[Mesa-dev] [PATCH] nir: add pass to move load_const
Ian Romanick
idr at freedesktop.org
Mon Jun 18 22:36:12 UTC 2018
On 06/11/2018 08:23 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.
>
> This helps reduce register usage in cases where the backend driver
> cannot lower the load_const to a uniform.
>
> v2: use nir_dominance_lca to figure out a better candidate block to
> move the load_const to, when all it's uses are not in the same
> block
>
> Signed-off-by: Rob Clark <robdclark at gmail.com>
I tried enabling this, and the results were a bit different than last
time. A bunch of shaders were hurt for spills and fills... I may end up
tweaking this a bit before we enable it in i965.
Reviewed-by: Ian Romanick <ian.d.romanick at intel.com>
> ---
> Ian, I played around with the following hack on top of this:
> -----------
> diff --git a/src/compiler/nir/nir_move_load_const.c b/src/compiler/nir/nir_move_load_const.c
> index abc53fdce6a..7a84cce2da1 100644
> --- a/src/compiler/nir/nir_move_load_const.c
> +++ b/src/compiler/nir/nir_move_load_const.c
> @@ -54,6 +54,10 @@ get_preferred_block(nir_ssa_def *def)
> nir_instr *instr = use->parent_instr;
> nir_block *use_block = instr->block;
>
> + // XXX hack:
> + if (use_block->imm_dom && exec_list_length(&use_block->instr_list) < 3)
> + use_block = use_block->imm_dom;
> +
> /*
> * Kind of an ugly special-case, but phi instructions
> * need to appear first in the block, so by definition
> -----------
>
> I just guessed that threshold at random. But it does seem to help
> improve the "total cycles in shared programs".. although my shaderdb
> numbers don't quite match yours (I guess I have an older copy of the
> closed db?)
>
> However I guess it is probably best to leave it to the backend compiler
> to move load_const to an earlier block when needed to hide latency.
>
> 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 | 141 +++++++++++++++++++++++++
> 4 files changed, 144 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 d629c2b8ec8..252cbed0d26 100644
> --- a/src/compiler/Makefile.sources
> +++ b/src/compiler/Makefile.sources
> @@ -254,6 +254,7 @@ 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 598c68aff9f..2b5da68c78b 100644
> --- a/src/compiler/nir/meson.build
> +++ b/src/compiler/nir/meson.build
> @@ -145,6 +145,7 @@ 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 bb477742dc6..2d620454796 100644
> --- a/src/compiler/nir/nir.h
> +++ b/src/compiler/nir/nir.h
> @@ -2630,6 +2630,7 @@ 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..abc53fdce6a
> --- /dev/null
> +++ b/src/compiler/nir/nir_move_load_const.c
> @@ -0,0 +1,141 @@
> +/*
> + * 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 try to find a more optimal block to
> + * move it to, using the dominance tree. In short, if all of the uses
> + * are contained in a single block, the load will be moved there,
> + * otherwise it will be move to the least common ancestor block of all
> + * the uses
> + */
> +static nir_block *
> +get_preferred_block(nir_ssa_def *def)
> +{
> + nir_block *lca = 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;
> + nir_block *use_block = instr->block;
> +
> + /*
> + * 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) {
> + nir_phi_instr *phi = nir_instr_as_phi(instr);
> + nir_block *phi_lca = NULL;
> + nir_foreach_phi_src(src, phi)
> + phi_lca = nir_dominance_lca(phi_lca, src->pred);
> + use_block = phi_lca;
> + }
> +
> + lca = nir_dominance_lca(lca, use_block);
> + }
> +
> + return lca;
> +}
> +
> +/* insert before first non-phi instruction: */
> +static void
> +insert_after_phi(nir_instr *instr, nir_block *block)
> +{
> + nir_foreach_instr(instr2, block) {
> + if (instr2->type == nir_instr_type_phi)
> + continue;
> +
> + exec_node_insert_node_before(&instr2->node,
> + &instr->node);
> +
> + return;
> + }
> +
> + /* if haven't inserted it, push to tail (ie. empty block or possibly
> + * a block only containing phi's?)
> + */
> + exec_list_push_tail(&block->instr_list, &instr->node);
> +}
> +
> +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_metadata_require(function->impl,
> + nir_metadata_block_index | nir_metadata_dominance);
> +
> + 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_preferred_block(&load->def);
> +
> + if (!use_block)
> + continue;
> +
> + if (use_block == load->instr.block)
> + continue;
> +
> + exec_node_remove(&load->instr.node);
> +
> + insert_after_phi(&load->instr, use_block);
> +
> + load->instr.block = use_block;
> +
> + progress = true;
> + }
> + }
> +
> + nir_metadata_preserve(function->impl,
> + nir_metadata_block_index | nir_metadata_dominance);
> + }
> +
> + return progress;
> +}
>
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