[Mesa-dev] [PATCH 15/16] i965/fs: Add support for removing MOV.NZ instructions.

[Matt Turner]

On Tue, Jan 20, 2015 at 3:17 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
>
>
> On Tue, Jan 20, 2015 at 3:09 PM, Matt Turner <mattst88 at gmail.com> wrote:
>>
>> On Tue, Jan 20, 2015 at 2:58 PM, Jason Ekstrand <jason at jlekstrand.net>
>> wrote:
>> > On Mon, Jan 19, 2015 at 3:31 PM, Matt Turner <mattst88 at gmail.com> wrote:
>> >>
>> >> For some reason, we occasionally write the flag register with a MOV.NZ
>> >> instruction:
>> >>
>> >>    add(8)          g25<1>F         -g6<0,1,0>F     g15<8,8,1>F
>> >>    cmp.l.f0(8)     g26<1>D         g25<8,8,1>F     0F
>> >>    mov.nz.f0(8)    null            g26<8,8,1>D
>> >>
>> >> A MOV.NZ instruction on the result of a CMP is like comparing for
>> >> equality with true in C. It's useless. Removing it allows us to
>> >> generate:
>> >>
>> >>    add.l.f0(8)     null            -g6<0,1,0>F     g15<8,8,1>F
>> >>
>> >> total instructions in shared programs: 5955701 -> 5951657 (-0.07%)
>> >> instructions in affected programs:     302910 -> 298866 (-1.34%)
>> >> GAINED:                                1
>> >> LOST:                                  0
>> >> ---
>> >>  .../drivers/dri/i965/brw_fs_cmod_propagation.cpp   | 23
>> >> ++++++++++++++--
>> >>  .../drivers/dri/i965/test_fs_cmod_propagation.cpp  | 32
>> >> ++++++++++++++++++++++
>> >>  2 files changed, 52 insertions(+), 3 deletions(-)
>> >>
>> >> diff --git a/src/mesa/drivers/dri/i965/brw_fs_cmod_propagation.cpp
>> >> b/src/mesa/drivers/dri/i965/brw_fs_cmod_propagation.cpp
>> >> index b521350..dd89512 100644
>> >> --- a/src/mesa/drivers/dri/i965/brw_fs_cmod_propagation.cpp
>> >> +++ b/src/mesa/drivers/dri/i965/brw_fs_cmod_propagation.cpp
>> >> @@ -57,12 +57,20 @@ opt_cmod_propagation_local(fs_visitor *v, bblock_t
>> >> *block)
>> >>     foreach_inst_in_block_reverse_safe(fs_inst, inst, block) {
>> >>        ip--;
>> >>
>> >> -      if (inst->opcode != BRW_OPCODE_CMP ||
>> >> +      if ((inst->opcode != BRW_OPCODE_CMP &&
>> >> +           inst->opcode != BRW_OPCODE_MOV) ||
>> >>            inst->predicate != BRW_PREDICATE_NONE ||
>> >>            !inst->dst.is_null() ||
>> >>            inst->src[0].file != GRF ||
>> >> -          inst->src[0].abs ||
>> >> -          !inst->src[1].is_zero())
>> >> +          inst->src[0].abs)
>> >> +         continue;
>> >> +
>> >> +      if (inst->opcode == BRW_OPCODE_CMP && !inst->src[1].is_zero())
>> >> +         continue;
>> >> +
>> >> +      if (inst->opcode == BRW_OPCODE_MOV &&
>> >> +          (inst->conditional_mod != BRW_CONDITIONAL_NZ ||
>> >> +           inst->src[0].negate))
>> >
>> >
>> > I think negate is ok here.  I'm not 100% sure on the symantics of
>> > move.nz,
>> > but if it's a "!= 0" then negation shouldn't matter.  If it only
>> > considers
>> > the bottom bit then negation shouldn't matter there either.
>>
>> The instruction "mov.nz.f0 null src0" sets f0 if src0 != 0.
>>
>> Hmm, you're right. Since we're only allowing NZ conditional modifiers
>> we can also allow negation. I don't think we'll ever generate that,
>> but okay. I'll remove the inst->src[0].negate check.
>
>
> Sure we will.  When we do older gens in NIR, we'll emit one of those after
> every cmp.  Still have to deal with the and though...

Emitting it after every comparison isn't what you want. We emit it
from resolve_bool_comparison() before we need the integer
representation of a bool for things like b2f. NIR -> FS should behave
the same way.