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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 "glsl_types.h"
46 class acp_entry : public exec_node
49 acp_entry(ir_variable *var, unsigned write_mask, ir_constant *constant)
54 this->write_mask = write_mask;
55 this->constant = constant;
56 this->initial_values = write_mask;
59 acp_entry(const acp_entry *src)
62 this->write_mask = src->write_mask;
63 this->constant = src->constant;
64 this->initial_values = src->initial_values;
68 ir_constant *constant;
71 /** Mask of values initially available in the constant. */
72 unsigned initial_values;
76 class kill_entry : public exec_node
79 kill_entry(ir_variable *var, unsigned write_mask)
83 this->write_mask = write_mask;
90 class ir_constant_propagation_visitor : public ir_rvalue_visitor {
92 ir_constant_propagation_visitor()
96 mem_ctx = ralloc_context(0);
97 this->acp = new(mem_ctx) exec_list;
98 this->kills = new(mem_ctx) exec_list;
100 ~ir_constant_propagation_visitor()
102 ralloc_free(mem_ctx);
105 virtual ir_visitor_status visit_enter(class ir_loop *);
106 virtual ir_visitor_status visit_enter(class ir_function_signature *);
107 virtual ir_visitor_status visit_enter(class ir_function *);
108 virtual ir_visitor_status visit_leave(class ir_assignment *);
109 virtual ir_visitor_status visit_enter(class ir_call *);
110 virtual ir_visitor_status visit_enter(class ir_if *);
112 void add_constant(ir_assignment *ir);
113 void kill(ir_variable *ir, unsigned write_mask);
114 void handle_if_block(exec_list *instructions);
115 void handle_rvalue(ir_rvalue **rvalue);
117 /** List of acp_entry: The available constants to propagate */
121 * List of kill_entry: The masks of variables whose values were
122 * killed in this block.
135 ir_constant_propagation_visitor::handle_rvalue(ir_rvalue **rvalue)
137 if (this->in_assignee || !*rvalue)
140 const glsl_type *type = (*rvalue)->type;
141 if (!type->is_scalar() && !type->is_vector())
144 ir_swizzle *swiz = NULL;
145 ir_dereference_variable *deref = (*rvalue)->as_dereference_variable();
147 swiz = (*rvalue)->as_swizzle();
151 deref = swiz->val->as_dereference_variable();
156 ir_constant_data data;
157 memset(&data, 0, sizeof(data));
159 for (unsigned int i = 0; i < type->components(); i++) {
161 acp_entry *found = NULL;
165 case 0: channel = swiz->mask.x; break;
166 case 1: channel = swiz->mask.y; break;
167 case 2: channel = swiz->mask.z; break;
168 case 3: channel = swiz->mask.w; break;
169 default: assert(!"shouldn't be reached"); channel = 0; break;
175 foreach_iter(exec_list_iterator, iter, *this->acp) {
176 acp_entry *entry = (acp_entry *)iter.get();
177 if (entry->var == deref->var && entry->write_mask & (1 << channel)) {
187 for (int j = 0; j < 4; j++) {
190 if (found->initial_values & (1 << j))
194 switch (type->base_type) {
195 case GLSL_TYPE_FLOAT:
196 data.f[i] = found->constant->value.f[rhs_channel];
199 data.i[i] = found->constant->value.i[rhs_channel];
202 data.u[i] = found->constant->value.u[rhs_channel];
205 data.b[i] = found->constant->value.b[rhs_channel];
208 assert(!"not reached");
213 *rvalue = new(ralloc_parent(deref)) ir_constant(type, &data);
214 this->progress = true;
218 ir_constant_propagation_visitor::visit_enter(ir_function_signature *ir)
220 /* Treat entry into a function signature as a completely separate
221 * block. Any instructions at global scope will be shuffled into
222 * main() at link time, so they're irrelevant to us.
224 exec_list *orig_acp = this->acp;
225 exec_list *orig_kills = this->kills;
226 bool orig_killed_all = this->killed_all;
228 this->acp = new(mem_ctx) exec_list;
229 this->kills = new(mem_ctx) exec_list;
230 this->killed_all = false;
232 visit_list_elements(this, &ir->body);
234 this->kills = orig_kills;
235 this->acp = orig_acp;
236 this->killed_all = orig_killed_all;
238 return visit_continue_with_parent;
242 ir_constant_propagation_visitor::visit_leave(ir_assignment *ir)
244 if (this->in_assignee)
245 return visit_continue;
247 unsigned kill_mask = ir->write_mask;
248 if (ir->lhs->as_dereference_array()) {
249 /* The LHS of the assignment uses an array indexing operator (e.g. v[i]
250 * = ...;). Since we only try to constant propagate vectors and
251 * scalars, this means that either (a) array indexing is being used to
252 * select a vector component, or (b) the variable in question is neither
253 * a scalar or a vector, so we don't care about it. In the former case,
254 * we want to kill the whole vector, since in general we can't predict
255 * which vector component will be selected by array indexing. In the
256 * latter case, it doesn't matter what we do, so go ahead and kill the
257 * whole variable anyway.
259 * Note that if the array index is constant (e.g. v[2] = ...;), we could
260 * in principle be smarter, but we don't need to, because a future
261 * optimization pass will convert it to a simple assignment with the
266 kill(ir->lhs->variable_referenced(), kill_mask);
270 return visit_continue;
274 ir_constant_propagation_visitor::visit_enter(ir_function *ir)
277 return visit_continue;
281 ir_constant_propagation_visitor::visit_enter(ir_call *ir)
283 /* Do constant propagation on call parameters, but skip any out params */
284 exec_list_iterator sig_param_iter = ir->callee->parameters.iterator();
285 foreach_iter(exec_list_iterator, iter, ir->actual_parameters) {
286 ir_variable *sig_param = (ir_variable *)sig_param_iter.get();
287 ir_rvalue *param = (ir_rvalue *)iter.get();
288 if (sig_param->mode != ir_var_out && sig_param->mode != ir_var_inout) {
289 ir_rvalue *new_param = param;
290 handle_rvalue(&new_param);
291 if (new_param != param)
292 param->replace_with(new_param);
296 sig_param_iter.next();
299 /* Since we're unlinked, we don't (necssarily) know the side effects of
300 * this call. So kill all copies.
303 this->killed_all = true;
305 return visit_continue_with_parent;
309 ir_constant_propagation_visitor::handle_if_block(exec_list *instructions)
311 exec_list *orig_acp = this->acp;
312 exec_list *orig_kills = this->kills;
313 bool orig_killed_all = this->killed_all;
315 this->acp = new(mem_ctx) exec_list;
316 this->kills = new(mem_ctx) exec_list;
317 this->killed_all = false;
319 /* Populate the initial acp with a constant of the original */
320 foreach_iter(exec_list_iterator, iter, *orig_acp) {
321 acp_entry *a = (acp_entry *)iter.get();
322 this->acp->push_tail(new(this->mem_ctx) acp_entry(a));
325 visit_list_elements(this, instructions);
327 if (this->killed_all) {
328 orig_acp->make_empty();
331 exec_list *new_kills = this->kills;
332 this->kills = orig_kills;
333 this->acp = orig_acp;
334 this->killed_all = this->killed_all || orig_killed_all;
336 foreach_iter(exec_list_iterator, iter, *new_kills) {
337 kill_entry *k = (kill_entry *)iter.get();
338 kill(k->var, k->write_mask);
343 ir_constant_propagation_visitor::visit_enter(ir_if *ir)
345 ir->condition->accept(this);
346 handle_rvalue(&ir->condition);
348 handle_if_block(&ir->then_instructions);
349 handle_if_block(&ir->else_instructions);
351 /* handle_if_block() already descended into the children. */
352 return visit_continue_with_parent;
356 ir_constant_propagation_visitor::visit_enter(ir_loop *ir)
358 exec_list *orig_acp = this->acp;
359 exec_list *orig_kills = this->kills;
360 bool orig_killed_all = this->killed_all;
362 /* FINISHME: For now, the initial acp for loops is totally empty.
363 * We could go through once, then go through again with the acp
364 * cloned minus the killed entries after the first run through.
366 this->acp = new(mem_ctx) exec_list;
367 this->kills = new(mem_ctx) exec_list;
368 this->killed_all = false;
370 visit_list_elements(this, &ir->body_instructions);
372 if (this->killed_all) {
373 orig_acp->make_empty();
376 exec_list *new_kills = this->kills;
377 this->kills = orig_kills;
378 this->acp = orig_acp;
379 this->killed_all = this->killed_all || orig_killed_all;
381 foreach_iter(exec_list_iterator, iter, *new_kills) {
382 kill_entry *k = (kill_entry *)iter.get();
383 kill(k->var, k->write_mask);
386 /* already descended into the children. */
387 return visit_continue_with_parent;
391 ir_constant_propagation_visitor::kill(ir_variable *var, unsigned write_mask)
395 /* We don't track non-vectors. */
396 if (!var->type->is_vector() && !var->type->is_scalar())
399 /* Remove any entries currently in the ACP for this kill. */
400 foreach_iter(exec_list_iterator, iter, *this->acp) {
401 acp_entry *entry = (acp_entry *)iter.get();
403 if (entry->var == var) {
404 entry->write_mask &= ~write_mask;
405 if (entry->write_mask == 0)
410 /* Add this writemask of the variable to the list of killed
411 * variables in this block.
413 foreach_iter(exec_list_iterator, iter, *this->kills) {
414 kill_entry *entry = (kill_entry *)iter.get();
416 if (entry->var == var) {
417 entry->write_mask |= write_mask;
421 /* Not already in the list. Make new entry. */
422 this->kills->push_tail(new(this->mem_ctx) kill_entry(var, write_mask));
426 * Adds an entry to the available constant list if it's a plain assignment
427 * of a variable to a variable.
430 ir_constant_propagation_visitor::add_constant(ir_assignment *ir)
440 ir_dereference_variable *deref = ir->lhs->as_dereference_variable();
441 ir_constant *constant = ir->rhs->as_constant();
443 if (!deref || !constant)
446 /* Only do constant propagation on vectors. Constant matrices,
447 * arrays, or structures would require more work elsewhere.
449 if (!deref->var->type->is_vector() && !deref->var->type->is_scalar())
452 entry = new(this->mem_ctx) acp_entry(deref->var, ir->write_mask, constant);
453 this->acp->push_tail(entry);
456 } /* unnamed namespace */
459 * Does a constant propagation pass on the code present in the instruction stream.
462 do_constant_propagation(exec_list *instructions)
464 ir_constant_propagation_visitor v;
466 visit_list_elements(&v, instructions);