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

[Francisco Jerez]

Matt Turner <mattst88 at gmail.com> writes:

> 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...
>

Heh, that explains why the code above wasn't getting rid of half the
control flow instructions from the program.  Thanks.

> 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.

I think DCE should just refuse to eliminate any control flow
instructions, it's wrong regardless of what the destination register
file is -- Or do BAD_FILE registers have any implications regarding the
kind of side effects the instructions that carry them may have?  I hope
not.
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