On Wed, Aug 10, 2016 at 11:14 AM, Ian Romanick <i...@freedesktop.org> wrote:
> 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? :)
>
Oh, yes! Not validating that would be baaaaad.
>
> 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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