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25 * \file opt_constant_propagation.cpp
27 * Tracks assignments of constants to channels of variables, and
28 * usage of those constant channels with direct usage of the constants.
30 * This can lead to constant folding and algebraic optimizations in
31 * those later expressions, while causing no increase in instruction
32 * count (due to constants being generally free to load from a
33 * constant push buffer or as instruction immediate values) and
34 * possibly reducing register pressure.
38 #include "ir_visitor.h"
39 #include "ir_rvalue_visitor.h"
40 #include "ir_basic_block.h"
41 #include "ir_optimization.h"
42 #include "compiler/glsl_types.h"
43 #include "util/hash_table.h"
47 class acp_entry : public exec_node
50 /* override operator new from exec_node */
51 DECLARE_LINEAR_ZALLOC_CXX_OPERATORS(acp_entry)
53 acp_entry(ir_variable *var, unsigned write_mask, ir_constant *constant)
58 this->write_mask = write_mask;
59 this->constant = constant;
60 this->initial_values = write_mask;
63 acp_entry(const acp_entry *src)
66 this->write_mask = src->write_mask;
67 this->constant = src->constant;
68 this->initial_values = src->initial_values;
72 ir_constant *constant;
75 /** Mask of values initially available in the constant. */
76 unsigned initial_values;
80 class kill_entry : public exec_node
83 /* override operator new from exec_node */
84 DECLARE_LINEAR_ZALLOC_CXX_OPERATORS(kill_entry)
86 kill_entry(ir_variable *var, unsigned write_mask)
90 this->write_mask = write_mask;
97 class ir_constant_propagation_visitor : public ir_rvalue_visitor {
99 ir_constant_propagation_visitor()
103 mem_ctx = ralloc_context(0);
104 this->lin_ctx = linear_alloc_parent(this->mem_ctx, 0);
105 this->acp = new(mem_ctx) exec_list;
106 this->kills = _mesa_hash_table_create(mem_ctx, _mesa_hash_pointer,
107 _mesa_key_pointer_equal);
109 ~ir_constant_propagation_visitor()
111 ralloc_free(mem_ctx);
114 virtual ir_visitor_status visit_enter(class ir_loop *);
115 virtual ir_visitor_status visit_enter(class ir_function_signature *);
116 virtual ir_visitor_status visit_enter(class ir_function *);
117 virtual ir_visitor_status visit_leave(class ir_assignment *);
118 virtual ir_visitor_status visit_enter(class ir_call *);
119 virtual ir_visitor_status visit_enter(class ir_if *);
121 void add_constant(ir_assignment *ir);
122 void constant_folding(ir_rvalue **rvalue);
123 void constant_propagation(ir_rvalue **rvalue);
124 void kill(ir_variable *ir, unsigned write_mask);
125 void handle_if_block(exec_list *instructions);
126 void handle_rvalue(ir_rvalue **rvalue);
128 /** List of acp_entry: The available constants to propagate */
132 * Hash table of kill_entry: The masks of variables whose values were
133 * killed in this block.
147 ir_constant_propagation_visitor::constant_folding(ir_rvalue **rvalue)
149 if (this->in_assignee || *rvalue == NULL)
152 if (ir_constant_fold(rvalue))
153 this->progress = true;
155 ir_dereference_variable *var_ref = (*rvalue)->as_dereference_variable();
156 if (var_ref && !var_ref->type->is_array()) {
157 ir_constant *constant = var_ref->constant_expression_value();
160 this->progress = true;
166 ir_constant_propagation_visitor::constant_propagation(ir_rvalue **rvalue) {
168 if (this->in_assignee || !*rvalue)
171 const glsl_type *type = (*rvalue)->type;
172 if (!type->is_scalar() && !type->is_vector())
175 ir_swizzle *swiz = NULL;
176 ir_dereference_variable *deref = (*rvalue)->as_dereference_variable();
178 swiz = (*rvalue)->as_swizzle();
182 deref = swiz->val->as_dereference_variable();
187 ir_constant_data data;
188 memset(&data, 0, sizeof(data));
190 for (unsigned int i = 0; i < type->components(); i++) {
192 acp_entry *found = NULL;
196 case 0: channel = swiz->mask.x; break;
197 case 1: channel = swiz->mask.y; break;
198 case 2: channel = swiz->mask.z; break;
199 case 3: channel = swiz->mask.w; break;
200 default: assert(!"shouldn't be reached"); channel = 0; break;
206 foreach_in_list(acp_entry, entry, this->acp) {
207 if (entry->var == deref->var && entry->write_mask & (1 << channel)) {
217 for (int j = 0; j < 4; j++) {
220 if (found->initial_values & (1 << j))
224 switch (type->base_type) {
225 case GLSL_TYPE_FLOAT:
226 data.f[i] = found->constant->value.f[rhs_channel];
228 case GLSL_TYPE_DOUBLE:
229 data.d[i] = found->constant->value.d[rhs_channel];
232 data.i[i] = found->constant->value.i[rhs_channel];
235 data.u[i] = found->constant->value.u[rhs_channel];
238 data.b[i] = found->constant->value.b[rhs_channel];
240 case GLSL_TYPE_UINT64:
241 data.u64[i] = found->constant->value.u64[rhs_channel];
243 case GLSL_TYPE_INT64:
244 data.i64[i] = found->constant->value.i64[rhs_channel];
247 assert(!"not reached");
252 *rvalue = new(ralloc_parent(deref)) ir_constant(type, &data);
253 this->progress = true;
257 ir_constant_propagation_visitor::handle_rvalue(ir_rvalue **rvalue)
259 constant_propagation(rvalue);
260 constant_folding(rvalue);
264 ir_constant_propagation_visitor::visit_enter(ir_function_signature *ir)
266 /* Treat entry into a function signature as a completely separate
267 * block. Any instructions at global scope will be shuffled into
268 * main() at link time, so they're irrelevant to us.
270 exec_list *orig_acp = this->acp;
271 hash_table *orig_kills = this->kills;
272 bool orig_killed_all = this->killed_all;
274 this->acp = new(mem_ctx) exec_list;
275 this->kills = _mesa_hash_table_create(mem_ctx, _mesa_hash_pointer,
276 _mesa_key_pointer_equal);
277 this->killed_all = false;
279 visit_list_elements(this, &ir->body);
281 this->kills = orig_kills;
282 this->acp = orig_acp;
283 this->killed_all = orig_killed_all;
285 return visit_continue_with_parent;
289 ir_constant_propagation_visitor::visit_leave(ir_assignment *ir)
291 constant_folding(&ir->rhs);
293 if (this->in_assignee)
294 return visit_continue;
296 unsigned kill_mask = ir->write_mask;
297 if (ir->lhs->as_dereference_array()) {
298 /* The LHS of the assignment uses an array indexing operator (e.g. v[i]
299 * = ...;). Since we only try to constant propagate vectors and
300 * scalars, this means that either (a) array indexing is being used to
301 * select a vector component, or (b) the variable in question is neither
302 * a scalar or a vector, so we don't care about it. In the former case,
303 * we want to kill the whole vector, since in general we can't predict
304 * which vector component will be selected by array indexing. In the
305 * latter case, it doesn't matter what we do, so go ahead and kill the
306 * whole variable anyway.
308 * Note that if the array index is constant (e.g. v[2] = ...;), we could
309 * in principle be smarter, but we don't need to, because a future
310 * optimization pass will convert it to a simple assignment with the
315 kill(ir->lhs->variable_referenced(), kill_mask);
319 return visit_continue;
323 ir_constant_propagation_visitor::visit_enter(ir_function *ir)
326 return visit_continue;
330 ir_constant_propagation_visitor::visit_enter(ir_call *ir)
332 /* Do constant propagation on call parameters, but skip any out params */
333 foreach_two_lists(formal_node, &ir->callee->parameters,
334 actual_node, &ir->actual_parameters) {
335 ir_variable *sig_param = (ir_variable *) formal_node;
336 ir_rvalue *param = (ir_rvalue *) actual_node;
337 if (sig_param->data.mode != ir_var_function_out
338 && sig_param->data.mode != ir_var_function_inout) {
339 ir_rvalue *new_param = param;
340 handle_rvalue(&new_param);
341 if (new_param != param)
342 param->replace_with(new_param);
348 /* Since we're unlinked, we don't (necssarily) know the side effects of
349 * this call. So kill all copies.
352 this->killed_all = true;
354 return visit_continue_with_parent;
358 ir_constant_propagation_visitor::handle_if_block(exec_list *instructions)
360 exec_list *orig_acp = this->acp;
361 hash_table *orig_kills = this->kills;
362 bool orig_killed_all = this->killed_all;
364 this->acp = new(mem_ctx) exec_list;
365 this->kills = _mesa_hash_table_create(mem_ctx, _mesa_hash_pointer,
366 _mesa_key_pointer_equal);
367 this->killed_all = false;
369 /* Populate the initial acp with a constant of the original */
370 foreach_in_list(acp_entry, a, orig_acp) {
371 this->acp->push_tail(new(this->lin_ctx) acp_entry(a));
374 visit_list_elements(this, instructions);
376 if (this->killed_all) {
377 orig_acp->make_empty();
380 hash_table *new_kills = this->kills;
381 this->kills = orig_kills;
382 this->acp = orig_acp;
383 this->killed_all = this->killed_all || orig_killed_all;
386 hash_table_foreach(new_kills, htk) {
387 kill_entry *k = (kill_entry *) htk->data;
388 kill(k->var, k->write_mask);
393 ir_constant_propagation_visitor::visit_enter(ir_if *ir)
395 ir->condition->accept(this);
396 handle_rvalue(&ir->condition);
398 handle_if_block(&ir->then_instructions);
399 handle_if_block(&ir->else_instructions);
401 /* handle_if_block() already descended into the children. */
402 return visit_continue_with_parent;
406 ir_constant_propagation_visitor::visit_enter(ir_loop *ir)
408 exec_list *orig_acp = this->acp;
409 hash_table *orig_kills = this->kills;
410 bool orig_killed_all = this->killed_all;
412 /* FINISHME: For now, the initial acp for loops is totally empty.
413 * We could go through once, then go through again with the acp
414 * cloned minus the killed entries after the first run through.
416 this->acp = new(mem_ctx) exec_list;
417 this->kills = _mesa_hash_table_create(mem_ctx, _mesa_hash_pointer,
418 _mesa_key_pointer_equal);
419 this->killed_all = false;
421 visit_list_elements(this, &ir->body_instructions);
423 if (this->killed_all) {
424 orig_acp->make_empty();
427 hash_table *new_kills = this->kills;
428 this->kills = orig_kills;
429 this->acp = orig_acp;
430 this->killed_all = this->killed_all || orig_killed_all;
433 hash_table_foreach(new_kills, htk) {
434 kill_entry *k = (kill_entry *) htk->data;
435 kill(k->var, k->write_mask);
438 /* already descended into the children. */
439 return visit_continue_with_parent;
443 ir_constant_propagation_visitor::kill(ir_variable *var, unsigned write_mask)
447 /* We don't track non-vectors. */
448 if (!var->type->is_vector() && !var->type->is_scalar())
451 /* Remove any entries currently in the ACP for this kill. */
452 foreach_in_list_safe(acp_entry, entry, this->acp) {
453 if (entry->var == var) {
454 entry->write_mask &= ~write_mask;
455 if (entry->write_mask == 0)
460 /* Add this writemask of the variable to the hash table of killed
461 * variables in this block.
463 hash_entry *kill_hash_entry = _mesa_hash_table_search(this->kills, var);
464 if (kill_hash_entry) {
465 kill_entry *entry = (kill_entry *) kill_hash_entry->data;
466 entry->write_mask |= write_mask;
469 /* Not already in the hash table. Make new entry. */
470 _mesa_hash_table_insert(this->kills, var,
471 new(this->lin_ctx) kill_entry(var, write_mask));
475 * Adds an entry to the available constant list if it's a plain assignment
476 * of a variable to a variable.
479 ir_constant_propagation_visitor::add_constant(ir_assignment *ir)
489 ir_dereference_variable *deref = ir->lhs->as_dereference_variable();
490 ir_constant *constant = ir->rhs->as_constant();
492 if (!deref || !constant)
495 /* Only do constant propagation on vectors. Constant matrices,
496 * arrays, or structures would require more work elsewhere.
498 if (!deref->var->type->is_vector() && !deref->var->type->is_scalar())
501 /* We can't do copy propagation on buffer variables, since the underlying
502 * memory storage is shared across multiple threads we can't be sure that
503 * the variable value isn't modified between this assignment and the next
504 * instruction where its value is read.
506 if (deref->var->data.mode == ir_var_shader_storage ||
507 deref->var->data.mode == ir_var_shader_shared)
510 entry = new(this->lin_ctx) acp_entry(deref->var, ir->write_mask, constant);
511 this->acp->push_tail(entry);
514 } /* unnamed namespace */
517 * Does a constant propagation pass on the code present in the instruction stream.
520 do_constant_propagation(exec_list *instructions)
522 ir_constant_propagation_visitor v;
524 visit_list_elements(&v, instructions);