[Mesa-dev] [PATCH] nir: add pass to move load_const
Rob Clark
robdclark at gmail.com
Mon Jun 11 15:23:26 UTC 2018
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>
---
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;
+}
--
2.17.0
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