1 /*
2 * Copyright © 2015 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 * Authors:
24 * Jason Ekstrand (jason@jlekstrand.net)
25 *
26 */
27
28 #include "nir.h"
29
30 /*
31 * Implements a pass that lowers vector phi nodes to scalar phi nodes when
32 * we don't think it will hurt anything.
33 */
34
35 struct lower_phis_to_scalar_state {
36 nir_shader *shader;
37 void *mem_ctx;
38 struct exec_list dead_instrs;
39
40 bool lower_all;
41
42 /* Hash table marking which phi nodes are scalarizable. The key is
43 * pointers to phi instructions and the entry is either NULL for not
44 * scalarizable or non-null for scalarizable.
45 */
46 struct hash_table *phi_table;
47 };
48
49 static bool
50 should_lower_phi(nir_phi_instr *phi, struct lower_phis_to_scalar_state *state);
51
52 static bool
is_phi_src_scalarizable(nir_phi_src * src,struct lower_phis_to_scalar_state * state)53 is_phi_src_scalarizable(nir_phi_src *src,
54 struct lower_phis_to_scalar_state *state)
55 {
56 /* Don't know what to do with non-ssa sources */
57 if (!src->src.is_ssa)
58 return false;
59
60 nir_instr *src_instr = src->src.ssa->parent_instr;
61 switch (src_instr->type) {
62 case nir_instr_type_alu: {
63 nir_alu_instr *src_alu = nir_instr_as_alu(src_instr);
64
65 /* ALU operations with output_size == 0 should be scalarized. We
66 * will also see a bunch of vecN operations from scalarizing ALU
67 * operations and, since they can easily be copy-propagated, they
68 * are ok too.
69 */
70 return nir_op_infos[src_alu->op].output_size == 0 ||
71 nir_op_is_vec(src_alu->op);
72 }
73
74 case nir_instr_type_phi:
75 /* A phi is scalarizable if we're going to lower it */
76 return should_lower_phi(nir_instr_as_phi(src_instr), state);
77
78 case nir_instr_type_load_const:
79 /* These are trivially scalarizable */
80 return true;
81
82 case nir_instr_type_ssa_undef:
83 /* The caller of this function is going to OR the results and we don't
84 * want undefs to count so we return false.
85 */
86 return false;
87
88 case nir_instr_type_intrinsic: {
89 nir_intrinsic_instr *src_intrin = nir_instr_as_intrinsic(src_instr);
90
91 switch (src_intrin->intrinsic) {
92 case nir_intrinsic_load_deref: {
93 /* Don't scalarize if we see a load of a local variable because it
94 * might turn into one of the things we can't scalarize.
95 */
96 nir_deref_instr *deref = nir_src_as_deref(src_intrin->src[0]);
97 return !nir_deref_mode_may_be(deref, nir_var_function_temp |
98 nir_var_shader_temp);
99 }
100
101 case nir_intrinsic_interp_deref_at_centroid:
102 case nir_intrinsic_interp_deref_at_sample:
103 case nir_intrinsic_interp_deref_at_offset:
104 case nir_intrinsic_interp_deref_at_vertex:
105 case nir_intrinsic_load_uniform:
106 case nir_intrinsic_load_ubo:
107 case nir_intrinsic_load_ssbo:
108 case nir_intrinsic_load_global:
109 case nir_intrinsic_load_global_constant:
110 case nir_intrinsic_load_input:
111 return true;
112 default:
113 break;
114 }
115 }
116 FALLTHROUGH;
117
118 default:
119 /* We can't scalarize this type of instruction */
120 return false;
121 }
122 }
123
124 /**
125 * Determines if the given phi node should be lowered. The only phi nodes
126 * we will scalarize at the moment are those where all of the sources are
127 * scalarizable, unless lower_all is set.
128 *
129 * The reason for this comes down to coalescing. Since phi sources can't
130 * swizzle, swizzles on phis have to be resolved by inserting a mov right
131 * before the phi. The choice then becomes between movs to pick off
132 * components for a scalar phi or potentially movs to recombine components
133 * for a vector phi. The problem is that the movs generated to pick off
134 * the components are almost uncoalescable. We can't coalesce them in NIR
135 * because we need them to pick off components and we can't coalesce them
136 * in the backend because the source register is a vector and the
137 * destination is a scalar that may be used at other places in the program.
138 * On the other hand, if we have a bunch of scalars going into a vector
139 * phi, the situation is much better. In this case, if the SSA def is
140 * generated in the predecessor block to the corresponding phi source, the
141 * backend code will be an ALU op into a temporary and then a mov into the
142 * given vector component; this move can almost certainly be coalesced
143 * away.
144 */
145 static bool
should_lower_phi(nir_phi_instr * phi,struct lower_phis_to_scalar_state * state)146 should_lower_phi(nir_phi_instr *phi, struct lower_phis_to_scalar_state *state)
147 {
148 /* Already scalar */
149 if (phi->dest.ssa.num_components == 1)
150 return false;
151
152 if (state->lower_all)
153 return true;
154
155 struct hash_entry *entry = _mesa_hash_table_search(state->phi_table, phi);
156 if (entry)
157 return entry->data != NULL;
158
159 /* Insert an entry and mark it as scalarizable for now. That way
160 * we don't recurse forever and a cycle in the dependence graph
161 * won't automatically make us fail to scalarize.
162 */
163 entry = _mesa_hash_table_insert(state->phi_table, phi, (void *)(intptr_t)1);
164
165 bool scalarizable = false;
166
167 nir_foreach_phi_src(src, phi) {
168 /* This loop ignores srcs that are not scalarizable because its likely
169 * still worth copying to temps if another phi source is scalarizable.
170 * This reduces register spilling by a huge amount in the i965 driver for
171 * Deus Ex: MD.
172 */
173 scalarizable = is_phi_src_scalarizable(src, state);
174 if (scalarizable)
175 break;
176 }
177
178 /* The hash table entry for 'phi' may have changed while recursing the
179 * dependence graph, so we need to reset it */
180 entry = _mesa_hash_table_search(state->phi_table, phi);
181 assert(entry);
182
183 entry->data = (void *)(intptr_t)scalarizable;
184
185 return scalarizable;
186 }
187
188 static bool
lower_phis_to_scalar_block(nir_block * block,struct lower_phis_to_scalar_state * state)189 lower_phis_to_scalar_block(nir_block *block,
190 struct lower_phis_to_scalar_state *state)
191 {
192 bool progress = false;
193
194 /* Find the last phi node in the block */
195 nir_phi_instr *last_phi = NULL;
196 nir_foreach_instr(instr, block) {
197 if (instr->type != nir_instr_type_phi)
198 break;
199
200 last_phi = nir_instr_as_phi(instr);
201 }
202
203 /* We have to handle the phi nodes in their own pass due to the way
204 * we're modifying the linked list of instructions.
205 */
206 nir_foreach_instr_safe(instr, block) {
207 if (instr->type != nir_instr_type_phi)
208 break;
209
210 nir_phi_instr *phi = nir_instr_as_phi(instr);
211
212 if (!should_lower_phi(phi, state))
213 continue;
214
215 unsigned bit_size = phi->dest.ssa.bit_size;
216
217 /* Create a vecN operation to combine the results. Most of these
218 * will be redundant, but copy propagation should clean them up for
219 * us. No need to add the complexity here.
220 */
221 nir_op vec_op = nir_op_vec(phi->dest.ssa.num_components);
222
223 nir_alu_instr *vec = nir_alu_instr_create(state->shader, vec_op);
224 nir_ssa_dest_init(&vec->instr, &vec->dest.dest,
225 phi->dest.ssa.num_components,
226 bit_size, NULL);
227 vec->dest.write_mask = (1 << phi->dest.ssa.num_components) - 1;
228
229 for (unsigned i = 0; i < phi->dest.ssa.num_components; i++) {
230 nir_phi_instr *new_phi = nir_phi_instr_create(state->shader);
231 nir_ssa_dest_init(&new_phi->instr, &new_phi->dest, 1,
232 phi->dest.ssa.bit_size, NULL);
233
234 vec->src[i].src = nir_src_for_ssa(&new_phi->dest.ssa);
235
236 nir_foreach_phi_src(src, phi) {
237 /* We need to insert a mov to grab the i'th component of src */
238 nir_alu_instr *mov = nir_alu_instr_create(state->shader,
239 nir_op_mov);
240 nir_ssa_dest_init(&mov->instr, &mov->dest.dest, 1, bit_size, NULL);
241 mov->dest.write_mask = 1;
242 nir_src_copy(&mov->src[0].src, &src->src);
243 mov->src[0].swizzle[0] = i;
244
245 /* Insert at the end of the predecessor but before the jump */
246 nir_instr *pred_last_instr = nir_block_last_instr(src->pred);
247 if (pred_last_instr && pred_last_instr->type == nir_instr_type_jump)
248 nir_instr_insert_before(pred_last_instr, &mov->instr);
249 else
250 nir_instr_insert_after_block(src->pred, &mov->instr);
251
252 nir_phi_instr_add_src(new_phi, src->pred, nir_src_for_ssa(&mov->dest.dest.ssa));
253 }
254
255 nir_instr_insert_before(&phi->instr, &new_phi->instr);
256 }
257
258 nir_instr_insert_after(&last_phi->instr, &vec->instr);
259
260 nir_ssa_def_rewrite_uses(&phi->dest.ssa,
261 &vec->dest.dest.ssa);
262
263 nir_instr_remove(&phi->instr);
264 exec_list_push_tail(&state->dead_instrs, &phi->instr.node);
265
266 progress = true;
267
268 /* We're using the safe iterator and inserting all the newly
269 * scalarized phi nodes before their non-scalarized version so that's
270 * ok. However, we are also inserting vec operations after all of
271 * the last phi node so once we get here, we can't trust even the
272 * safe iterator to stop properly. We have to break manually.
273 */
274 if (instr == &last_phi->instr)
275 break;
276 }
277
278 return progress;
279 }
280
281 static bool
lower_phis_to_scalar_impl(nir_function_impl * impl,bool lower_all)282 lower_phis_to_scalar_impl(nir_function_impl *impl, bool lower_all)
283 {
284 struct lower_phis_to_scalar_state state;
285 bool progress = false;
286
287 state.shader = impl->function->shader;
288 state.mem_ctx = ralloc_parent(impl);
289 exec_list_make_empty(&state.dead_instrs);
290 state.phi_table = _mesa_pointer_hash_table_create(NULL);
291 state.lower_all = lower_all;
292
293 nir_foreach_block(block, impl) {
294 progress = lower_phis_to_scalar_block(block, &state) || progress;
295 }
296
297 nir_metadata_preserve(impl, nir_metadata_block_index |
298 nir_metadata_dominance);
299
300 nir_instr_free_list(&state.dead_instrs);
301
302 ralloc_free(state.phi_table);
303
304 return progress;
305 }
306
307 /** A pass that lowers vector phi nodes to scalar
308 *
309 * This pass loops through the blocks and lowers looks for vector phi nodes
310 * it can lower to scalar phi nodes. Not all phi nodes are lowered. For
311 * instance, if one of the sources is a non-scalarizable vector, then we
312 * don't bother lowering because that would generate hard-to-coalesce movs.
313 */
314 bool
nir_lower_phis_to_scalar(nir_shader * shader,bool lower_all)315 nir_lower_phis_to_scalar(nir_shader *shader, bool lower_all)
316 {
317 bool progress = false;
318
319 nir_foreach_function(function, shader) {
320 if (function->impl)
321 progress = lower_phis_to_scalar_impl(function->impl, lower_all) || progress;
322 }
323
324 return progress;
325 }
326