making dead code
>> elimination null out the destination avoids this issue.
>>
>> This patch cuts one shader's estimated cycles by -98.39%!  Removing the
>> message response should also help with data cluster bandwidth.
>>
>> On Skylake:
>>
>> total instructions in shared programs: 13366907 -> 13363051 (-0.03%)
>> instructions in affected programs: 49635 -> 45779 (-7.77%)
>> helped: 133
>> HURT: 0
>>
>> total cycles in shared programs: 255433388 -> 248081818 (-2.88%)
>> cycles in affected programs: 12370702 -> 5019132 (-59.43%)
>> helped: 100
>> HURT: 24
>>
>> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
>> ---
>>  .../dri/i965/brw_fs_dead_code_eliminate.cpp        | 56 ++++++++++++++++------
>>  1 file changed, 41 insertions(+), 15 deletions(-)
>>
>> 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 930dc733b45..885ae2638a8 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
>> @@ -34,6 +34,43 @@
>>   * yet in the tail end of this block.
>>   */
>>
>> +static bool
>> +can_eliminate(const fs_inst *inst, BITSET_WORD *flag_live)
>> +{
>> +    return inst->opcode != BRW_OPCODE_IF &&
>> +           inst->opcode != BRW_OPCODE_WHILE &&
>
> Isn't this missing a bunch of other control flow instructions that
> shouldn't be eliminated?  What's going on here, are they also being
> DCE'ed?  Shouldn't this be !inst->is_control_flow()?

IF/WHILE were checked specifically because on Sandybridge they can
take a conditional-mod and a null destination.

Other control-flow instructions (or even IF & WHILE without cmod) have
BAD_FILE as a destination. Now that I think about it, I'm not really
sure why if.cmod wouldn't have had a BAD_FILE destination as well...

Since we never emit IF/WHILE with cmod anymore, it doesn't matter what
we do with the code. We should probably just make sure those
instructions have BAD_FILE dest. I'll be happy to clean this up after
these patches go in.