[Mesa-dev] [PATCH] i965/vec4: Reduce working set size of live variables computation.

[Paul Berry]

On 29 October 2013 00:28, Eric Anholt <eric at anholt.net> wrote:

> Paul Berry <stereotype441 at gmail.com> writes:
>
> > On 21 October 2013 11:20, Eric Anholt <eric at anholt.net> wrote:
> >
> >> Orbital Explorer was generating a 4000 instruction geometry shader,
> which
> >> was taking 275 trips through dead code elimination and register
> >> coalescing, each of which updated live variables to get its work done,
> and
> >> invalidated those live variables afterwards.
> >>
> >> By using bitfields instead of bools (reducing the working set size by a
> >> factor of 8) in live variables analysis, it drops from 88% of the
> profile
> >> to 57%, and reduces overall runtime from I-got-bored-and-killed-it (Paul
> >> says 3+ minutes) to 10.5 seconds.
> >>
> >> Compare to f179f419d1d0a03fad36c2b0a58e8b853bae6118 on the FS side.
> >> ---
> >>  .../drivers/dri/i965/brw_vec4_live_variables.cpp   | 41
> >> ++++++++++++----------
> >>  .../drivers/dri/i965/brw_vec4_live_variables.h     | 10 +++---
> >>  2 files changed, 28 insertions(+), 23 deletions(-)
> >>
> >> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_live_variables.cpp
> >> b/src/mesa/drivers/dri/i965/brw_vec4_live_variables.cpp
> >> index db3787b..f6675c8 100644
> >> --- a/src/mesa/drivers/dri/i965/brw_vec4_live_variables.cpp
> >> +++ b/src/mesa/drivers/dri/i965/brw_vec4_live_variables.cpp
> >> @@ -83,8 +83,8 @@ vec4_live_variables::setup_def_use()
> >>
> >>                 for (int j = 0; j < 4; j++) {
> >>                    int c = BRW_GET_SWZ(inst->src[i].swizzle, j);
> >> -                  if (!bd[b].def[reg * 4 + c])
> >> -                     bd[b].use[reg * 4 + c] = true;
> >> +                  if (!BITSET_TEST(bd[b].def, reg * 4 + c))
> >> +                     BITSET_SET(bd[b].use, reg * 4 + c);
> >>                 }
> >>             }
> >>          }
> >> @@ -99,8 +99,8 @@ vec4_live_variables::setup_def_use()
> >>              for (int c = 0; c < 4; c++) {
> >>                 if (inst->dst.writemask & (1 << c)) {
> >>                    int reg = inst->dst.reg;
> >> -                  if (!bd[b].use[reg * 4 + c])
> >> -                     bd[b].def[reg * 4 + c] = true;
> >> +                  if (!BITSET_TEST(bd[b].use, reg * 4 + c))
> >> +                     BITSET_SET(bd[b].def, reg * 4 + c);
> >>                 }
> >>              }
> >>           }
> >> @@ -126,12 +126,12 @@ vec4_live_variables::compute_live_variables()
> >>
> >>        for (int b = 0; b < cfg->num_blocks; b++) {
> >>          /* Update livein */
> >> -        for (int i = 0; i < num_vars; i++) {
> >> -           if (bd[b].use[i] || (bd[b].liveout[i] && !bd[b].def[i])) {
> >> -              if (!bd[b].livein[i]) {
> >> -                 bd[b].livein[i] = true;
> >> -                 cont = true;
> >> -              }
> >> +        for (int i = 0; i < bitset_words; i++) {
> >> +            BITSET_WORD new_livein = (bd[b].use[i] |
> >> +                                      (bd[b].liveout[i] &
> ~bd[b].def[i]));
> >> +            if (new_livein & ~bd[b].livein[i]) {
> >> +               bd[b].livein[i] |= new_livein;
> >> +               cont = true;
> >>
> >
> > Personally, I think this would be slightly clearer as:
> >
> > BITSET_WORD new_livein = bd[b].livein[i] | bd[b].use[i] |
> (bd[b].liveout[i]
> > & ~bd[b].def[i]);
> > if (new_livein != bd[b].livein[i]) {
> >    bd[b].livein[i] = new_livein;
> >    cont = true;
> > }
> >
> >
> >>             }
> >>          }
> >>
> >> @@ -140,9 +140,11 @@ vec4_live_variables::compute_live_variables()
> >>             bblock_link *link = (bblock_link *)block_node;
> >>             bblock_t *block = link->block;
> >>
> >> -           for (int i = 0; i < num_vars; i++) {
> >> -              if (bd[block->block_num].livein[i] && !bd[b].liveout[i])
> {
> >> -                 bd[b].liveout[i] = true;
> >> +           for (int i = 0; i < bitset_words; i++) {
> >> +               BITSET_WORD new_liveout =
> (bd[block->block_num].livein[i] &
> >> +                                          ~bd[b].liveout[i]);
> >> +               if (new_liveout) {
> >> +                  bd[b].liveout[i] |= new_liveout;
> >>                   cont = true;
> >>                }
> >>
> >
> > Similarly, here I think it would be clearer to do:
> >
> > BITSET_WORD new_liveout = bd[block->block_num].livein[i];
> > if (new_liveout != bd[b].liveout[i]) {
> >    bd[b].liveout[i] |= new_liveout;
> >    cont = true;
> > }
>
> I don't think this second block is correct -- you're missing a
> "bd[b].liveout | " in setting up new_liveout.  I'm leaving it as is to
> keep it matching the FS side for now, at least.
>

Yeah, you're right.  I changed my approach halfway through writing it and
failed to update it properly.  FTR, what I was going for was:

BITSET_WORD new_liveout = bd[b].liveout[i] | bd[block->block_num].livein[i];
if (new_liveout != bd[b].liveout[i]) {
   bd[b].liveout[i] = new_liveout;
   cont = true;
}

I'm fine leaving it as is, though.


>
> > Either way, the patch is:
> >
> > Reviewed-by: Paul Berry <stereotype441 at gmail.com>
>
>
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