2 * Copyright © 2014 Intel Corporation
4 * Permission is hereby granted, free of charge, to any person obtaining a
5 * copy of this software and associated documentation files (the "Software"),
6 * to deal in the Software without restriction, including without limitation
7 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
8 * and/or sell copies of the Software, and to permit persons to whom the
9 * Software is furnished to do so, subject to the following conditions:
11 * The above copyright notice and this permission notice (including the next
12 * paragraph) shall be included in all copies or substantial portions of the
15 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
16 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
17 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
18 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
19 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
20 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
24 * Jason Ekstrand (jason@jlekstrand.net)
29 #include "nir_instr_set.h"
32 * Implements Global Code Motion. A description of GCM can be found in
33 * "Global Code Motion; Global Value Numbering" by Cliff Click.
34 * Unfortunately, the algorithm presented in the paper is broken in a
35 * number of ways. The algorithm used here differs substantially from the
36 * one in the paper but it is, in my opinion, much easier to read and
40 struct gcm_block_info
{
41 /* Number of loops this block is inside */
44 /* The last instruction inserted into this block. This is used as we
45 * traverse the instructions and insert them back into the program to
46 * put them in the right order.
48 nir_instr
*last_instr
;
51 struct gcm_instr_info
{
52 nir_block
*early_block
;
55 /* Flags used in the instr->pass_flags field for various instruction states */
57 GCM_INSTR_PINNED
= (1 << 0),
58 GCM_INSTR_SCHEDULE_EARLIER_ONLY
= (1 << 1),
59 GCM_INSTR_SCHEDULED_EARLY
= (1 << 2),
60 GCM_INSTR_SCHEDULED_LATE
= (1 << 3),
61 GCM_INSTR_PLACED
= (1 << 4),
65 nir_function_impl
*impl
;
70 /* The list of non-pinned instructions. As we do the late scheduling,
71 * we pull non-pinned instructions out of their blocks and place them in
72 * this list. This saves us from having linked-list problems when we go
73 * to put instructions back in their blocks.
75 struct exec_list instrs
;
77 struct gcm_block_info
*blocks
;
80 struct gcm_instr_info
*instr_infos
;
83 /* Recursively walks the CFG and builds the block_info structure */
85 gcm_build_block_info(struct exec_list
*cf_list
, struct gcm_state
*state
,
88 foreach_list_typed(nir_cf_node
, node
, node
, cf_list
) {
90 case nir_cf_node_block
: {
91 nir_block
*block
= nir_cf_node_as_block(node
);
92 state
->blocks
[block
->index
].loop_depth
= loop_depth
;
95 case nir_cf_node_if
: {
96 nir_if
*if_stmt
= nir_cf_node_as_if(node
);
97 gcm_build_block_info(&if_stmt
->then_list
, state
, loop_depth
);
98 gcm_build_block_info(&if_stmt
->else_list
, state
, loop_depth
);
101 case nir_cf_node_loop
: {
102 nir_loop
*loop
= nir_cf_node_as_loop(node
);
103 gcm_build_block_info(&loop
->body
, state
, loop_depth
+ 1);
107 unreachable("Invalid CF node type");
112 /* Walks the instruction list and marks immovable instructions as pinned
114 * This function also serves to initialize the instr->pass_flags field.
115 * After this is completed, all instructions' pass_flags fields will be set
116 * to either GCM_INSTR_PINNED or 0.
119 gcm_pin_instructions(nir_function_impl
*impl
, struct gcm_state
*state
)
121 state
->num_instrs
= 0;
123 nir_foreach_block(block
, impl
) {
124 nir_foreach_instr_safe(instr
, block
) {
125 /* Index the instructions for use in gcm_state::instrs */
126 instr
->index
= state
->num_instrs
++;
128 switch (instr
->type
) {
129 case nir_instr_type_alu
:
130 switch (nir_instr_as_alu(instr
)->op
) {
133 case nir_op_fddx_fine
:
134 case nir_op_fddy_fine
:
135 case nir_op_fddx_coarse
:
136 case nir_op_fddy_coarse
:
137 /* These can only go in uniform control flow */
138 instr
->pass_flags
= GCM_INSTR_SCHEDULE_EARLIER_ONLY
;
142 instr
->pass_flags
= 0;
147 case nir_instr_type_tex
:
148 if (nir_tex_instr_has_implicit_derivative(nir_instr_as_tex(instr
)))
149 instr
->pass_flags
= GCM_INSTR_SCHEDULE_EARLIER_ONLY
;
152 case nir_instr_type_deref
:
153 case nir_instr_type_load_const
:
154 instr
->pass_flags
= 0;
157 case nir_instr_type_intrinsic
: {
158 if (nir_intrinsic_can_reorder(nir_instr_as_intrinsic(instr
))) {
159 instr
->pass_flags
= 0;
161 instr
->pass_flags
= GCM_INSTR_PINNED
;
166 case nir_instr_type_jump
:
167 case nir_instr_type_ssa_undef
:
168 case nir_instr_type_phi
:
169 instr
->pass_flags
= GCM_INSTR_PINNED
;
173 unreachable("Invalid instruction type in GCM");
176 if (!(instr
->pass_flags
& GCM_INSTR_PINNED
)) {
177 /* If this is an unpinned instruction, go ahead and pull it out of
178 * the program and put it on the instrs list. This has a couple
179 * of benifits. First, it makes the scheduling algorithm more
180 * efficient because we can avoid walking over basic blocks and
181 * pinned instructions. Second, it keeps us from causing linked
182 * list confusion when we're trying to put everything in its
183 * proper place at the end of the pass.
185 * Note that we don't use nir_instr_remove here because that also
186 * cleans up uses and defs and we want to keep that information.
188 exec_node_remove(&instr
->node
);
189 exec_list_push_tail(&state
->instrs
, &instr
->node
);
196 gcm_schedule_early_instr(nir_instr
*instr
, struct gcm_state
*state
);
198 /** Update an instructions schedule for the given source
200 * This function is called iteratively as we walk the sources of an
201 * instruction. It ensures that the given source instruction has been
202 * scheduled and then update this instruction's block if the source
203 * instruction is lower down the tree.
206 gcm_schedule_early_src(nir_src
*src
, void *void_state
)
208 struct gcm_state
*state
= void_state
;
209 nir_instr
*instr
= state
->instr
;
213 gcm_schedule_early_instr(src
->ssa
->parent_instr
, void_state
);
215 /* While the index isn't a proper dominance depth, it does have the
216 * property that if A dominates B then A->index <= B->index. Since we
217 * know that this instruction must have been dominated by all of its
218 * sources at some point (even if it's gone through value-numbering),
219 * all of the sources must lie on the same branch of the dominance tree.
220 * Therefore, we can just go ahead and just compare indices.
222 struct gcm_instr_info
*src_info
=
223 &state
->instr_infos
[src
->ssa
->parent_instr
->index
];
224 struct gcm_instr_info
*info
= &state
->instr_infos
[instr
->index
];
225 if (info
->early_block
->index
< src_info
->early_block
->index
)
226 info
->early_block
= src_info
->early_block
;
228 /* We need to restore the state instruction because it may have been
229 * changed through the gcm_schedule_early_instr call above. Since we
230 * may still be iterating through sources and future calls to
231 * gcm_schedule_early_src for the same instruction will still need it.
233 state
->instr
= instr
;
238 /** Schedules an instruction early
240 * This function performs a recursive depth-first search starting at the
241 * given instruction and proceeding through the sources to schedule
242 * instructions as early as they can possibly go in the dominance tree.
243 * The instructions are "scheduled" by updating the early_block field of
244 * the corresponding gcm_instr_state entry.
247 gcm_schedule_early_instr(nir_instr
*instr
, struct gcm_state
*state
)
249 if (instr
->pass_flags
& GCM_INSTR_SCHEDULED_EARLY
)
252 instr
->pass_flags
|= GCM_INSTR_SCHEDULED_EARLY
;
254 /* Pinned instructions always get scheduled in their original block so we
255 * don't need to do anything. Also, bailing here keeps us from ever
256 * following the sources of phi nodes which can be back-edges.
258 if (instr
->pass_flags
& GCM_INSTR_PINNED
) {
259 state
->instr_infos
[instr
->index
].early_block
= instr
->block
;
263 /* Start with the instruction at the top. As we iterate over the
264 * sources, it will get moved down as needed.
266 state
->instr_infos
[instr
->index
].early_block
= nir_start_block(state
->impl
);
267 state
->instr
= instr
;
269 nir_foreach_src(instr
, gcm_schedule_early_src
, state
);
273 gcm_choose_block_for_instr(nir_instr
*instr
, nir_block
*early_block
,
274 nir_block
*late_block
, struct gcm_state
*state
)
276 assert(nir_block_dominates(early_block
, late_block
));
278 nir_block
*best
= late_block
;
279 for (nir_block
*block
= late_block
; block
!= NULL
; block
= block
->imm_dom
) {
280 /* Being too aggressive with how we pull instructions out of loops can
281 * result in extra register pressure and spilling. For example its fairly
282 * common for loops in compute shaders to calculate SSBO offsets using
283 * the workgroup id, subgroup id and subgroup invocation, pulling all
284 * these calculations outside the loop causes register pressure.
286 * To work around these issues for now we only allow constant and texture
287 * instructions to be moved outside their original loops.
289 * TODO: figure out some heuristics to allow more to be moved out of loops.
291 if (state
->blocks
[block
->index
].loop_depth
<
292 state
->blocks
[best
->index
].loop_depth
&&
293 (nir_block_dominates(instr
->block
, block
) ||
294 instr
->type
== nir_instr_type_load_const
||
295 instr
->type
== nir_instr_type_tex
))
297 else if (block
== instr
->block
)
300 if (block
== early_block
)
308 gcm_schedule_late_instr(nir_instr
*instr
, struct gcm_state
*state
);
310 /** Schedules the instruction associated with the given SSA def late
312 * This function works by first walking all of the uses of the given SSA
313 * definition, ensuring that they are scheduled, and then computing the LCA
314 * (least common ancestor) of its uses. It then schedules this instruction
315 * as close to the LCA as possible while trying to stay out of loops.
318 gcm_schedule_late_def(nir_ssa_def
*def
, void *void_state
)
320 struct gcm_state
*state
= void_state
;
322 nir_block
*lca
= NULL
;
324 nir_foreach_use(use_src
, def
) {
325 nir_instr
*use_instr
= use_src
->parent_instr
;
327 gcm_schedule_late_instr(use_instr
, state
);
329 /* Phi instructions are a bit special. SSA definitions don't have to
330 * dominate the sources of the phi nodes that use them; instead, they
331 * have to dominate the predecessor block corresponding to the phi
332 * source. We handle this by looking through the sources, finding
333 * any that are usingg this SSA def, and using those blocks instead
334 * of the one the phi lives in.
336 if (use_instr
->type
== nir_instr_type_phi
) {
337 nir_phi_instr
*phi
= nir_instr_as_phi(use_instr
);
339 nir_foreach_phi_src(phi_src
, phi
) {
340 if (phi_src
->src
.ssa
== def
)
341 lca
= nir_dominance_lca(lca
, phi_src
->pred
);
344 lca
= nir_dominance_lca(lca
, use_instr
->block
);
348 nir_foreach_if_use(use_src
, def
) {
349 nir_if
*if_stmt
= use_src
->parent_if
;
351 /* For if statements, we consider the block to be the one immediately
352 * preceding the if CF node.
354 nir_block
*pred_block
=
355 nir_cf_node_as_block(nir_cf_node_prev(&if_stmt
->cf_node
));
357 lca
= nir_dominance_lca(lca
, pred_block
);
360 nir_block
*early_block
=
361 state
->instr_infos
[def
->parent_instr
->index
].early_block
;
363 /* Some instructions may never be used. Flag them and the instruction
364 * placement code will get rid of them for us.
367 def
->parent_instr
->block
= NULL
;
371 if (def
->parent_instr
->pass_flags
& GCM_INSTR_SCHEDULE_EARLIER_ONLY
&&
372 lca
!= def
->parent_instr
->block
&&
373 nir_block_dominates(def
->parent_instr
->block
, lca
)) {
374 lca
= def
->parent_instr
->block
;
377 /* We now have the LCA of all of the uses. If our invariants hold,
378 * this is dominated by the block that we chose when scheduling early.
379 * We now walk up the dominance tree and pick the lowest block that is
380 * as far outside loops as we can get.
382 nir_block
*best_block
=
383 gcm_choose_block_for_instr(def
->parent_instr
, early_block
, lca
, state
);
385 if (def
->parent_instr
->block
!= best_block
)
386 state
->progress
= true;
388 def
->parent_instr
->block
= best_block
;
393 /** Schedules an instruction late
395 * This function performs a depth-first search starting at the given
396 * instruction and proceeding through its uses to schedule instructions as
397 * late as they can reasonably go in the dominance tree. The instructions
398 * are "scheduled" by updating their instr->block field.
400 * The name of this function is actually a bit of a misnomer as it doesn't
401 * schedule them "as late as possible" as the paper implies. Instead, it
402 * first finds the lates possible place it can schedule the instruction and
403 * then possibly schedules it earlier than that. The actual location is as
404 * far down the tree as we can go while trying to stay out of loops.
407 gcm_schedule_late_instr(nir_instr
*instr
, struct gcm_state
*state
)
409 if (instr
->pass_flags
& GCM_INSTR_SCHEDULED_LATE
)
412 instr
->pass_flags
|= GCM_INSTR_SCHEDULED_LATE
;
414 /* Pinned instructions are already scheduled so we don't need to do
415 * anything. Also, bailing here keeps us from ever following phi nodes
416 * which can be back-edges.
418 if (instr
->pass_flags
& GCM_INSTR_PINNED
)
421 nir_foreach_ssa_def(instr
, gcm_schedule_late_def
, state
);
425 gcm_place_instr(nir_instr
*instr
, struct gcm_state
*state
);
428 gcm_place_instr_def(nir_ssa_def
*def
, void *state
)
430 nir_foreach_use(use_src
, def
)
431 gcm_place_instr(use_src
->parent_instr
, state
);
437 gcm_replace_def_with_undef(nir_ssa_def
*def
, void *void_state
)
439 struct gcm_state
*state
= void_state
;
441 if (list_is_empty(&def
->uses
) && list_is_empty(&def
->if_uses
))
444 nir_ssa_undef_instr
*undef
=
445 nir_ssa_undef_instr_create(state
->impl
->function
->shader
,
446 def
->num_components
, def
->bit_size
);
447 nir_instr_insert(nir_before_cf_list(&state
->impl
->body
), &undef
->instr
);
448 nir_ssa_def_rewrite_uses(def
, nir_src_for_ssa(&undef
->def
));
453 /** Places an instrution back into the program
455 * The earlier passes of GCM simply choose blocks for each instruction and
456 * otherwise leave them alone. This pass actually places the instructions
457 * into their chosen blocks.
459 * To do so, we use a standard post-order depth-first search linearization
460 * algorithm. We walk over the uses of the given instruction and ensure
461 * that they are placed and then place this instruction. Because we are
462 * working on multiple blocks at a time, we keep track of the last inserted
463 * instruction per-block in the state structure's block_info array. When
464 * we insert an instruction in a block we insert it before the last
465 * instruction inserted in that block rather than the last instruction
469 gcm_place_instr(nir_instr
*instr
, struct gcm_state
*state
)
471 if (instr
->pass_flags
& GCM_INSTR_PLACED
)
474 instr
->pass_flags
|= GCM_INSTR_PLACED
;
476 if (instr
->block
== NULL
) {
477 nir_foreach_ssa_def(instr
, gcm_replace_def_with_undef
, state
);
478 nir_instr_remove(instr
);
482 /* Phi nodes are our once source of back-edges. Since right now we are
483 * only doing scheduling within blocks, we don't need to worry about
484 * them since they are always at the top. Just skip them completely.
486 if (instr
->type
== nir_instr_type_phi
) {
487 assert(instr
->pass_flags
& GCM_INSTR_PINNED
);
491 nir_foreach_ssa_def(instr
, gcm_place_instr_def
, state
);
493 if (instr
->pass_flags
& GCM_INSTR_PINNED
) {
494 /* Pinned instructions have an implicit dependence on the pinned
495 * instructions that come after them in the block. Since the pinned
496 * instructions will naturally "chain" together, we only need to
497 * explicitly visit one of them.
499 for (nir_instr
*after
= nir_instr_next(instr
);
501 after
= nir_instr_next(after
)) {
502 if (after
->pass_flags
& GCM_INSTR_PINNED
) {
503 gcm_place_instr(after
, state
);
509 struct gcm_block_info
*block_info
= &state
->blocks
[instr
->block
->index
];
510 if (!(instr
->pass_flags
& GCM_INSTR_PINNED
)) {
511 exec_node_remove(&instr
->node
);
513 if (block_info
->last_instr
) {
514 exec_node_insert_node_before(&block_info
->last_instr
->node
,
517 /* Schedule it at the end of the block */
518 nir_instr
*jump_instr
= nir_block_last_instr(instr
->block
);
519 if (jump_instr
&& jump_instr
->type
== nir_instr_type_jump
) {
520 exec_node_insert_node_before(&jump_instr
->node
, &instr
->node
);
522 exec_list_push_tail(&instr
->block
->instr_list
, &instr
->node
);
527 block_info
->last_instr
= instr
;
531 opt_gcm_impl(nir_function_impl
*impl
, bool value_number
)
533 nir_metadata_require(impl
, nir_metadata_block_index
|
534 nir_metadata_dominance
);
536 struct gcm_state state
;
540 state
.progress
= false;
541 exec_list_make_empty(&state
.instrs
);
542 state
.blocks
= rzalloc_array(NULL
, struct gcm_block_info
, impl
->num_blocks
);
544 gcm_build_block_info(&impl
->body
, &state
, 0);
546 gcm_pin_instructions(impl
, &state
);
549 rzalloc_array(NULL
, struct gcm_instr_info
, state
.num_instrs
);
552 struct set
*gvn_set
= nir_instr_set_create(NULL
);
553 foreach_list_typed_safe(nir_instr
, instr
, node
, &state
.instrs
) {
554 if (nir_instr_set_add_or_rewrite(gvn_set
, instr
)) {
555 nir_instr_remove(instr
);
556 state
.progress
= true;
559 nir_instr_set_destroy(gvn_set
);
562 foreach_list_typed(nir_instr
, instr
, node
, &state
.instrs
)
563 gcm_schedule_early_instr(instr
, &state
);
565 foreach_list_typed(nir_instr
, instr
, node
, &state
.instrs
)
566 gcm_schedule_late_instr(instr
, &state
);
568 while (!exec_list_is_empty(&state
.instrs
)) {
569 nir_instr
*instr
= exec_node_data(nir_instr
,
570 state
.instrs
.tail_sentinel
.prev
, node
);
571 gcm_place_instr(instr
, &state
);
574 ralloc_free(state
.blocks
);
575 ralloc_free(state
.instr_infos
);
577 nir_metadata_preserve(impl
, nir_metadata_block_index
|
578 nir_metadata_dominance
);
580 return state
.progress
;
584 nir_opt_gcm(nir_shader
*shader
, bool value_number
)
586 bool progress
= false;
588 nir_foreach_function(function
, shader
) {
590 progress
|= opt_gcm_impl(function
->impl
, value_number
);