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

[Jason Ekstrand]

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...
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