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->progress = false;
69 this->stage = stage;
70 this->max_depth = max_depth;
71 this->min_branch_cost = min_branch_cost;
72 this->depth = 0;
73
74 this->condition_variables =
75 _mesa_set_create(NULL, _mesa_hash_pointer,
76 _mesa_key_pointer_equal);
77 }
78
~ir_if_to_cond_assign_visitor()79 ~ir_if_to_cond_assign_visitor()
80 {
81 _mesa_set_destroy(this->condition_variables, NULL);
82 }
83
84 ir_visitor_status visit_enter(ir_if *);
85 ir_visitor_status visit_leave(ir_if *);
86
87 bool found_unsupported_op;
88 bool found_expensive_op;
89 bool found_dynamic_arrayref;
90 bool is_then;
91 bool progress;
92 gl_shader_stage stage;
93 unsigned then_cost;
94 unsigned else_cost;
95 unsigned min_branch_cost;
96 unsigned max_depth;
97 unsigned depth;
98
99 struct set *condition_variables;
100 };
101
102 } /* anonymous namespace */
103
104 bool
lower_if_to_cond_assign(gl_shader_stage stage,exec_list * instructions,unsigned max_depth,unsigned min_branch_cost)105 lower_if_to_cond_assign(gl_shader_stage stage, exec_list *instructions,
106 unsigned max_depth, unsigned min_branch_cost)
107 {
108 if (max_depth == UINT_MAX)
109 return false;
110
111 ir_if_to_cond_assign_visitor v(stage, max_depth, min_branch_cost);
112
113 visit_list_elements(&v, instructions);
114
115 return v.progress;
116 }
117
118 void
check_ir_node(ir_instruction * ir,void * data)119 check_ir_node(ir_instruction *ir, void *data)
120 {
121 ir_if_to_cond_assign_visitor *v = (ir_if_to_cond_assign_visitor *)data;
122
123 switch (ir->ir_type) {
124 case ir_type_call:
125 case ir_type_discard:
126 case ir_type_loop:
127 case ir_type_loop_jump:
128 case ir_type_return:
129 case ir_type_emit_vertex:
130 case ir_type_end_primitive:
131 case ir_type_barrier:
132 v->found_unsupported_op = true;
133 break;
134
135 case ir_type_dereference_variable: {
136 ir_variable *var = ir->as_dereference_variable()->variable_referenced();
137
138 /* Lowering branches with TCS output accesses breaks many piglit tests,
139 * so don't touch them for now.
140 */
141 if (v->stage == MESA_SHADER_TESS_CTRL &&
142 var->data.mode == ir_var_shader_out)
143 v->found_unsupported_op = true;
144 break;
145 }
146
147 /* SSBO, images, atomic counters are handled by ir_type_call */
148 case ir_type_texture:
149 v->found_expensive_op = true;
150 break;
151
152 case ir_type_dereference_array: {
153 ir_dereference_array *deref = ir->as_dereference_array();
154
155 if (deref->array_index->ir_type != ir_type_constant)
156 v->found_dynamic_arrayref = true;
157 } /* fall-through */
158 case ir_type_expression:
159 case ir_type_dereference_record:
160 if (v->is_then)
161 v->then_cost++;
162 else
163 v->else_cost++;
164 break;
165
166 default:
167 break;
168 }
169 }
170
171 void
move_block_to_cond_assign(void * mem_ctx,ir_if * if_ir,ir_rvalue * cond_expr,exec_list * instructions,struct set * set)172 move_block_to_cond_assign(void *mem_ctx,
173 ir_if *if_ir, ir_rvalue *cond_expr,
174 exec_list *instructions,
175 struct set *set)
176 {
177 foreach_in_list_safe(ir_instruction, ir, instructions) {
178 if (ir->ir_type == ir_type_assignment) {
179 ir_assignment *assign = (ir_assignment *)ir;
180
181 if (_mesa_set_search(set, assign) == NULL) {
182 _mesa_set_add(set, assign);
183
184 /* If the LHS of the assignment is a condition variable that was
185 * previously added, insert an additional assignment of false to
186 * the variable.
187 */
188 const bool assign_to_cv =
189 _mesa_set_search(
190 set, assign->lhs->variable_referenced()) != NULL;
191
192 if (!assign->condition) {
193 if (assign_to_cv) {
194 assign->rhs =
195 new(mem_ctx) ir_expression(ir_binop_logic_and,
196 glsl_type::bool_type,
197 cond_expr->clone(mem_ctx, NULL),
198 assign->rhs);
199 } else {
200 assign->condition = cond_expr->clone(mem_ctx, NULL);
201 }
202 } else {
203 assign->condition =
204 new(mem_ctx) ir_expression(ir_binop_logic_and,
205 glsl_type::bool_type,
206 cond_expr->clone(mem_ctx, NULL),
207 assign->condition);
208 }
209 }
210 }
211
212 /* Now, move from the if block to the block surrounding it. */
213 ir->remove();
214 if_ir->insert_before(ir);
215 }
216 }
217
218 ir_visitor_status
visit_enter(ir_if * ir)219 ir_if_to_cond_assign_visitor::visit_enter(ir_if *ir)
220 {
221 (void) ir;
222 this->depth++;
223
224 return visit_continue;
225 }
226
227 ir_visitor_status
visit_leave(ir_if * ir)228 ir_if_to_cond_assign_visitor::visit_leave(ir_if *ir)
229 {
230 bool must_lower = this->depth-- > this->max_depth;
231
232 /* Only flatten when beyond the GPU's maximum supported nesting depth. */
233 if (!must_lower && this->min_branch_cost == 0)
234 return visit_continue;
235
236 this->found_unsupported_op = false;
237 this->found_expensive_op = false;
238 this->found_dynamic_arrayref = false;
239 this->then_cost = 0;
240 this->else_cost = 0;
241
242 ir_assignment *assign;
243
244 /* Check that both blocks don't contain anything we can't support. */
245 this->is_then = true;
246 foreach_in_list(ir_instruction, then_ir, &ir->then_instructions) {
247 visit_tree(then_ir, check_ir_node, this);
248 }
249
250 this->is_then = false;
251 foreach_in_list(ir_instruction, else_ir, &ir->else_instructions) {
252 visit_tree(else_ir, check_ir_node, this);
253 }
254
255 if (this->found_unsupported_op)
256 return visit_continue; /* can't handle inner unsupported opcodes */
257
258 /* Skip if the branch cost is high enough or if there's an expensive op.
259 *
260 * Also skip if non-constant array indices were encountered, since those
261 * can be out-of-bounds for a not-taken branch, and so generating an
262 * assignment would be incorrect. In the case of must_lower, it's up to the
263 * backend to deal with any potential fall-out (perhaps by translating the
264 * assignments to hardware-predicated moves).
265 */
266 if (!must_lower &&
267 (this->found_expensive_op ||
268 this->found_dynamic_arrayref ||
269 MAX2(this->then_cost, this->else_cost) >= this->min_branch_cost))
270 return visit_continue;
271
272 void *mem_ctx = ralloc_parent(ir);
273
274 /* Store the condition to a variable. Move all of the instructions from
275 * the then-clause of the if-statement. Use the condition variable as a
276 * condition for all assignments.
277 */
278 ir_variable *const then_var =
279 new(mem_ctx) ir_variable(glsl_type::bool_type,
280 "if_to_cond_assign_then",
281 ir_var_temporary);
282 ir->insert_before(then_var);
283
284 ir_dereference_variable *then_cond =
285 new(mem_ctx) ir_dereference_variable(then_var);
286
287 assign = new(mem_ctx) ir_assignment(then_cond, ir->condition);
288 ir->insert_before(assign);
289
290 move_block_to_cond_assign(mem_ctx, ir, then_cond,
291 &ir->then_instructions,
292 this->condition_variables);
293
294 /* Add the new condition variable to the hash table. This allows us to
295 * find this variable when lowering other (enclosing) if-statements.
296 */
297 _mesa_set_add(this->condition_variables, then_var);
298
299 /* If there are instructions in the else-clause, store the inverse of the
300 * condition to a variable. Move all of the instructions from the
301 * else-clause if the if-statement. Use the (inverse) condition variable
302 * as a condition for all assignments.
303 */
304 if (!ir->else_instructions.is_empty()) {
305 ir_variable *const else_var =
306 new(mem_ctx) ir_variable(glsl_type::bool_type,
307 "if_to_cond_assign_else",
308 ir_var_temporary);
309 ir->insert_before(else_var);
310
311 ir_dereference_variable *else_cond =
312 new(mem_ctx) ir_dereference_variable(else_var);
313
314 ir_rvalue *inverse =
315 new(mem_ctx) ir_expression(ir_unop_logic_not,
316 then_cond->clone(mem_ctx, NULL));
317
318 assign = new(mem_ctx) ir_assignment(else_cond, inverse);
319 ir->insert_before(assign);
320
321 move_block_to_cond_assign(mem_ctx, ir, else_cond,
322 &ir->else_instructions,
323 this->condition_variables);
324
325 /* Add the new condition variable to the hash table. This allows us to
326 * find this variable when lowering other (enclosing) if-statements.
327 */
328 _mesa_set_add(this->condition_variables, else_var);
329 }
330
331 ir->remove();
332
333 this->progress = true;
334
335 return visit_continue;
336 }
337