2 * Copyright © 2014 Connor Abbott
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 * Connor Abbott (cwabbott0@gmail.com)
29 #include "nir_control_flow.h"
32 * This file implements an optimization that deletes statically
33 * unreachable/dead code. In NIR, one way this can happen if if an if
34 * statement has a constant condition:
40 * We delete the if statement and paste the contents of the always-executed
41 * branch into the surrounding control flow, possibly removing more code if
42 * the branch had a jump at the end.
44 * Another way is that control flow can end in a jump so that code after it
45 * never gets executed. In particular, this can happen after optimizing
54 * We also consider the case where both branches of an if end in a jump, e.g.:
63 * Finally, we also handle removing useless loops, i.e. loops with no side
64 * effects and without any definitions that are used elsewhere. This case is a
65 * little different from the first two in that the code is actually run (it
66 * just never does anything), but there are similar issues with needing to
67 * be careful with restarting after deleting the cf_node (see dead_cf_list())
68 * so this is a convenient place to remove them.
72 remove_after_cf_node(nir_cf_node
*node
)
74 nir_cf_node
*end
= node
;
75 while (!nir_cf_node_is_last(end
))
76 end
= nir_cf_node_next(end
);
79 nir_cf_extract(&list
, nir_after_cf_node(node
), nir_after_cf_node(end
));
84 opt_constant_if(nir_if
*if_stmt
, bool condition
)
86 /* First, we need to remove any phi nodes after the if by rewriting uses to
87 * point to the correct source.
89 nir_block
*after
= nir_cf_node_as_block(nir_cf_node_next(&if_stmt
->cf_node
));
90 nir_block
*last_block
=
91 nir_cf_node_as_block(condition
? nir_if_last_then_node(if_stmt
)
92 : nir_if_last_else_node(if_stmt
));
94 nir_foreach_instr_safe(after
, instr
) {
95 if (instr
->type
!= nir_instr_type_phi
)
98 nir_phi_instr
*phi
= nir_instr_as_phi(instr
);
99 nir_ssa_def
*def
= NULL
;
100 nir_foreach_phi_src(phi
, phi_src
) {
101 if (phi_src
->pred
!= last_block
)
104 assert(phi_src
->src
.is_ssa
);
105 def
= phi_src
->src
.ssa
;
109 assert(phi
->dest
.is_ssa
);
110 nir_ssa_def_rewrite_uses(&phi
->dest
.ssa
, nir_src_for_ssa(def
));
111 nir_instr_remove(instr
);
114 /* The control flow list we're about to paste in may include a jump at the
115 * end, and in that case we have to delete the rest of the control flow
116 * list after the if since it's unreachable and the validator will balk if
120 if (!exec_list_is_empty(&last_block
->instr_list
)) {
121 nir_instr
*last_instr
= nir_block_last_instr(last_block
);
122 if (last_instr
->type
== nir_instr_type_jump
)
123 remove_after_cf_node(&if_stmt
->cf_node
);
126 /* Finally, actually paste in the then or else branch and delete the if. */
127 struct exec_list
*cf_list
= condition
? &if_stmt
->then_list
128 : &if_stmt
->else_list
;
131 nir_cf_extract(&list
, nir_before_cf_list(cf_list
),
132 nir_after_cf_list(cf_list
));
133 nir_cf_reinsert(&list
, nir_after_cf_node(&if_stmt
->cf_node
));
134 nir_cf_node_remove(&if_stmt
->cf_node
);
138 block_has_no_side_effects(nir_block
*block
, void *state
)
142 nir_foreach_instr(block
, instr
) {
143 if (instr
->type
== nir_instr_type_call
)
146 /* Return instructions can cause us to skip over other side-effecting
147 * instructions after the loop, so consider them to have side effects
151 if (instr
->type
== nir_instr_type_jump
&&
152 nir_instr_as_jump(instr
)->type
== nir_jump_return
)
155 if (instr
->type
!= nir_instr_type_intrinsic
)
158 nir_intrinsic_instr
*intrin
= nir_instr_as_intrinsic(instr
);
159 if (!nir_intrinsic_infos
[intrin
->intrinsic
].flags
&
160 NIR_INTRINSIC_CAN_ELIMINATE
)
168 def_not_live_out(nir_ssa_def
*def
, void *state
)
170 nir_block
*after
= state
;
172 return !BITSET_TEST(after
->live_in
, def
->live_index
);
176 * Test if a loop is dead. A loop is dead if:
178 * 1) It has no side effects (i.e. intrinsics which could possibly affect the
179 * state of the program aside from producing an SSA value, indicated by a lack
180 * of NIR_INTRINSIC_CAN_ELIMINATE).
182 * 2) It has no phi nodes after it, since those indicate values inside the
183 * loop being used after the loop.
185 * 3) If there are no phi nodes after the loop, then the only way a value
186 * defined inside the loop can be used outside the loop is if its definition
187 * dominates the block after the loop. If none of the definitions that
188 * dominate the loop exit are used outside the loop, then the loop is dead
189 * and it can be deleted.
193 loop_is_dead(nir_loop
*loop
)
195 nir_block
*before
= nir_cf_node_as_block(nir_cf_node_prev(&loop
->cf_node
));
196 nir_block
*after
= nir_cf_node_as_block(nir_cf_node_next(&loop
->cf_node
));
198 if (!exec_list_is_empty(&after
->instr_list
) &&
199 nir_block_first_instr(after
)->type
== nir_instr_type_phi
)
202 if (!nir_foreach_block_in_cf_node(&loop
->cf_node
, block_has_no_side_effects
,
206 nir_function_impl
*impl
= nir_cf_node_get_function(&loop
->cf_node
);
207 nir_metadata_require(impl
, nir_metadata_live_ssa_defs
|
208 nir_metadata_dominance
);
210 for (nir_block
*cur
= after
->imm_dom
; cur
!= before
; cur
= cur
->imm_dom
) {
211 nir_foreach_instr(cur
, instr
) {
212 if (!nir_foreach_ssa_def(instr
, def_not_live_out
, after
))
221 dead_cf_block(nir_block
*block
)
223 nir_if
*following_if
= nir_block_get_following_if(block
);
225 nir_const_value
*const_value
=
226 nir_src_as_const_value(following_if
->condition
);
231 opt_constant_if(following_if
, const_value
->u
[0] != 0);
235 nir_loop
*following_loop
= nir_block_get_following_loop(block
);
239 if (!loop_is_dead(following_loop
))
242 nir_cf_node_remove(&following_loop
->cf_node
);
247 ends_in_jump(nir_block
*block
)
249 if (exec_list_is_empty(&block
->instr_list
))
252 nir_instr
*instr
= nir_block_last_instr(block
);
253 return instr
->type
== nir_instr_type_jump
;
257 dead_cf_list(struct exec_list
*list
, bool *list_ends_in_jump
)
259 bool progress
= false;
260 *list_ends_in_jump
= false;
262 nir_cf_node
*prev
= NULL
;
264 foreach_list_typed(nir_cf_node
, cur
, node
, list
) {
266 case nir_cf_node_block
: {
267 nir_block
*block
= nir_cf_node_as_block(cur
);
268 if (dead_cf_block(block
)) {
269 /* We just deleted the if or loop after this block, so we may have
270 * deleted the block before or after it -- which one is an
271 * implementation detail. Therefore, to recover the place we were
272 * at, we have to use the previous cf_node.
276 cur
= nir_cf_node_next(prev
);
278 cur
= exec_node_data(nir_cf_node
, exec_list_get_head(list
),
282 block
= nir_cf_node_as_block(cur
);
287 if (ends_in_jump(block
)) {
288 *list_ends_in_jump
= true;
290 if (!exec_node_is_tail_sentinel(cur
->node
.next
)) {
291 remove_after_cf_node(cur
);
299 case nir_cf_node_if
: {
300 nir_if
*if_stmt
= nir_cf_node_as_if(cur
);
301 bool then_ends_in_jump
, else_ends_in_jump
;
302 progress
|= dead_cf_list(&if_stmt
->then_list
, &then_ends_in_jump
);
303 progress
|= dead_cf_list(&if_stmt
->else_list
, &else_ends_in_jump
);
305 if (then_ends_in_jump
&& else_ends_in_jump
) {
306 *list_ends_in_jump
= true;
307 nir_block
*next
= nir_cf_node_as_block(nir_cf_node_next(cur
));
308 if (!exec_list_is_empty(&next
->instr_list
) ||
309 !exec_node_is_tail_sentinel(next
->cf_node
.node
.next
)) {
310 remove_after_cf_node(cur
);
318 case nir_cf_node_loop
: {
319 nir_loop
*loop
= nir_cf_node_as_loop(cur
);
321 progress
|= dead_cf_list(&loop
->body
, &dummy
);
327 unreachable("unknown cf node type");
337 opt_dead_cf_impl(nir_function_impl
*impl
)
340 bool progress
= dead_cf_list(&impl
->body
, &dummy
);
343 nir_metadata_preserve(impl
, nir_metadata_none
);
349 nir_opt_dead_cf(nir_shader
*shader
)
351 bool progress
= false;
353 nir_foreach_overload(shader
, overload
)
355 progress
|= opt_dead_cf_impl(overload
->impl
);