1 /*
2 * Copyright © 2010 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
21 * DEALINGS IN THE SOFTWARE.
22 */
23
24 /**
25 * \file lower_if_to_cond_assign.cpp
26 *
27 * This flattens if-statements to conditional assignments if:
28 *
29 * - the GPU has limited or no flow control support
30 * (controlled by max_depth)
31 *
32 * - small conditional branches are more expensive than conditional assignments
33 * (controlled by min_branch_cost, that's the cost for a branch to be
34 * preserved)
35 *
36 * It can't handle other control flow being inside of its block, such
37 * as calls or loops. Hopefully loop unrolling and inlining will take
38 * care of those.
39 *
40 * Drivers for GPUs with no control flow support should simply call
41 *
42 * lower_if_to_cond_assign(instructions)
43 *
44 * to attempt to flatten all if-statements.
45 *
46 * Some GPUs (such as i965 prior to gen6) do support control flow, but have a
47 * maximum nesting depth N. Drivers for such hardware can call
48 *
49 * lower_if_to_cond_assign(instructions, N)
50 *
51 * to attempt to flatten any if-statements appearing at depth > N.
52 */
53
54 #include "compiler/glsl_types.h"
55 #include "ir.h"
56 #include "util/set.h"
57 #include "util/hash_table.h" /* Needed for the hashing functions */
58 #include "main/macros.h" /* for MAX2 */
59
60 namespace {
61
62 class ir_if_to_cond_assign_visitor : public ir_hierarchical_visitor {
63 public:
ir_if_to_cond_assign_visitor(gl_shader_stage stage,unsigned max_depth,unsigned min_branch_cost)64 ir_if_to_cond_assign_visitor(gl_shader_stage stage,
65 unsigned max_depth,
66 unsigned min_branch_cost)
67 {
68 this->found_unsupported_op = false;
69 this->found_expensive_op = false;
70 this->found_dynamic_arrayref = false;
71 this->is_then = false;
72 this->progress = false;
73 this->stage = stage;
74 this->then_cost = 0;
75 this->else_cost = 0;
76 this->max_depth = max_depth;
77 this->min_branch_cost = min_branch_cost;
78 this->depth = 0;
79
80 this->condition_variables = _mesa_pointer_set_create(NULL);
81 }
82
~ir_if_to_cond_assign_visitor()83 ~ir_if_to_cond_assign_visitor()
84 {
85 _mesa_set_destroy(this->condition_variables, NULL);
86 }
87
88 ir_visitor_status visit_enter(ir_if *);
89 ir_visitor_status visit_leave(ir_if *);
90
91 bool found_unsupported_op;
92 bool found_expensive_op;
93 bool found_dynamic_arrayref;
94 bool is_then;
95 bool progress;
96 gl_shader_stage stage;
97 unsigned then_cost;
98 unsigned else_cost;
99 unsigned min_branch_cost;
100 unsigned max_depth;
101 unsigned depth;
102
103 struct set *condition_variables;
104 };
105
106 } /* anonymous namespace */
107
108 bool
lower_if_to_cond_assign(gl_shader_stage stage,exec_list * instructions,unsigned max_depth,unsigned min_branch_cost)109 lower_if_to_cond_assign(gl_shader_stage stage, exec_list *instructions,
110 unsigned max_depth, unsigned min_branch_cost)
111 {
112 if (max_depth == UINT_MAX)
113 return false;
114
115 ir_if_to_cond_assign_visitor v(stage, max_depth, min_branch_cost);
116
117 visit_list_elements(&v, instructions);
118
119 return v.progress;
120 }
121
122 static void
check_ir_node(ir_instruction * ir,void * data)123 check_ir_node(ir_instruction *ir, void *data)
124 {
125 ir_if_to_cond_assign_visitor *v = (ir_if_to_cond_assign_visitor *)data;
126
127 switch (ir->ir_type) {
128 case ir_type_call:
129 case ir_type_discard:
130 case ir_type_loop:
131 case ir_type_loop_jump:
132 case ir_type_return:
133 case ir_type_emit_vertex:
134 case ir_type_end_primitive:
135 case ir_type_barrier:
136 v->found_unsupported_op = true;
137 break;
138
139 case ir_type_dereference_variable: {
140 ir_variable *var = ir->as_dereference_variable()->variable_referenced();
141
142 /* Lowering branches with TCS output accesses breaks many piglit tests,
143 * so don't touch them for now.
144 */
145 if (v->stage == MESA_SHADER_TESS_CTRL &&
146 var->data.mode == ir_var_shader_out)
147 v->found_unsupported_op = true;
148 break;
149 }
150
151 /* SSBO, images, atomic counters are handled by ir_type_call */
152 case ir_type_texture:
153 v->found_expensive_op = true;
154 break;
155
156 case ir_type_dereference_array: {
157 ir_dereference_array *deref = ir->as_dereference_array();
158
159 if (deref->array_index->ir_type != ir_type_constant)
160 v->found_dynamic_arrayref = true;
161 } /* fall-through */
162 case ir_type_expression:
163 case ir_type_dereference_record:
164 if (v->is_then)
165 v->then_cost++;
166 else
167 v->else_cost++;
168 break;
169
170 default:
171 break;
172 }
173 }
174
175 static void
move_block_to_cond_assign(void * mem_ctx,ir_if * if_ir,ir_rvalue * cond_expr,exec_list * instructions,struct set * set)176 move_block_to_cond_assign(void *mem_ctx,
177 ir_if *if_ir, ir_rvalue *cond_expr,
178 exec_list *instructions,
179 struct set *set)
180 {
181 foreach_in_list_safe(ir_instruction, ir, instructions) {
182 if (ir->ir_type == ir_type_assignment) {
183 ir_assignment *assign = (ir_assignment *)ir;
184
185 if (_mesa_set_search(set, assign) == NULL) {
186 _mesa_set_add(set, assign);
187
188 /* If the LHS of the assignment is a condition variable that was
189 * previously added, insert an additional assignment of false to
190 * the variable.
191 */
192 const bool assign_to_cv =
193 _mesa_set_search(
194 set, assign->lhs->variable_referenced()) != NULL;
195
196 if (!assign->condition) {
197 if (assign_to_cv) {
198 assign->rhs =
199 new(mem_ctx) ir_expression(ir_binop_logic_and,
200 glsl_type::bool_type,
201 cond_expr->clone(mem_ctx, NULL),
202 assign->rhs);
203 } else {
204 assign->condition = cond_expr->clone(mem_ctx, NULL);
205 }
206 } else {
207 assign->condition =
208 new(mem_ctx) ir_expression(ir_binop_logic_and,
209 glsl_type::bool_type,
210 cond_expr->clone(mem_ctx, NULL),
211 assign->condition);
212 }
213 }
214 }
215
216 /* Now, move from the if block to the block surrounding it. */
217 ir->remove();
218 if_ir->insert_before(ir);
219 }
220 }
221
222 ir_visitor_status
visit_enter(ir_if *)223 ir_if_to_cond_assign_visitor::visit_enter(ir_if *)
224 {
225 this->depth++;
226
227 return visit_continue;
228 }
229
230 ir_visitor_status
visit_leave(ir_if * ir)231 ir_if_to_cond_assign_visitor::visit_leave(ir_if *ir)
232 {
233 bool must_lower = this->depth-- > this->max_depth;
234
235 /* Only flatten when beyond the GPU's maximum supported nesting depth. */
236 if (!must_lower && this->min_branch_cost == 0)
237 return visit_continue;
238
239 this->found_unsupported_op = false;
240 this->found_expensive_op = false;
241 this->found_dynamic_arrayref = false;
242 this->then_cost = 0;
243 this->else_cost = 0;
244
245 ir_assignment *assign;
246
247 /* Check that both blocks don't contain anything we can't support. */
248 this->is_then = true;
249 foreach_in_list(ir_instruction, then_ir, &ir->then_instructions) {
250 visit_tree(then_ir, check_ir_node, this);
251 }
252
253 this->is_then = false;
254 foreach_in_list(ir_instruction, else_ir, &ir->else_instructions) {
255 visit_tree(else_ir, check_ir_node, this);
256 }
257
258 if (this->found_unsupported_op)
259 return visit_continue; /* can't handle inner unsupported opcodes */
260
261 /* Skip if the branch cost is high enough or if there's an expensive op.
262 *
263 * Also skip if non-constant array indices were encountered, since those
264 * can be out-of-bounds for a not-taken branch, and so generating an
265 * assignment would be incorrect. In the case of must_lower, it's up to the
266 * backend to deal with any potential fall-out (perhaps by translating the
267 * assignments to hardware-predicated moves).
268 */
269 if (!must_lower &&
270 (this->found_expensive_op ||
271 this->found_dynamic_arrayref ||
272 MAX2(this->then_cost, this->else_cost) >= this->min_branch_cost))
273 return visit_continue;
274
275 void *mem_ctx = ralloc_parent(ir);
276
277 /* Store the condition to a variable. Move all of the instructions from
278 * the then-clause of the if-statement. Use the condition variable as a
279 * condition for all assignments.
280 */
281 ir_variable *const then_var =
282 new(mem_ctx) ir_variable(glsl_type::bool_type,
283 "if_to_cond_assign_then",
284 ir_var_temporary);
285 ir->insert_before(then_var);
286
287 ir_dereference_variable *then_cond =
288 new(mem_ctx) ir_dereference_variable(then_var);
289
290 assign = new(mem_ctx) ir_assignment(then_cond, ir->condition);
291 ir->insert_before(assign);
292
293 move_block_to_cond_assign(mem_ctx, ir, then_cond,
294 &ir->then_instructions,
295 this->condition_variables);
296
297 /* Add the new condition variable to the hash table. This allows us to
298 * find this variable when lowering other (enclosing) if-statements.
299 */
300 _mesa_set_add(this->condition_variables, then_var);
301
302 /* If there are instructions in the else-clause, store the inverse of the
303 * condition to a variable. Move all of the instructions from the
304 * else-clause if the if-statement. Use the (inverse) condition variable
305 * as a condition for all assignments.
306 */
307 if (!ir->else_instructions.is_empty()) {
308 ir_variable *const else_var =
309 new(mem_ctx) ir_variable(glsl_type::bool_type,
310 "if_to_cond_assign_else",
311 ir_var_temporary);
312 ir->insert_before(else_var);
313
314 ir_dereference_variable *else_cond =
315 new(mem_ctx) ir_dereference_variable(else_var);
316
317 ir_rvalue *inverse =
318 new(mem_ctx) ir_expression(ir_unop_logic_not,
319 then_cond->clone(mem_ctx, NULL));
320
321 assign = new(mem_ctx) ir_assignment(else_cond, inverse);
322 ir->insert_before(assign);
323
324 move_block_to_cond_assign(mem_ctx, ir, else_cond,
325 &ir->else_instructions,
326 this->condition_variables);
327
328 /* Add the new condition variable to the hash table. This allows us to
329 * find this variable when lowering other (enclosing) if-statements.
330 */
331 _mesa_set_add(this->condition_variables, else_var);
332 }
333
334 ir->remove();
335
336 this->progress = true;
337
338 return visit_continue;
339 }
340