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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 ir_constant_propagation_visitor
: public ir_rvalue_visitor
{
82 ir_constant_propagation_visitor()
86 mem_ctx
= ralloc_context(0);
87 this->lin_ctx
= linear_alloc_parent(this->mem_ctx
, 0);
88 this->acp
= new(mem_ctx
) exec_list
;
89 this->kills
= _mesa_pointer_hash_table_create(mem_ctx
);
91 ~ir_constant_propagation_visitor()
96 virtual ir_visitor_status
visit_enter(class ir_loop
*);
97 virtual ir_visitor_status
visit_enter(class ir_function_signature
*);
98 virtual ir_visitor_status
visit_enter(class ir_function
*);
99 virtual ir_visitor_status
visit_leave(class ir_assignment
*);
100 virtual ir_visitor_status
visit_enter(class ir_call
*);
101 virtual ir_visitor_status
visit_enter(class ir_if
*);
103 void add_constant(ir_assignment
*ir
);
104 void constant_folding(ir_rvalue
**rvalue
);
105 void constant_propagation(ir_rvalue
**rvalue
);
106 void kill(ir_variable
*ir
, unsigned write_mask
);
107 void handle_if_block(exec_list
*instructions
, hash_table
*kills
, bool *killed_all
);
108 void handle_loop(class ir_loop
*, bool keep_acp
);
109 void handle_rvalue(ir_rvalue
**rvalue
);
111 /** List of acp_entry: The available constants to propagate */
115 * Hash table of killed entries: maps variables to the mask of killed channels.
129 ir_constant_propagation_visitor::constant_folding(ir_rvalue
**rvalue
)
131 if (this->in_assignee
|| *rvalue
== NULL
)
134 if (ir_constant_fold(rvalue
))
135 this->progress
= true;
137 ir_dereference_variable
*var_ref
= (*rvalue
)->as_dereference_variable();
138 if (var_ref
&& !var_ref
->type
->is_array()) {
139 ir_constant
*constant
=
140 var_ref
->constant_expression_value(ralloc_parent(var_ref
));
143 this->progress
= true;
149 ir_constant_propagation_visitor::constant_propagation(ir_rvalue
**rvalue
) {
151 if (this->in_assignee
|| !*rvalue
)
154 const glsl_type
*type
= (*rvalue
)->type
;
155 if (!type
->is_scalar() && !type
->is_vector())
158 ir_swizzle
*swiz
= NULL
;
159 ir_dereference_variable
*deref
= (*rvalue
)->as_dereference_variable();
161 swiz
= (*rvalue
)->as_swizzle();
165 deref
= swiz
->val
->as_dereference_variable();
170 ir_constant_data data
;
171 memset(&data
, 0, sizeof(data
));
173 for (unsigned int i
= 0; i
< type
->components(); i
++) {
175 acp_entry
*found
= NULL
;
179 case 0: channel
= swiz
->mask
.x
; break;
180 case 1: channel
= swiz
->mask
.y
; break;
181 case 2: channel
= swiz
->mask
.z
; break;
182 case 3: channel
= swiz
->mask
.w
; break;
183 default: assert(!"shouldn't be reached"); channel
= 0; break;
189 foreach_in_list(acp_entry
, entry
, this->acp
) {
190 if (entry
->var
== deref
->var
&& entry
->write_mask
& (1 << channel
)) {
200 for (int j
= 0; j
< 4; j
++) {
203 if (found
->initial_values
& (1 << j
))
207 switch (type
->base_type
) {
208 case GLSL_TYPE_FLOAT
:
209 data
.f
[i
] = found
->constant
->value
.f
[rhs_channel
];
211 case GLSL_TYPE_DOUBLE
:
212 data
.d
[i
] = found
->constant
->value
.d
[rhs_channel
];
215 data
.i
[i
] = found
->constant
->value
.i
[rhs_channel
];
218 data
.u
[i
] = found
->constant
->value
.u
[rhs_channel
];
221 data
.b
[i
] = found
->constant
->value
.b
[rhs_channel
];
223 case GLSL_TYPE_UINT64
:
224 data
.u64
[i
] = found
->constant
->value
.u64
[rhs_channel
];
226 case GLSL_TYPE_INT64
:
227 data
.i64
[i
] = found
->constant
->value
.i64
[rhs_channel
];
230 assert(!"not reached");
235 *rvalue
= new(ralloc_parent(deref
)) ir_constant(type
, &data
);
236 this->progress
= true;
240 ir_constant_propagation_visitor::handle_rvalue(ir_rvalue
**rvalue
)
242 constant_propagation(rvalue
);
243 constant_folding(rvalue
);
247 ir_constant_propagation_visitor::visit_enter(ir_function_signature
*ir
)
249 /* Treat entry into a function signature as a completely separate
250 * block. Any instructions at global scope will be shuffled into
251 * main() at link time, so they're irrelevant to us.
253 exec_list
*orig_acp
= this->acp
;
254 hash_table
*orig_kills
= this->kills
;
255 bool orig_killed_all
= this->killed_all
;
257 this->acp
= new(mem_ctx
) exec_list
;
258 this->kills
= _mesa_pointer_hash_table_create(mem_ctx
);
259 this->killed_all
= false;
261 visit_list_elements(this, &ir
->body
);
263 this->kills
= orig_kills
;
264 this->acp
= orig_acp
;
265 this->killed_all
= orig_killed_all
;
267 return visit_continue_with_parent
;
271 ir_constant_propagation_visitor::visit_leave(ir_assignment
*ir
)
273 constant_folding(&ir
->rhs
);
275 if (this->in_assignee
)
276 return visit_continue
;
278 unsigned kill_mask
= ir
->write_mask
;
279 if (ir
->lhs
->as_dereference_array()) {
280 /* The LHS of the assignment uses an array indexing operator (e.g. v[i]
281 * = ...;). Since we only try to constant propagate vectors and
282 * scalars, this means that either (a) array indexing is being used to
283 * select a vector component, or (b) the variable in question is neither
284 * a scalar or a vector, so we don't care about it. In the former case,
285 * we want to kill the whole vector, since in general we can't predict
286 * which vector component will be selected by array indexing. In the
287 * latter case, it doesn't matter what we do, so go ahead and kill the
288 * whole variable anyway.
290 * Note that if the array index is constant (e.g. v[2] = ...;), we could
291 * in principle be smarter, but we don't need to, because a future
292 * optimization pass will convert it to a simple assignment with the
297 kill(ir
->lhs
->variable_referenced(), kill_mask
);
301 return visit_continue
;
305 ir_constant_propagation_visitor::visit_enter(ir_function
*ir
)
308 return visit_continue
;
312 ir_constant_propagation_visitor::visit_enter(ir_call
*ir
)
314 /* Do constant propagation on call parameters, but skip any out params */
315 foreach_two_lists(formal_node
, &ir
->callee
->parameters
,
316 actual_node
, &ir
->actual_parameters
) {
317 ir_variable
*sig_param
= (ir_variable
*) formal_node
;
318 ir_rvalue
*param
= (ir_rvalue
*) actual_node
;
319 if (sig_param
->data
.mode
!= ir_var_function_out
320 && sig_param
->data
.mode
!= ir_var_function_inout
) {
321 ir_rvalue
*new_param
= param
;
322 handle_rvalue(&new_param
);
323 if (new_param
!= param
)
324 param
->replace_with(new_param
);
330 /* Since we're unlinked, we don't (necssarily) know the side effects of
331 * this call. So kill all copies.
334 this->killed_all
= true;
336 return visit_continue_with_parent
;
340 ir_constant_propagation_visitor::handle_if_block(exec_list
*instructions
, hash_table
*kills
, bool *killed_all
)
342 exec_list
*orig_acp
= this->acp
;
343 hash_table
*orig_kills
= this->kills
;
344 bool orig_killed_all
= this->killed_all
;
346 this->acp
= new(mem_ctx
) exec_list
;
348 this->killed_all
= false;
350 /* Populate the initial acp with a constant of the original */
351 foreach_in_list(acp_entry
, a
, orig_acp
) {
352 this->acp
->push_tail(new(this->lin_ctx
) acp_entry(a
));
355 visit_list_elements(this, instructions
);
357 *killed_all
= this->killed_all
;
358 this->kills
= orig_kills
;
359 this->acp
= orig_acp
;
360 this->killed_all
= orig_killed_all
;
364 ir_constant_propagation_visitor::visit_enter(ir_if
*ir
)
366 ir
->condition
->accept(this);
367 handle_rvalue(&ir
->condition
);
369 hash_table
*new_kills
= _mesa_pointer_hash_table_create(mem_ctx
);
370 bool then_killed_all
= false;
371 bool else_killed_all
= false;
373 handle_if_block(&ir
->then_instructions
, new_kills
, &then_killed_all
);
374 handle_if_block(&ir
->else_instructions
, new_kills
, &else_killed_all
);
376 if (then_killed_all
|| else_killed_all
) {
380 hash_table_foreach(new_kills
, htk
)
381 kill((ir_variable
*) htk
->key
, (uintptr_t) htk
->data
);
384 _mesa_hash_table_destroy(new_kills
, NULL
);
386 /* handle_if_block() already descended into the children. */
387 return visit_continue_with_parent
;
391 ir_constant_propagation_visitor::handle_loop(ir_loop
*ir
, bool keep_acp
)
393 exec_list
*orig_acp
= this->acp
;
394 hash_table
*orig_kills
= this->kills
;
395 bool orig_killed_all
= this->killed_all
;
397 this->acp
= new(mem_ctx
) exec_list
;
398 this->kills
= _mesa_pointer_hash_table_create(mem_ctx
);
399 this->killed_all
= false;
402 foreach_in_list(acp_entry
, a
, orig_acp
) {
403 this->acp
->push_tail(new(this->lin_ctx
) acp_entry(a
));
407 visit_list_elements(this, &ir
->body_instructions
);
409 if (this->killed_all
) {
410 orig_acp
->make_empty();
413 hash_table
*new_kills
= this->kills
;
414 this->kills
= orig_kills
;
415 this->acp
= orig_acp
;
416 this->killed_all
= this->killed_all
|| orig_killed_all
;
418 hash_table_foreach(new_kills
, htk
) {
419 kill((ir_variable
*) htk
->key
, (uintptr_t) htk
->data
);
424 ir_constant_propagation_visitor::visit_enter(ir_loop
*ir
)
426 /* Make a conservative first pass over the loop with an empty ACP set.
427 * This also removes any killed entries from the original ACP set.
429 handle_loop(ir
, false);
431 /* Then, run it again with the real ACP set, minus any killed entries.
432 * This takes care of propagating values from before the loop into it.
434 handle_loop(ir
, true);
436 /* already descended into the children. */
437 return visit_continue_with_parent
;
441 ir_constant_propagation_visitor::kill(ir_variable
*var
, unsigned write_mask
)
445 /* We don't track non-vectors. */
446 if (!var
->type
->is_vector() && !var
->type
->is_scalar())
449 /* Remove any entries currently in the ACP for this kill. */
450 foreach_in_list_safe(acp_entry
, entry
, this->acp
) {
451 if (entry
->var
== var
) {
452 entry
->write_mask
&= ~write_mask
;
453 if (entry
->write_mask
== 0)
458 /* Add this writemask of the variable to the hash table of killed
459 * variables in this block.
461 hash_entry
*kill_hash_entry
= _mesa_hash_table_search(this->kills
, var
);
462 if (kill_hash_entry
) {
463 uintptr_t new_write_mask
= ((uintptr_t) kill_hash_entry
->data
) | write_mask
;
464 kill_hash_entry
->data
= (void *) new_write_mask
;
467 /* Not already in the hash table. Make new entry. */
468 _mesa_hash_table_insert(this->kills
, var
, (void *) uintptr_t(write_mask
));
472 * Adds an entry to the available constant list if it's a plain assignment
473 * of a variable to a variable.
476 ir_constant_propagation_visitor::add_constant(ir_assignment
*ir
)
486 ir_dereference_variable
*deref
= ir
->lhs
->as_dereference_variable();
487 ir_constant
*constant
= ir
->rhs
->as_constant();
489 if (!deref
|| !constant
)
492 /* Only do constant propagation on vectors. Constant matrices,
493 * arrays, or structures would require more work elsewhere.
495 if (!deref
->var
->type
->is_vector() && !deref
->var
->type
->is_scalar())
498 /* We can't do copy propagation on buffer variables, since the underlying
499 * memory storage is shared across multiple threads we can't be sure that
500 * the variable value isn't modified between this assignment and the next
501 * instruction where its value is read.
503 if (deref
->var
->data
.mode
== ir_var_shader_storage
||
504 deref
->var
->data
.mode
== ir_var_shader_shared
)
507 entry
= new(this->lin_ctx
) acp_entry(deref
->var
, ir
->write_mask
, constant
);
508 this->acp
->push_tail(entry
);
511 } /* unnamed namespace */
514 * Does a constant propagation pass on the code present in the instruction stream.
517 do_constant_propagation(exec_list
*instructions
)
519 ir_constant_propagation_visitor v
;
521 visit_list_elements(&v
, instructions
);