[Mesa-dev] [PATCH 4/6] nir: Introduce a nir_opt_move_comparisons() pass.
Kenneth Graunke
kenneth at whitecape.org
Wed Aug 10 02:30:17 UTC 2016
This tries to move comparisons (a common source of boolean values)
closer to their first use. For GPUs which use condition codes,
this can eliminate a lot of temporary booleans and comparisons
which reload the condition code register based on a boolean.
Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
src/compiler/Makefile.sources | 1 +
src/compiler/nir/nir.h | 2 +
src/compiler/nir/nir_opt_move_comparisons.c | 173 ++++++++++++++++++++++++++++
3 files changed, 176 insertions(+)
create mode 100644 src/compiler/nir/nir_opt_move_comparisons.c
diff --git a/src/compiler/Makefile.sources b/src/compiler/Makefile.sources
index 0ff9b23..008a101 100644
--- a/src/compiler/Makefile.sources
+++ b/src/compiler/Makefile.sources
@@ -226,6 +226,7 @@ NIR_FILES = \
nir/nir_opt_gcm.c \
nir/nir_opt_global_to_local.c \
nir/nir_opt_peephole_select.c \
+ nir/nir_opt_move_comparisons.c \
nir/nir_opt_remove_phis.c \
nir/nir_opt_undef.c \
nir/nir_phi_builder.c \
diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
index 9ce5be2..79511a7 100644
--- a/src/compiler/nir/nir.h
+++ b/src/compiler/nir/nir.h
@@ -2574,6 +2574,8 @@ bool nir_opt_dead_cf(nir_shader *shader);
void nir_opt_gcm(nir_shader *shader);
+bool nir_opt_move_comparisons(nir_shader *shader);
+
bool nir_opt_peephole_select(nir_shader *shader);
bool nir_opt_remove_phis(nir_shader *shader);
diff --git a/src/compiler/nir/nir_opt_move_comparisons.c b/src/compiler/nir/nir_opt_move_comparisons.c
new file mode 100644
index 0000000..74927c9
--- /dev/null
+++ b/src/compiler/nir/nir_opt_move_comparisons.c
@@ -0,0 +1,173 @@
+/*
+ * Copyright © 2016 Intel Corporation
+ *
+ * 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.
+ */
+
+#include "nir.h"
+
+/**
+ * \file nir_opt_move_comparisons.c
+ *
+ * This pass moves ALU comparison operations just before their first use.
+ *
+ * It only moves instructions within a single basic block; cross-block
+ * movement is left to global code motion.
+ *
+ * Many GPUs generate condition codes for comparisons, and use predication
+ * for conditional selects and control flow. In a sequence such as:
+ *
+ * vec1 32 ssa_1 = flt a b
+ * <some other operations>
+ * vec1 32 ssa_2 = bcsel ssa_1 c d
+ *
+ * the backend would likely do the comparison, producing condition codes,
+ * then save those to a boolean value. The intervening operations might
+ * trash the condition codes. Then, in order to do the bcsel, it would
+ * need to re-populate the condition code register based on the boolean.
+ *
+ * By moving the comparison just before the bcsel, the condition codes could
+ * be used directly. This eliminates the need to reload them from the boolean
+ * (generally eliminating an instruction). It may also eliminate the need to
+ * create a boolean value altogether (unless it's used elsewhere), which could
+ * lower register pressure.
+ */
+
+static bool
+is_comparison(nir_op op)
+{
+ switch (op) {
+ case nir_op_flt:
+ case nir_op_fge:
+ case nir_op_feq:
+ case nir_op_fne:
+ case nir_op_ilt:
+ case nir_op_ult:
+ case nir_op_ige:
+ case nir_op_uge:
+ case nir_op_ieq:
+ case nir_op_ine:
+ return true;
+ default:
+ return false;
+ }
+}
+
+static bool
+move_comparison_source(nir_src *src, nir_block *block, struct exec_node *before)
+{
+ if (src->is_ssa && src->ssa->parent_instr->block == block &&
+ src->ssa->parent_instr->type == nir_instr_type_alu &&
+ is_comparison(nir_instr_as_alu(src->ssa->parent_instr)->op)) {
+
+ struct exec_node *src_node = &src->ssa->parent_instr->node;
+ exec_node_remove(src_node);
+
+ if (before)
+ exec_node_insert_node_before(before, src_node);
+ else
+ exec_list_push_tail(&block->instr_list, src_node);
+
+ return true;
+ }
+
+ return false;
+}
+
+/* nir_foreach_src callback boilerplate */
+struct nomc_tuple
+{
+ nir_instr *instr;
+ bool progress;
+};
+
+static bool
+move_comparison_source_cb(nir_src *src, void *data)
+{
+ struct nomc_tuple *tuple = data;
+
+ if (move_comparison_source(src, tuple->instr->block, &tuple->instr->node))
+ tuple->progress = true;
+
+ return true; /* nir_foreach_src should keep going */
+}
+
+static bool
+move_comparisons(nir_block *block)
+{
+ bool progress = false;
+
+ /* We use a simple approach: walk instructions backwards.
+ *
+ * If the instruction's source is a comparison from the same block,
+ * simply move it here. This may break SSA if it's used earlier in
+ * the block as well. However, as we walk backwards, we'll find the
+ * earlier use and move it again, further up. It eventually ends up
+ * dominating all uses again, restoring SSA form.
+ *
+ * Before walking instructions, we consider the if-condition at the
+ * end of the block, if one exists. It's effectively a use at the
+ * bottom of the block.
+ */
+ nir_if *iff = nir_block_get_following_if(block);
+ if (iff) {
+ progress |= move_comparison_source(&iff->condition, block, NULL);
+ }
+
+ nir_foreach_instr_reverse(instr, block) {
+ if (instr->type == nir_instr_type_alu) {
+ /* Walk ALU instruction sources backwards so that bcsel's boolean
+ * condition is processed last.
+ */
+ nir_alu_instr *alu = nir_instr_as_alu(instr);
+ for (int i = nir_op_infos[alu->op].num_inputs - 1; i >= 0; i--) {
+ progress |= move_comparison_source(&alu->src[i].src,
+ block, &instr->node);
+ }
+ } else {
+ struct nomc_tuple tuple = { instr, false };
+ nir_foreach_src(instr, move_comparison_source_cb, &tuple);
+ progress |= tuple.progress;
+ }
+ }
+
+ return progress;
+}
+
+bool
+nir_opt_move_comparisons(nir_shader *shader)
+{
+ bool progress = false;
+
+ nir_foreach_function(func, shader) {
+ if (func->impl) {
+ nir_foreach_block(block, func->impl) {
+ if (move_comparisons(block)) {
+ nir_metadata_preserve(func->impl, nir_metadata_block_index |
+ nir_metadata_dominance |
+ nir_metadata_live_ssa_defs);
+ progress = true;
+ }
+ }
+ }
+ }
+
+ return progress;
+}
--
2.9.0
More information about the mesa-dev
mailing list