[Mesa-dev] [PATCH 1/2] nir: add pass to move load_const

[Ian Romanick]

On 06/06/2018 07:43 AM, Rob Clark wrote:
> Run this pass late (after opt loop) to move load_const instructions back
> into the basic blocks which use the result, in cases where a load_const
> is only consumed in a single block.

If the load_const is used in more than one block, you could use it to
the block that dominates all of the block that contain a use.  I suspect
in many cases that will just be the first block, but it's probably worth
trying.

> This helps reduce register usage in cases where the backend driver
> cannot lower the load_const to a uniform.

The trade off is that now you might not be able to hide the latency of
the load.  I tried this on i965, and the results (below) support this
idea a bit.  If you have a structured backend IR, this seems like a
thing better done there... like, if you can't register allocate without
spilling, try moving the load.  But that's really just a scheduling
heuristic.  Hmm...

total instructions in shared programs: 14398410 -> 14397918 (<.01%)
instructions in affected programs: 134856 -> 134364 (-0.36%)
helped: 53
HURT: 10
helped stats (abs) min: 1 max: 30 x̄: 9.47 x̃: 7
helped stats (rel) min: 0.16% max: 2.74% x̄: 0.64% x̃: 0.40%
HURT stats (abs)   min: 1 max: 1 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 0.03% max: 0.21% x̄: 0.05% x̃: 0.03%
95% mean confidence interval for instructions value: -9.61 -6.01
95% mean confidence interval for instructions %-change: -0.68% -0.38%
Instructions are helped.

total cycles in shared programs: 532949823 -> 532851352 (-0.02%)
cycles in affected programs: 260693023 -> 260594552 (-0.04%)
helped: 122
HURT: 88
helped stats (abs) min: 1 max: 24933 x̄: 1080.30 x̃: 20
helped stats (rel) min: <.01% max: 34.94% x̄: 2.20% x̃: 0.32%
HURT stats (abs)   min: 1 max: 1884 x̄: 378.69 x̃: 29
HURT stats (rel)   min: <.01% max: 10.23% x̄: 0.64% x̃: 0.09%
95% mean confidence interval for cycles value: -943.71 5.89
95% mean confidence interval for cycles %-change: -1.71% -0.32%
Inconclusive result (value mean confidence interval includes 0).

total spills in shared programs: 8044 -> 7975 (-0.86%)
spills in affected programs: 2391 -> 2322 (-2.89%)
helped: 42
HURT: 0

total fills in shared programs: 10950 -> 10884 (-0.60%)
fills in affected programs: 2999 -> 2933 (-2.20%)
helped: 42
HURT: 0

LOST:   4
GAINED: 7

> Signed-off-by: Rob Clark <robdclark at gmail.com>
> ---
>  src/compiler/Makefile.sources          |   1 +
>  src/compiler/nir/meson.build           |   1 +
>  src/compiler/nir/nir.h                 |   1 +
>  src/compiler/nir/nir_move_load_const.c | 131 +++++++++++++++++++++++++
>  4 files changed, 134 insertions(+)
>  create mode 100644 src/compiler/nir/nir_move_load_const.c
> 
> diff --git a/src/compiler/Makefile.sources b/src/compiler/Makefile.sources
> index 3daa2c51334..cb8a2875a84 100644
> --- a/src/compiler/Makefile.sources
> +++ b/src/compiler/Makefile.sources
> @@ -253,6 +253,7 @@ NIR_FILES = \
>  	nir/nir_lower_wpos_center.c \
>  	nir/nir_lower_wpos_ytransform.c \
>  	nir/nir_metadata.c \
> +	nir/nir_move_load_const.c \
>  	nir/nir_move_vec_src_uses_to_dest.c \
>  	nir/nir_normalize_cubemap_coords.c \
>  	nir/nir_opt_conditional_discard.c \
> diff --git a/src/compiler/nir/meson.build b/src/compiler/nir/meson.build
> index 3fec363691d..373a47fd155 100644
> --- a/src/compiler/nir/meson.build
> +++ b/src/compiler/nir/meson.build
> @@ -144,6 +144,7 @@ files_libnir = files(
>    'nir_lower_wpos_ytransform.c',
>    'nir_lower_bit_size.c',
>    'nir_metadata.c',
> +  'nir_move_load_const.c',
>    'nir_move_vec_src_uses_to_dest.c',
>    'nir_normalize_cubemap_coords.c',
>    'nir_opt_conditional_discard.c',
> diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
> index 5a1f79515ad..073ab4e82ea 100644
> --- a/src/compiler/nir/nir.h
> +++ b/src/compiler/nir/nir.h
> @@ -2612,6 +2612,7 @@ bool nir_remove_dead_variables(nir_shader *shader, nir_variable_mode modes);
>  bool nir_lower_constant_initializers(nir_shader *shader,
>                                       nir_variable_mode modes);
>  
> +bool nir_move_load_const(nir_shader *shader);
>  bool nir_move_vec_src_uses_to_dest(nir_shader *shader);
>  bool nir_lower_vec_to_movs(nir_shader *shader);
>  void nir_lower_alpha_test(nir_shader *shader, enum compare_func func,
> diff --git a/src/compiler/nir/nir_move_load_const.c b/src/compiler/nir/nir_move_load_const.c
> new file mode 100644
> index 00000000000..c0750035fbb
> --- /dev/null
> +++ b/src/compiler/nir/nir_move_load_const.c
> @@ -0,0 +1,131 @@
> +/*
> + * Copyright © 2018 Red Hat
> + *
> + * Permission is hereby granted, free of charge, to any person obtaining a
> + * copy of this software and associated documentation files (the "Software"),
> + * to deal in the Software without restriction, including without limitation
> + * the rights to use, copy, modify, merge, publish, distribute, sublicense,
> + * and/or sell copies of the Software, and to permit persons to whom the
> + * Software is furnished to do so, subject to the following conditions:
> + *
> + * The above copyright notice and this permission notice (including the next
> + * paragraph) shall be included in all copies or substantial portions of the
> + * Software.
> + *
> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
> + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
> + * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
> + * IN THE SOFTWARE.
> + *
> + * Authors:
> + *    Rob Clark (robdclark at gmail.com>
> + *
> + */
> +
> +#include "nir.h"
> +
> +
> +/*
> + * A simple pass that moves load_const's into consuming block if
> + * they are only consumed in a single block, to try to counter-
> + * act nir's tendency to move all load_const to the top of the
> + * first block.
> + */
> +
> +/* iterate a ssa def's use's and if all uses are within a single
> + * block, return it, otherwise return null.
> + */
> +static nir_block *
> +get_single_block(nir_ssa_def *def)
> +{
> +   nir_block *b = NULL;
> +
> +   /* hmm, probably ignore if-uses: */
> +   if (!list_empty(&def->if_uses))
> +      return NULL;
> +
> +   nir_foreach_use(use, def) {
> +      nir_instr *instr = use->parent_instr;
> +
> +      /*
> +       * Kind of an ugly special-case, but phi instructions
> +       * need to appear first in the block, so by definition
> +       * we can't move a load_immed into a block where it is
> +       * consumed by a phi instruction.  We could conceivably
> +       * move it into a dominator block.
> +       */
> +      if (instr->type == nir_instr_type_phi)
> +         return NULL;
> +
> +      nir_block *use_block = instr->block;
> +
> +      if (!b) {
> +         b = use_block;
> +      } else if (b != use_block) {
> +         return NULL;
> +      }
> +   }
> +
> +   return b;
> +}
> +
> +bool
> +nir_move_load_const(nir_shader *shader)
> +{
> +   bool progress = false;
> +
> +   nir_foreach_function(function, shader) {
> +      if (!function->impl)
> +         continue;
> +
> +      nir_foreach_block(block, function->impl) {
> +         nir_foreach_instr_safe(instr, block) {
> +            if (instr->type != nir_instr_type_load_const)
> +               continue;
> +
> +            nir_load_const_instr *load =
> +                  nir_instr_as_load_const(instr);
> +            nir_block *use_block =
> +                  get_single_block(&load->def);
> +
> +            /* if consumed in more than one block, ignore.  Possibly
> +             * there should be a threshold, ie. if used in a small #
> +             * of blocks in a large shader, it could be better to
> +             * split the load_const.  For now ignore that and focus
> +             * on the clear wins:
> +             */
> +            if (!use_block)
> +               continue;
> +
> +            if (use_block == load->instr.block)
> +               continue;
> +
> +            exec_node_remove(&load->instr.node);
> +
> +            /* insert before first non-phi instruction: */
> +            nir_foreach_instr(instr2, use_block) {
> +               if (instr2->type == nir_instr_type_phi)
> +                  continue;
> +
> +               exec_node_insert_node_before(&instr2->node,
> +                                            &load->instr.node);
> +
> +               break;
> +            }
> +
> +            load->instr.block = use_block;
> +
> +            progress = true;
> +         }
> +      }
> +
> +      nir_metadata_preserve(function->impl,
> +                            nir_metadata_block_index | nir_metadata_dominance);
> +   }
> +
> +
> +   return progress;
> +}
>