On 08/09/2016 07:30 PM, Kenneth Graunke wrote:
> 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 <kenn...@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.
I assume the NIR validation passes double-check that for us? :)
Reviewed-by: Ian Romanick <ian.d.roman...@intel.com>
> + *
> + * 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;
> +}
>
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