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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
=
158 var_ref
->constant_expression_value(ralloc_parent(var_ref
));
161 this->progress
= true;
167 ir_constant_propagation_visitor::constant_propagation(ir_rvalue
**rvalue
) {
169 if (this->in_assignee
|| !*rvalue
)
172 const glsl_type
*type
= (*rvalue
)->type
;
173 if (!type
->is_scalar() && !type
->is_vector())
176 ir_swizzle
*swiz
= NULL
;
177 ir_dereference_variable
*deref
= (*rvalue
)->as_dereference_variable();
179 swiz
= (*rvalue
)->as_swizzle();
183 deref
= swiz
->val
->as_dereference_variable();
188 ir_constant_data data
;
189 memset(&data
, 0, sizeof(data
));
191 for (unsigned int i
= 0; i
< type
->components(); i
++) {
193 acp_entry
*found
= NULL
;
197 case 0: channel
= swiz
->mask
.x
; break;
198 case 1: channel
= swiz
->mask
.y
; break;
199 case 2: channel
= swiz
->mask
.z
; break;
200 case 3: channel
= swiz
->mask
.w
; break;
201 default: assert(!"shouldn't be reached"); channel
= 0; break;
207 foreach_in_list(acp_entry
, entry
, this->acp
) {
208 if (entry
->var
== deref
->var
&& entry
->write_mask
& (1 << channel
)) {
218 for (int j
= 0; j
< 4; j
++) {
221 if (found
->initial_values
& (1 << j
))
225 switch (type
->base_type
) {
226 case GLSL_TYPE_FLOAT
:
227 data
.f
[i
] = found
->constant
->value
.f
[rhs_channel
];
229 case GLSL_TYPE_DOUBLE
:
230 data
.d
[i
] = found
->constant
->value
.d
[rhs_channel
];
233 data
.i
[i
] = found
->constant
->value
.i
[rhs_channel
];
236 data
.u
[i
] = found
->constant
->value
.u
[rhs_channel
];
239 data
.b
[i
] = found
->constant
->value
.b
[rhs_channel
];
242 assert(!"not reached");
247 *rvalue
= new(ralloc_parent(deref
)) ir_constant(type
, &data
);
248 this->progress
= true;
252 ir_constant_propagation_visitor::handle_rvalue(ir_rvalue
**rvalue
)
254 constant_propagation(rvalue
);
255 constant_folding(rvalue
);
259 ir_constant_propagation_visitor::visit_enter(ir_function_signature
*ir
)
261 /* Treat entry into a function signature as a completely separate
262 * block. Any instructions at global scope will be shuffled into
263 * main() at link time, so they're irrelevant to us.
265 exec_list
*orig_acp
= this->acp
;
266 hash_table
*orig_kills
= this->kills
;
267 bool orig_killed_all
= this->killed_all
;
269 this->acp
= new(mem_ctx
) exec_list
;
270 this->kills
= _mesa_hash_table_create(mem_ctx
, _mesa_hash_pointer
,
271 _mesa_key_pointer_equal
);
272 this->killed_all
= false;
274 visit_list_elements(this, &ir
->body
);
276 this->kills
= orig_kills
;
277 this->acp
= orig_acp
;
278 this->killed_all
= orig_killed_all
;
280 return visit_continue_with_parent
;
284 ir_constant_propagation_visitor::visit_leave(ir_assignment
*ir
)
286 constant_folding(&ir
->rhs
);
288 if (this->in_assignee
)
289 return visit_continue
;
291 unsigned kill_mask
= ir
->write_mask
;
292 if (ir
->lhs
->as_dereference_array()) {
293 /* The LHS of the assignment uses an array indexing operator (e.g. v[i]
294 * = ...;). Since we only try to constant propagate vectors and
295 * scalars, this means that either (a) array indexing is being used to
296 * select a vector component, or (b) the variable in question is neither
297 * a scalar or a vector, so we don't care about it. In the former case,
298 * we want to kill the whole vector, since in general we can't predict
299 * which vector component will be selected by array indexing. In the
300 * latter case, it doesn't matter what we do, so go ahead and kill the
301 * whole variable anyway.
303 * Note that if the array index is constant (e.g. v[2] = ...;), we could
304 * in principle be smarter, but we don't need to, because a future
305 * optimization pass will convert it to a simple assignment with the
310 kill(ir
->lhs
->variable_referenced(), kill_mask
);
314 return visit_continue
;
318 ir_constant_propagation_visitor::visit_enter(ir_function
*ir
)
321 return visit_continue
;
325 ir_constant_propagation_visitor::visit_enter(ir_call
*ir
)
327 /* Do constant propagation on call parameters, but skip any out params */
328 foreach_two_lists(formal_node
, &ir
->callee
->parameters
,
329 actual_node
, &ir
->actual_parameters
) {
330 ir_variable
*sig_param
= (ir_variable
*) formal_node
;
331 ir_rvalue
*param
= (ir_rvalue
*) actual_node
;
332 if (sig_param
->data
.mode
!= ir_var_function_out
333 && sig_param
->data
.mode
!= ir_var_function_inout
) {
334 ir_rvalue
*new_param
= param
;
335 handle_rvalue(&new_param
);
336 if (new_param
!= param
)
337 param
->replace_with(new_param
);
343 /* Since we're unlinked, we don't (necssarily) know the side effects of
344 * this call. So kill all copies.
347 this->killed_all
= true;
349 return visit_continue_with_parent
;
353 ir_constant_propagation_visitor::handle_if_block(exec_list
*instructions
)
355 exec_list
*orig_acp
= this->acp
;
356 hash_table
*orig_kills
= this->kills
;
357 bool orig_killed_all
= this->killed_all
;
359 this->acp
= new(mem_ctx
) exec_list
;
360 this->kills
= _mesa_hash_table_create(mem_ctx
, _mesa_hash_pointer
,
361 _mesa_key_pointer_equal
);
362 this->killed_all
= false;
364 /* Populate the initial acp with a constant of the original */
365 foreach_in_list(acp_entry
, a
, orig_acp
) {
366 this->acp
->push_tail(new(this->lin_ctx
) acp_entry(a
));
369 visit_list_elements(this, instructions
);
371 if (this->killed_all
) {
372 orig_acp
->make_empty();
375 hash_table
*new_kills
= this->kills
;
376 this->kills
= orig_kills
;
377 this->acp
= orig_acp
;
378 this->killed_all
= this->killed_all
|| orig_killed_all
;
381 hash_table_foreach(new_kills
, htk
) {
382 kill_entry
*k
= (kill_entry
*) htk
->data
;
383 kill(k
->var
, k
->write_mask
);
388 ir_constant_propagation_visitor::visit_enter(ir_if
*ir
)
390 ir
->condition
->accept(this);
391 handle_rvalue(&ir
->condition
);
393 handle_if_block(&ir
->then_instructions
);
394 handle_if_block(&ir
->else_instructions
);
396 /* handle_if_block() already descended into the children. */
397 return visit_continue_with_parent
;
401 ir_constant_propagation_visitor::visit_enter(ir_loop
*ir
)
403 exec_list
*orig_acp
= this->acp
;
404 hash_table
*orig_kills
= this->kills
;
405 bool orig_killed_all
= this->killed_all
;
407 /* FINISHME: For now, the initial acp for loops is totally empty.
408 * We could go through once, then go through again with the acp
409 * cloned minus the killed entries after the first run through.
411 this->acp
= new(mem_ctx
) exec_list
;
412 this->kills
= _mesa_hash_table_create(mem_ctx
, _mesa_hash_pointer
,
413 _mesa_key_pointer_equal
);
414 this->killed_all
= false;
416 visit_list_elements(this, &ir
->body_instructions
);
418 if (this->killed_all
) {
419 orig_acp
->make_empty();
422 hash_table
*new_kills
= this->kills
;
423 this->kills
= orig_kills
;
424 this->acp
= orig_acp
;
425 this->killed_all
= this->killed_all
|| orig_killed_all
;
428 hash_table_foreach(new_kills
, htk
) {
429 kill_entry
*k
= (kill_entry
*) htk
->data
;
430 kill(k
->var
, k
->write_mask
);
433 /* already descended into the children. */
434 return visit_continue_with_parent
;
438 ir_constant_propagation_visitor::kill(ir_variable
*var
, unsigned write_mask
)
442 /* We don't track non-vectors. */
443 if (!var
->type
->is_vector() && !var
->type
->is_scalar())
446 /* Remove any entries currently in the ACP for this kill. */
447 foreach_in_list_safe(acp_entry
, entry
, this->acp
) {
448 if (entry
->var
== var
) {
449 entry
->write_mask
&= ~write_mask
;
450 if (entry
->write_mask
== 0)
455 /* Add this writemask of the variable to the hash table of killed
456 * variables in this block.
458 hash_entry
*kill_hash_entry
= _mesa_hash_table_search(this->kills
, var
);
459 if (kill_hash_entry
) {
460 kill_entry
*entry
= (kill_entry
*) kill_hash_entry
->data
;
461 entry
->write_mask
|= write_mask
;
464 /* Not already in the hash table. Make new entry. */
465 _mesa_hash_table_insert(this->kills
, var
,
466 new(this->lin_ctx
) kill_entry(var
, write_mask
));
470 * Adds an entry to the available constant list if it's a plain assignment
471 * of a variable to a variable.
474 ir_constant_propagation_visitor::add_constant(ir_assignment
*ir
)
484 ir_dereference_variable
*deref
= ir
->lhs
->as_dereference_variable();
485 ir_constant
*constant
= ir
->rhs
->as_constant();
487 if (!deref
|| !constant
)
490 /* Only do constant propagation on vectors. Constant matrices,
491 * arrays, or structures would require more work elsewhere.
493 if (!deref
->var
->type
->is_vector() && !deref
->var
->type
->is_scalar())
496 /* We can't do copy propagation on buffer variables, since the underlying
497 * memory storage is shared across multiple threads we can't be sure that
498 * the variable value isn't modified between this assignment and the next
499 * instruction where its value is read.
501 if (deref
->var
->data
.mode
== ir_var_shader_storage
||
502 deref
->var
->data
.mode
== ir_var_shader_shared
)
505 entry
= new(this->lin_ctx
) acp_entry(deref
->var
, ir
->write_mask
, constant
);
506 this->acp
->push_tail(entry
);
509 } /* unnamed namespace */
512 * Does a constant propagation pass on the code present in the instruction stream.
515 do_constant_propagation(exec_list
*instructions
)
517 ir_constant_propagation_visitor v
;
519 visit_list_elements(&v
, instructions
);