1 /*
2 * Copyright © 2018 Intel Corporation
3 *
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:
10 *
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
13 * Software.
14 *
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
21 * IN THE SOFTWARE.
22 */
23
24 #include "nir.h"
25 #include "nir_builder.h"
26 #include "nir_deref.h"
27
28 #include "util/u_dynarray.h"
29
30 /**
31 * Elimination of dead writes based on derefs.
32 *
33 * Dead writes are stores and copies that write to a deref, which then gets
34 * another write before it was used (read or sourced for a copy). Those
35 * writes can be removed since they don't affect anything.
36 *
37 * For derefs that refer to a memory area that can be read after the program,
38 * the last write is considered used. The presence of certain instructions
39 * may also cause writes to be considered used, e.g. memory barrier (in this case
40 * the value must be written as other thread might use it).
41 *
42 * The write mask for store instructions is considered, so it is possible that
43 * a store is removed because of the combination of other stores overwritten
44 * its value.
45 */
46
47 /* Entry for unused_writes arrays. */
48 struct write_entry {
49 /* If NULL indicates the entry is free to be reused. */
50 nir_intrinsic_instr *intrin;
51 nir_component_mask_t mask;
52 nir_deref_instr *dst;
53 };
54
55 static void
clear_unused_for_modes(struct util_dynarray * unused_writes,nir_variable_mode modes)56 clear_unused_for_modes(struct util_dynarray *unused_writes, nir_variable_mode modes)
57 {
58 util_dynarray_foreach_reverse(unused_writes, struct write_entry, entry) {
59 if (nir_deref_mode_may_be(entry->dst, modes))
60 *entry = util_dynarray_pop(unused_writes, struct write_entry);
61 }
62 }
63
64 static void
clear_unused_for_read(struct util_dynarray * unused_writes,nir_deref_instr * src)65 clear_unused_for_read(struct util_dynarray *unused_writes, nir_deref_instr *src)
66 {
67 util_dynarray_foreach_reverse(unused_writes, struct write_entry, entry) {
68 if (nir_compare_derefs(src, entry->dst) & nir_derefs_may_alias_bit)
69 *entry = util_dynarray_pop(unused_writes, struct write_entry);
70 }
71 }
72
73 static bool
update_unused_writes(struct util_dynarray * unused_writes,nir_intrinsic_instr * intrin,nir_deref_instr * dst,nir_component_mask_t mask)74 update_unused_writes(struct util_dynarray *unused_writes,
75 nir_intrinsic_instr *intrin,
76 nir_deref_instr *dst, nir_component_mask_t mask)
77 {
78 bool progress = false;
79
80 /* This pass assumes that destination of copies and stores are derefs that
81 * end in a vector or scalar (it is OK to have wildcards or indirects for
82 * arrays).
83 */
84 assert(glsl_type_is_vector_or_scalar(dst->type));
85
86 /* Find writes that are unused and can be removed. */
87 util_dynarray_foreach_reverse(unused_writes, struct write_entry, entry) {
88 nir_deref_compare_result comp = nir_compare_derefs(dst, entry->dst);
89 if (comp & nir_derefs_a_contains_b_bit) {
90 entry->mask &= ~mask;
91 if (entry->mask == 0) {
92 nir_instr_remove(&entry->intrin->instr);
93 *entry = util_dynarray_pop(unused_writes, struct write_entry);
94 progress = true;
95 }
96 }
97 }
98
99 /* Add the new write to the unused array. */
100 struct write_entry new_entry = {
101 .intrin = intrin,
102 .mask = mask,
103 .dst = dst,
104 };
105
106 util_dynarray_append(unused_writes, struct write_entry, new_entry);
107
108 return progress;
109 }
110
111 static bool
remove_dead_write_vars_local(void * mem_ctx,nir_shader * shader,nir_block * block)112 remove_dead_write_vars_local(void *mem_ctx, nir_shader *shader, nir_block *block)
113 {
114 bool progress = false;
115
116 struct util_dynarray unused_writes;
117 util_dynarray_init(&unused_writes, mem_ctx);
118
119 nir_foreach_instr_safe(instr, block) {
120 if (instr->type == nir_instr_type_call) {
121 clear_unused_for_modes(&unused_writes, nir_var_shader_out |
122 nir_var_shader_temp |
123 nir_var_function_temp |
124 nir_var_mem_ssbo |
125 nir_var_mem_shared |
126 nir_var_mem_global);
127 continue;
128 }
129
130 if (instr->type != nir_instr_type_intrinsic)
131 continue;
132
133 nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
134 switch (intrin->intrinsic) {
135 case nir_intrinsic_barrier: {
136 if (nir_intrinsic_memory_semantics(intrin) & NIR_MEMORY_RELEASE) {
137 clear_unused_for_modes(&unused_writes,
138 nir_intrinsic_memory_modes(intrin));
139 }
140 break;
141 }
142
143 case nir_intrinsic_emit_vertex:
144 case nir_intrinsic_emit_vertex_with_counter: {
145 clear_unused_for_modes(&unused_writes, nir_var_shader_out);
146 break;
147 }
148
149 case nir_intrinsic_execute_callable:
150 case nir_intrinsic_rt_execute_callable: {
151 /* Mark payload as it can be used by the callee */
152 nir_deref_instr *src = nir_src_as_deref(intrin->src[1]);
153 clear_unused_for_read(&unused_writes, src);
154 break;
155 }
156
157 case nir_intrinsic_trace_ray:
158 case nir_intrinsic_rt_trace_ray: {
159 /* Mark payload as it can be used by the callees */
160 nir_deref_instr *src = nir_src_as_deref(intrin->src[10]);
161 clear_unused_for_read(&unused_writes, src);
162 break;
163 }
164
165 case nir_intrinsic_load_deref: {
166 nir_deref_instr *src = nir_src_as_deref(intrin->src[0]);
167 if (nir_deref_mode_must_be(src, nir_var_read_only_modes))
168 break;
169 clear_unused_for_read(&unused_writes, src);
170 break;
171 }
172
173 case nir_intrinsic_store_deref: {
174 nir_deref_instr *dst = nir_src_as_deref(intrin->src[0]);
175
176 if (nir_intrinsic_access(intrin) & ACCESS_VOLATILE) {
177 /* Consider a volatile write to also be a sort of read. This
178 * prevents us from deleting a non-volatile write just before a
179 * volatile write thanks to a non-volatile write afterwards. It's
180 * quite the corner case, but this should be safer and more
181 * predictable for the programmer than allowing two non-volatile
182 * writes to be combined with a volatile write between them.
183 */
184 clear_unused_for_read(&unused_writes, dst);
185 break;
186 }
187
188 nir_component_mask_t mask = nir_intrinsic_write_mask(intrin);
189 progress |= update_unused_writes(&unused_writes, intrin, dst, mask);
190 break;
191 }
192
193 case nir_intrinsic_copy_deref: {
194 nir_deref_instr *src = nir_src_as_deref(intrin->src[1]);
195 nir_deref_instr *dst = nir_src_as_deref(intrin->src[0]);
196
197 if (nir_intrinsic_dst_access(intrin) & ACCESS_VOLATILE) {
198 clear_unused_for_read(&unused_writes, src);
199 clear_unused_for_read(&unused_writes, dst);
200 break;
201 }
202
203 /* Self-copy is removed. */
204 if (nir_compare_derefs(src, dst) & nir_derefs_equal_bit) {
205 nir_instr_remove(instr);
206 progress = true;
207 break;
208 }
209
210 clear_unused_for_read(&unused_writes, src);
211 nir_component_mask_t mask = (1 << glsl_get_vector_elements(dst->type)) - 1;
212 progress |= update_unused_writes(&unused_writes, intrin, dst, mask);
213 break;
214 }
215
216 default:
217 break;
218 }
219 }
220
221 /* All unused writes at the end of the block are kept, since we can't be
222 * sure they'll be overwritten or not with local analysis only.
223 */
224
225 return progress;
226 }
227
228 static bool
remove_dead_write_vars_impl(void * mem_ctx,nir_shader * shader,nir_function_impl * impl)229 remove_dead_write_vars_impl(void *mem_ctx, nir_shader *shader, nir_function_impl *impl)
230 {
231 bool progress = false;
232
233 nir_metadata_require(impl, nir_metadata_block_index);
234
235 nir_foreach_block(block, impl)
236 progress |= remove_dead_write_vars_local(mem_ctx, shader, block);
237
238 if (progress) {
239 nir_metadata_preserve(impl, nir_metadata_control_flow);
240 } else {
241 nir_metadata_preserve(impl, nir_metadata_all);
242 }
243
244 return progress;
245 }
246
247 bool
nir_opt_dead_write_vars(nir_shader * shader)248 nir_opt_dead_write_vars(nir_shader *shader)
249 {
250 void *mem_ctx = ralloc_context(NULL);
251 bool progress = false;
252
253 nir_foreach_function_impl(impl, shader) {
254 progress |= remove_dead_write_vars_impl(mem_ctx, shader, impl);
255 }
256
257 ralloc_free(mem_ctx);
258 return progress;
259 }
260