1 /*
2 +----------------------------------------------------------------------+
3 | Zend Engine, DCE - Dead Code Elimination |
4 +----------------------------------------------------------------------+
5 | Copyright (c) The PHP Group |
6 +----------------------------------------------------------------------+
7 | This source file is subject to version 3.01 of the PHP license, |
8 | that is bundled with this package in the file LICENSE, and is |
9 | available through the world-wide-web at the following url: |
10 | https://www.php.net/license/3_01.txt |
11 | If you did not receive a copy of the PHP license and are unable to |
12 | obtain it through the world-wide-web, please send a note to |
13 | license@php.net so we can mail you a copy immediately. |
14 +----------------------------------------------------------------------+
15 | Authors: Nikita Popov <nikic@php.net> |
16 | Dmitry Stogov <dmitry@php.net> |
17 +----------------------------------------------------------------------+
18 */
19
20 #include "Optimizer/zend_optimizer_internal.h"
21 #include "Optimizer/zend_inference.h"
22 #include "Optimizer/zend_ssa.h"
23 #include "Optimizer/zend_func_info.h"
24 #include "Optimizer/zend_call_graph.h"
25 #include "zend_bitset.h"
26
27 /* This pass implements a form of dead code elimination (DCE). The algorithm optimistically assumes
28 * that all instructions and phis are dead. Instructions with immediate side-effects are then marked
29 * as live. We then recursively (using a worklist) propagate liveness to the instructions that def
30 * the used operands.
31 *
32 * Notes:
33 * * This pass does not perform unreachable code elimination. This happens as part of the SCCP
34 * pass.
35 * * The DCE is performed without taking control-dependence into account, i.e. all conditional
36 * branches are assumed to be live. It's possible to take control-dependence into account using
37 * the DCE algorithm described by Cytron et al., however it requires the construction of a
38 * postdominator tree and of postdominance frontiers, which does not seem worthwhile at this
39 * point.
40 * * We separate intrinsic side-effects from potential side-effects in the form of notices thrown
41 * by the instruction (in case we want to make this configurable). See may_have_side_effects() and
42 * zend_may_throw().
43 * * We often cannot DCE assignments and unsets while guaranteeing that dtors run in the same
44 * order. There is an optimization option to allow reordering of dtor effects.
45 * * The algorithm is able to eliminate dead modifications of non-escaping arrays
46 * and objects as well as dead arrays and objects allocations.
47 */
48
49 typedef struct {
50 zend_ssa *ssa;
51 zend_op_array *op_array;
52 zend_bitset instr_dead;
53 zend_bitset phi_dead;
54 zend_bitset instr_worklist;
55 zend_bitset phi_worklist;
56 zend_bitset phi_worklist_no_val;
57 uint32_t instr_worklist_len;
58 uint32_t phi_worklist_len;
59 unsigned reorder_dtor_effects : 1;
60 } context;
61
is_bad_mod(const zend_ssa * ssa,int use,int def)62 static inline bool is_bad_mod(const zend_ssa *ssa, int use, int def) {
63 if (def < 0) {
64 /* This modification is not tracked by SSA, assume the worst */
65 return 1;
66 }
67 if (ssa->var_info[use].type & MAY_BE_REF) {
68 /* Modification of reference may have side-effect */
69 return 1;
70 }
71 return 0;
72 }
73
may_have_side_effects(zend_op_array * op_array,zend_ssa * ssa,const zend_op * opline,const zend_ssa_op * ssa_op,bool reorder_dtor_effects)74 static inline bool may_have_side_effects(
75 zend_op_array *op_array, zend_ssa *ssa,
76 const zend_op *opline, const zend_ssa_op *ssa_op,
77 bool reorder_dtor_effects) {
78 switch (opline->opcode) {
79 case ZEND_NOP:
80 case ZEND_IS_IDENTICAL:
81 case ZEND_IS_NOT_IDENTICAL:
82 case ZEND_QM_ASSIGN:
83 case ZEND_FREE:
84 case ZEND_FE_FREE:
85 case ZEND_TYPE_CHECK:
86 case ZEND_DEFINED:
87 case ZEND_ADD:
88 case ZEND_SUB:
89 case ZEND_MUL:
90 case ZEND_POW:
91 case ZEND_BW_OR:
92 case ZEND_BW_AND:
93 case ZEND_BW_XOR:
94 case ZEND_CONCAT:
95 case ZEND_FAST_CONCAT:
96 case ZEND_DIV:
97 case ZEND_MOD:
98 case ZEND_BOOL_XOR:
99 case ZEND_BOOL:
100 case ZEND_BOOL_NOT:
101 case ZEND_BW_NOT:
102 case ZEND_SL:
103 case ZEND_SR:
104 case ZEND_IS_EQUAL:
105 case ZEND_IS_NOT_EQUAL:
106 case ZEND_IS_SMALLER:
107 case ZEND_IS_SMALLER_OR_EQUAL:
108 case ZEND_CASE:
109 case ZEND_CASE_STRICT:
110 case ZEND_CAST:
111 case ZEND_ROPE_INIT:
112 case ZEND_ROPE_ADD:
113 case ZEND_INIT_ARRAY:
114 case ZEND_SPACESHIP:
115 case ZEND_STRLEN:
116 case ZEND_COUNT:
117 case ZEND_GET_TYPE:
118 case ZEND_ISSET_ISEMPTY_THIS:
119 case ZEND_ISSET_ISEMPTY_DIM_OBJ:
120 case ZEND_FETCH_DIM_IS:
121 case ZEND_ISSET_ISEMPTY_CV:
122 case ZEND_ISSET_ISEMPTY_VAR:
123 case ZEND_FETCH_IS:
124 case ZEND_IN_ARRAY:
125 case ZEND_FUNC_NUM_ARGS:
126 case ZEND_FUNC_GET_ARGS:
127 case ZEND_ARRAY_KEY_EXISTS:
128 /* No side effects */
129 return 0;
130 case ZEND_ADD_ARRAY_ELEMENT:
131 /* TODO: We can't free two vars. Keep instruction alive. <?php [0, "$a" => "$b"]; */
132 if ((opline->op1_type & (IS_VAR|IS_TMP_VAR)) && (opline->op2_type & (IS_VAR|IS_TMP_VAR))) {
133 return 1;
134 }
135 return 0;
136 case ZEND_ROPE_END:
137 /* TODO: Rope dce optimization, see #76446 */
138 return 1;
139 case ZEND_JMP:
140 case ZEND_JMPZ:
141 case ZEND_JMPNZ:
142 case ZEND_JMPZ_EX:
143 case ZEND_JMPNZ_EX:
144 case ZEND_JMP_SET:
145 case ZEND_COALESCE:
146 case ZEND_ASSERT_CHECK:
147 case ZEND_JMP_NULL:
148 case ZEND_BIND_INIT_STATIC_OR_JMP:
149 case ZEND_JMP_FRAMELESS:
150 /* For our purposes a jumps and branches are side effects. */
151 return 1;
152 case ZEND_BEGIN_SILENCE:
153 case ZEND_END_SILENCE:
154 case ZEND_ECHO:
155 case ZEND_INCLUDE_OR_EVAL:
156 case ZEND_THROW:
157 case ZEND_MATCH_ERROR:
158 case ZEND_EXT_STMT:
159 case ZEND_EXT_FCALL_BEGIN:
160 case ZEND_EXT_FCALL_END:
161 case ZEND_TICKS:
162 case ZEND_YIELD:
163 case ZEND_YIELD_FROM:
164 case ZEND_VERIFY_NEVER_TYPE:
165 /* Intrinsic side effects */
166 return 1;
167 case ZEND_DO_FCALL:
168 case ZEND_DO_FCALL_BY_NAME:
169 case ZEND_DO_ICALL:
170 case ZEND_DO_UCALL:
171 case ZEND_FRAMELESS_ICALL_0:
172 case ZEND_FRAMELESS_ICALL_1:
173 case ZEND_FRAMELESS_ICALL_2:
174 case ZEND_FRAMELESS_ICALL_3:
175 /* For now assume all calls have side effects */
176 return 1;
177 case ZEND_RECV:
178 case ZEND_RECV_INIT:
179 /* Even though RECV_INIT can be side-effect free, these cannot be simply dropped
180 * due to the prologue skipping code. */
181 return 1;
182 case ZEND_ASSIGN_REF:
183 return 1;
184 case ZEND_ASSIGN:
185 {
186 if (is_bad_mod(ssa, ssa_op->op1_use, ssa_op->op1_def)) {
187 return 1;
188 }
189 if (!reorder_dtor_effects) {
190 if (opline->op2_type != IS_CONST
191 && (OP2_INFO() & MAY_HAVE_DTOR)
192 && ssa->vars[ssa_op->op2_use].escape_state != ESCAPE_STATE_NO_ESCAPE) {
193 /* DCE might shorten lifetime */
194 return 1;
195 }
196 }
197 return 0;
198 }
199 case ZEND_UNSET_VAR:
200 return 1;
201 case ZEND_UNSET_CV:
202 {
203 uint32_t t1 = OP1_INFO();
204 if (t1 & MAY_BE_REF) {
205 /* We don't consider uses as the LHS of an assignment as real uses during DCE, so
206 * an unset may be considered dead even if there is a later assignment to the
207 * variable. Removing the unset in this case would not be correct if the variable
208 * is a reference, because unset breaks references. */
209 return 1;
210 }
211 return 0;
212 }
213 case ZEND_PRE_INC:
214 case ZEND_POST_INC:
215 case ZEND_PRE_DEC:
216 case ZEND_POST_DEC:
217 return is_bad_mod(ssa, ssa_op->op1_use, ssa_op->op1_def);
218 case ZEND_ASSIGN_OP:
219 return is_bad_mod(ssa, ssa_op->op1_use, ssa_op->op1_def)
220 || ssa->vars[ssa_op->op1_def].escape_state != ESCAPE_STATE_NO_ESCAPE;
221 case ZEND_ASSIGN_DIM:
222 case ZEND_ASSIGN_OBJ:
223 if (is_bad_mod(ssa, ssa_op->op1_use, ssa_op->op1_def)
224 || ssa->vars[ssa_op->op1_def].escape_state != ESCAPE_STATE_NO_ESCAPE) {
225 return 1;
226 }
227 if (!reorder_dtor_effects) {
228 opline++;
229 ssa_op++;
230 if (opline->op1_type != IS_CONST
231 && (OP1_INFO() & MAY_HAVE_DTOR)) {
232 /* DCE might shorten lifetime */
233 return 1;
234 }
235 }
236 return 0;
237 case ZEND_PRE_INC_OBJ:
238 case ZEND_PRE_DEC_OBJ:
239 case ZEND_POST_INC_OBJ:
240 case ZEND_POST_DEC_OBJ:
241 if (is_bad_mod(ssa, ssa_op->op1_use, ssa_op->op1_def)
242 || ssa->vars[ssa_op->op1_def].escape_state != ESCAPE_STATE_NO_ESCAPE) {
243 return 1;
244 }
245 return 0;
246 case ZEND_BIND_STATIC:
247 if (op_array->static_variables) {
248 /* Implicit and Explicit bind static is effectively prologue of closure so
249 report it has side effects like RECV, RECV_INIT; This allows us to
250 reflect on the closure and discover used variable at runtime */
251 if ((opline->extended_value & (ZEND_BIND_IMPLICIT|ZEND_BIND_EXPLICIT))) {
252 return 1;
253 }
254 /* Modifies static variables which are observable through reflection */
255 if ((opline->extended_value & ZEND_BIND_REF) && opline->op2_type != IS_UNUSED) {
256 return 1;
257 }
258 }
259 return 0;
260 case ZEND_CHECK_VAR:
261 return (OP1_INFO() & MAY_BE_UNDEF) != 0;
262 case ZEND_FE_RESET_R:
263 case ZEND_FE_RESET_RW:
264 /* Model as not having side-effects -- let the side-effect be introduced by
265 * FE_FETCH if the array is not known to be non-empty. */
266 return (OP1_INFO() & MAY_BE_ANY) != MAY_BE_ARRAY;
267 default:
268 /* For everything we didn't handle, assume a side-effect */
269 return 1;
270 }
271 }
272
add_to_worklists(context * ctx,int var_num,int check)273 static zend_always_inline void add_to_worklists(context *ctx, int var_num, int check) {
274 zend_ssa_var *var = &ctx->ssa->vars[var_num];
275 if (var->definition >= 0) {
276 if (!check || zend_bitset_in(ctx->instr_dead, var->definition)) {
277 zend_bitset_incl(ctx->instr_worklist, var->definition);
278 }
279 } else if (var->definition_phi) {
280 if (!check || zend_bitset_in(ctx->phi_dead, var_num)) {
281 zend_bitset_incl(ctx->phi_worklist, var_num);
282 }
283 }
284 }
285
add_to_phi_worklist_no_val(context * ctx,int var_num)286 static inline void add_to_phi_worklist_no_val(context *ctx, int var_num) {
287 zend_ssa_var *var = &ctx->ssa->vars[var_num];
288 if (var->definition_phi && zend_bitset_in(ctx->phi_dead, var_num)) {
289 zend_bitset_incl(ctx->phi_worklist_no_val, var_num);
290 }
291 }
292
add_operands_to_worklists(context * ctx,zend_op * opline,zend_ssa_op * ssa_op,zend_ssa * ssa,int check)293 static zend_always_inline void add_operands_to_worklists(context *ctx, zend_op *opline, zend_ssa_op *ssa_op, zend_ssa *ssa, int check) {
294 if (ssa_op->result_use >= 0) {
295 add_to_worklists(ctx, ssa_op->result_use, check);
296 }
297 if (ssa_op->op1_use >= 0) {
298 if (!zend_ssa_is_no_val_use(opline, ssa_op, ssa_op->op1_use)
299 || (opline->opcode == ZEND_ASSIGN
300 && (ssa->var_info[ssa_op->op1_use].type & MAY_BE_REF) != 0)) {
301 add_to_worklists(ctx, ssa_op->op1_use, check);
302 } else {
303 add_to_phi_worklist_no_val(ctx, ssa_op->op1_use);
304 }
305 }
306 if (ssa_op->op2_use >= 0) {
307 if (!zend_ssa_is_no_val_use(opline, ssa_op, ssa_op->op2_use)
308 || (opline->opcode == ZEND_FE_FETCH_R
309 && (ssa->var_info[ssa_op->op2_use].type & MAY_BE_REF) != 0)) {
310 add_to_worklists(ctx, ssa_op->op2_use, check);
311 } else {
312 add_to_phi_worklist_no_val(ctx, ssa_op->op2_use);
313 }
314 }
315 }
316
add_phi_sources_to_worklists(context * ctx,zend_ssa_phi * phi,int check)317 static zend_always_inline void add_phi_sources_to_worklists(context *ctx, zend_ssa_phi *phi, int check) {
318 zend_ssa *ssa = ctx->ssa;
319 int source;
320 FOREACH_PHI_SOURCE(phi, source) {
321 add_to_worklists(ctx, source, check);
322 } FOREACH_PHI_SOURCE_END();
323 }
324
is_var_dead(context * ctx,int var_num)325 static inline bool is_var_dead(context *ctx, int var_num) {
326 zend_ssa_var *var = &ctx->ssa->vars[var_num];
327 if (var->definition_phi) {
328 return zend_bitset_in(ctx->phi_dead, var_num);
329 } else if (var->definition >= 0) {
330 return zend_bitset_in(ctx->instr_dead, var->definition);
331 } else {
332 /* Variable has no definition, so either the definition has already been removed (var is
333 * dead) or this is one of the implicit variables at the start of the function (for our
334 * purposes live) */
335 return var_num >= ctx->op_array->last_var;
336 }
337 }
338
339 // Sometimes we can mark the var as EXT_UNUSED
try_remove_var_def(context * ctx,int free_var,int use_chain,zend_op * opline)340 static bool try_remove_var_def(context *ctx, int free_var, int use_chain, zend_op *opline) {
341 if (use_chain >= 0) {
342 return 0;
343 }
344 zend_ssa_var *var = &ctx->ssa->vars[free_var];
345 int def = var->definition;
346
347 if (def >= 0) {
348 zend_ssa_op *def_op = &ctx->ssa->ops[def];
349
350 if (def_op->result_def == free_var
351 && var->phi_use_chain == NULL
352 && var->use_chain == (opline - ctx->op_array->opcodes)) {
353 zend_op *def_opline = &ctx->op_array->opcodes[def];
354
355 switch (def_opline->opcode) {
356 case ZEND_ASSIGN:
357 case ZEND_ASSIGN_REF:
358 case ZEND_ASSIGN_DIM:
359 case ZEND_ASSIGN_OBJ:
360 case ZEND_ASSIGN_OBJ_REF:
361 case ZEND_ASSIGN_STATIC_PROP:
362 case ZEND_ASSIGN_STATIC_PROP_REF:
363 case ZEND_ASSIGN_OP:
364 case ZEND_ASSIGN_DIM_OP:
365 case ZEND_ASSIGN_OBJ_OP:
366 case ZEND_ASSIGN_STATIC_PROP_OP:
367 case ZEND_PRE_INC:
368 case ZEND_PRE_DEC:
369 case ZEND_PRE_INC_OBJ:
370 case ZEND_PRE_DEC_OBJ:
371 case ZEND_DO_ICALL:
372 case ZEND_DO_UCALL:
373 case ZEND_DO_FCALL_BY_NAME:
374 case ZEND_DO_FCALL:
375 case ZEND_INCLUDE_OR_EVAL:
376 case ZEND_YIELD:
377 case ZEND_YIELD_FROM:
378 case ZEND_ASSERT_CHECK:
379 def_opline->result_type = IS_UNUSED;
380 def_opline->result.var = 0;
381 def_op->result_def = -1;
382 var->definition = -1;
383 return 1;
384 default:
385 break;
386 }
387 }
388 }
389 return 0;
390 }
391
may_be_refcounted(uint32_t type)392 static zend_always_inline bool may_be_refcounted(uint32_t type) {
393 return (type & (MAY_BE_STRING|MAY_BE_ARRAY|MAY_BE_OBJECT|MAY_BE_RESOURCE|MAY_BE_REF)) != 0;
394 }
395
is_free_of_live_var(context * ctx,zend_op * opline,zend_ssa_op * ssa_op)396 static inline bool is_free_of_live_var(context *ctx, zend_op *opline, zend_ssa_op *ssa_op) {
397 switch (opline->opcode) {
398 case ZEND_FREE:
399 /* It is always safe to remove FREEs of non-refcounted values, even if they are live. */
400 if ((ctx->ssa->var_info[ssa_op->op1_use].type & (MAY_BE_REF|MAY_BE_ANY|MAY_BE_UNDEF)) != 0
401 && !may_be_refcounted(ctx->ssa->var_info[ssa_op->op1_use].type)) {
402 return 0;
403 }
404 ZEND_FALLTHROUGH;
405 case ZEND_FE_FREE:
406 return !is_var_dead(ctx, ssa_op->op1_use);
407 default:
408 return 0;
409 }
410 }
411
412 /* Returns whether the instruction has been DCEd */
dce_instr(context * ctx,zend_op * opline,zend_ssa_op * ssa_op)413 static bool dce_instr(context *ctx, zend_op *opline, zend_ssa_op *ssa_op) {
414 zend_ssa *ssa = ctx->ssa;
415 int free_var = -1;
416 uint8_t free_var_type;
417
418 if (opline->opcode == ZEND_NOP) {
419 return 0;
420 }
421
422 /* We mark FREEs as dead, but they're only really dead if the destroyed var is dead */
423 if (is_free_of_live_var(ctx, opline, ssa_op)) {
424 return 0;
425 }
426
427 if ((opline->op1_type & (IS_VAR|IS_TMP_VAR))&& !is_var_dead(ctx, ssa_op->op1_use)) {
428 if (!try_remove_var_def(ctx, ssa_op->op1_use, ssa_op->op1_use_chain, opline)) {
429 if (may_be_refcounted(ssa->var_info[ssa_op->op1_use].type)
430 && opline->opcode != ZEND_CASE && opline->opcode != ZEND_CASE_STRICT) {
431 free_var = ssa_op->op1_use;
432 free_var_type = opline->op1_type;
433 }
434 }
435 }
436 if ((opline->op2_type & (IS_VAR|IS_TMP_VAR)) && !is_var_dead(ctx, ssa_op->op2_use)) {
437 if (!try_remove_var_def(ctx, ssa_op->op2_use, ssa_op->op2_use_chain, opline)) {
438 if (may_be_refcounted(ssa->var_info[ssa_op->op2_use].type)) {
439 if (free_var >= 0) {
440 // TODO: We can't free two vars. Keep instruction alive.
441 zend_bitset_excl(ctx->instr_dead, opline - ctx->op_array->opcodes);
442 return 0;
443 }
444 free_var = ssa_op->op2_use;
445 free_var_type = opline->op2_type;
446 }
447 }
448 }
449
450 zend_ssa_rename_defs_of_instr(ctx->ssa, ssa_op);
451 zend_ssa_remove_instr(ctx->ssa, opline, ssa_op);
452
453 if (free_var >= 0) {
454 opline->opcode = ZEND_FREE;
455 opline->op1.var = EX_NUM_TO_VAR(ssa->vars[free_var].var);
456 opline->op1_type = free_var_type;
457
458 ssa_op->op1_use = free_var;
459 ssa_op->op1_use_chain = ssa->vars[free_var].use_chain;
460 ssa->vars[free_var].use_chain = ssa_op - ssa->ops;
461 return 0;
462 }
463 return 1;
464 }
465
get_common_phi_source(zend_ssa * ssa,zend_ssa_phi * phi)466 static inline int get_common_phi_source(zend_ssa *ssa, zend_ssa_phi *phi) {
467 int common_source = -1;
468 int source;
469 FOREACH_PHI_SOURCE(phi, source) {
470 if (source == phi->ssa_var) {
471 continue;
472 }
473 if (common_source == -1) {
474 common_source = source;
475 } else if (common_source != source) {
476 return -1;
477 }
478 } FOREACH_PHI_SOURCE_END();
479
480 /* If all sources are phi->ssa_var this phi must be in an unreachable cycle.
481 * We can't easily drop the phi in that case, as we don't have something to replace it with.
482 * Ideally SCCP would eliminate the whole cycle. */
483 return common_source;
484 }
485
try_remove_trivial_phi(context * ctx,zend_ssa_phi * phi)486 static void try_remove_trivial_phi(context *ctx, zend_ssa_phi *phi) {
487 zend_ssa *ssa = ctx->ssa;
488 if (phi->pi < 0) {
489 /* Phi assignment with identical source operands */
490 int common_source = get_common_phi_source(ssa, phi);
491 if (common_source >= 0) {
492 zend_ssa_rename_var_uses(ssa, phi->ssa_var, common_source, 1);
493 zend_ssa_remove_phi(ssa, phi);
494 }
495 } else {
496 /* Pi assignment that is only used in Phi/Pi assignments */
497 // TODO What if we want to rerun type inference after DCE? Maybe separate this?
498 /*ZEND_ASSERT(phi->sources[0] != -1);
499 if (ssa->vars[phi->ssa_var].use_chain < 0) {
500 zend_ssa_rename_var_uses_keep_types(ssa, phi->ssa_var, phi->sources[0], 1);
501 zend_ssa_remove_phi(ssa, phi);
502 }*/
503 }
504 }
505
may_break_varargs(const zend_op_array * op_array,const zend_ssa * ssa,const zend_ssa_op * ssa_op)506 static inline bool may_break_varargs(const zend_op_array *op_array, const zend_ssa *ssa, const zend_ssa_op *ssa_op) {
507 if (ssa_op->op1_def >= 0
508 && ssa->vars[ssa_op->op1_def].var < op_array->num_args) {
509 return 1;
510 }
511 if (ssa_op->op2_def >= 0
512 && ssa->vars[ssa_op->op2_def].var < op_array->num_args) {
513 return 1;
514 }
515 if (ssa_op->result_def >= 0
516 && ssa->vars[ssa_op->result_def].var < op_array->num_args) {
517 return 1;
518 }
519 return 0;
520 }
521
may_throw_dce_exception(const zend_op * opline)522 static inline bool may_throw_dce_exception(const zend_op *opline) {
523 return opline->opcode == ZEND_ADD_ARRAY_ELEMENT && opline->op2_type == IS_UNUSED;
524 }
525
dce_optimize_op_array(zend_op_array * op_array,zend_optimizer_ctx * optimizer_ctx,zend_ssa * ssa,bool reorder_dtor_effects)526 int dce_optimize_op_array(zend_op_array *op_array, zend_optimizer_ctx *optimizer_ctx, zend_ssa *ssa, bool reorder_dtor_effects) {
527 int i;
528 zend_ssa_phi *phi;
529 int removed_ops = 0;
530
531 /* DCE of CV operations that changes arguments may affect vararg functions. */
532 bool has_varargs = (ssa->cfg.flags & ZEND_FUNC_VARARG) != 0;
533
534 context ctx;
535 ctx.ssa = ssa;
536 ctx.op_array = op_array;
537 ctx.reorder_dtor_effects = reorder_dtor_effects;
538
539 void *checkpoint = zend_arena_checkpoint(optimizer_ctx->arena);
540 /* We have no dedicated phi vector, so we use the whole ssa var vector instead */
541 ctx.instr_worklist_len = zend_bitset_len(op_array->last);
542 ctx.instr_worklist = zend_arena_calloc(&optimizer_ctx->arena, ctx.instr_worklist_len, sizeof(zend_ulong));
543 ctx.phi_worklist_len = zend_bitset_len(ssa->vars_count);
544 ctx.phi_worklist = zend_arena_calloc(&optimizer_ctx->arena, ctx.phi_worklist_len, sizeof(zend_ulong));
545 ctx.phi_worklist_no_val = zend_arena_calloc(&optimizer_ctx->arena, ctx.phi_worklist_len, sizeof(zend_ulong));
546
547 /* Optimistically assume all instructions and phis to be dead */
548 ctx.instr_dead = zend_arena_calloc(&optimizer_ctx->arena, ctx.instr_worklist_len, sizeof(zend_ulong));
549 ctx.phi_dead = zend_arena_alloc(&optimizer_ctx->arena, ctx.phi_worklist_len * sizeof(zend_ulong));
550 memset(ctx.phi_dead, 0xff, sizeof(zend_ulong) * ctx.phi_worklist_len);
551
552 /* Mark non-CV phis as live. Even if the result is unused, we generally cannot remove one
553 * of the producing instructions, as it combines producing the result with control flow.
554 * This can be made more precise if there are any cases where this is not the case. */
555 FOREACH_PHI(phi) {
556 if (phi->var >= op_array->last_var
557 && may_be_refcounted(ssa->var_info[phi->ssa_var].type)) {
558 zend_bitset_excl(ctx.phi_dead, phi->ssa_var);
559 add_phi_sources_to_worklists(&ctx, phi, 0);
560 }
561 } FOREACH_PHI_END();
562
563 /* Mark reachable instruction without side effects as dead */
564 int b = ssa->cfg.blocks_count;
565 while (b > 0) {
566 int op_data = -1;
567
568 b--;
569 zend_basic_block *block = &ssa->cfg.blocks[b];
570 if (!(block->flags & ZEND_BB_REACHABLE)) {
571 continue;
572 }
573 i = block->start + block->len;
574 while (i > block->start) {
575 i--;
576
577 if (op_array->opcodes[i].opcode == ZEND_OP_DATA) {
578 op_data = i;
579 continue;
580 }
581
582 if (zend_bitset_in(ctx.instr_worklist, i)) {
583 zend_bitset_excl(ctx.instr_worklist, i);
584 add_operands_to_worklists(&ctx, &op_array->opcodes[i], &ssa->ops[i], ssa, 0);
585 if (op_data >= 0) {
586 add_operands_to_worklists(&ctx, &op_array->opcodes[op_data], &ssa->ops[op_data], ssa, 0);
587 }
588 } else if (may_have_side_effects(op_array, ssa, &op_array->opcodes[i], &ssa->ops[i], ctx.reorder_dtor_effects)
589 || (zend_may_throw(&op_array->opcodes[i], &ssa->ops[i], op_array, ssa)
590 && !may_throw_dce_exception(&op_array->opcodes[i]))
591 || (has_varargs && may_break_varargs(op_array, ssa, &ssa->ops[i]))) {
592 if (op_array->opcodes[i].opcode == ZEND_NEW
593 && op_array->opcodes[i+1].opcode == ZEND_DO_FCALL
594 && ssa->ops[i].result_def >= 0
595 && ssa->vars[ssa->ops[i].result_def].escape_state == ESCAPE_STATE_NO_ESCAPE) {
596 zend_bitset_incl(ctx.instr_dead, i);
597 zend_bitset_incl(ctx.instr_dead, i+1);
598 } else {
599 add_operands_to_worklists(&ctx, &op_array->opcodes[i], &ssa->ops[i], ssa, 0);
600 if (op_data >= 0) {
601 add_operands_to_worklists(&ctx, &op_array->opcodes[op_data], &ssa->ops[op_data], ssa, 0);
602 }
603 }
604 } else {
605 zend_bitset_incl(ctx.instr_dead, i);
606 if (op_data >= 0) {
607 zend_bitset_incl(ctx.instr_dead, op_data);
608 }
609 }
610 op_data = -1;
611 }
612 }
613
614 /* Propagate liveness backwards to all definitions of used vars */
615 while (!zend_bitset_empty(ctx.instr_worklist, ctx.instr_worklist_len)
616 || !zend_bitset_empty(ctx.phi_worklist, ctx.phi_worklist_len)) {
617 while ((i = zend_bitset_pop_first(ctx.instr_worklist, ctx.instr_worklist_len)) >= 0) {
618 zend_bitset_excl(ctx.instr_dead, i);
619 add_operands_to_worklists(&ctx, &op_array->opcodes[i], &ssa->ops[i], ssa, 1);
620 if (i < op_array->last
621 && (op_array->opcodes[i+1].opcode == ZEND_OP_DATA
622 || (op_array->opcodes[i].opcode == ZEND_NEW
623 && op_array->opcodes[i+1].opcode == ZEND_DO_FCALL))) {
624 zend_bitset_excl(ctx.instr_dead, i+1);
625 add_operands_to_worklists(&ctx, &op_array->opcodes[i+1], &ssa->ops[i+1], ssa, 1);
626 }
627 }
628 while ((i = zend_bitset_pop_first(ctx.phi_worklist, ctx.phi_worklist_len)) >= 0) {
629 zend_bitset_excl(ctx.phi_dead, i);
630 zend_bitset_excl(ctx.phi_worklist_no_val, i);
631 add_phi_sources_to_worklists(&ctx, ssa->vars[i].definition_phi, 1);
632 }
633 }
634
635 /* Eliminate dead instructions */
636 ZEND_BITSET_FOREACH(ctx.instr_dead, ctx.instr_worklist_len, i) {
637 removed_ops += dce_instr(&ctx, &op_array->opcodes[i], &ssa->ops[i]);
638 } ZEND_BITSET_FOREACH_END();
639
640 /* Improper uses don't count as "uses" for the purpose of instruction elimination,
641 * but we have to retain phis defining them.
642 * Propagate this information backwards, marking any phi with an improperly used
643 * target as non-dead. */
644 while ((i = zend_bitset_pop_first(ctx.phi_worklist_no_val, ctx.phi_worklist_len)) >= 0) {
645 zend_ssa_phi *phi = ssa->vars[i].definition_phi;
646 int source;
647 zend_bitset_excl(ctx.phi_dead, i);
648 FOREACH_PHI_SOURCE(phi, source) {
649 add_to_phi_worklist_no_val(&ctx, source);
650 } FOREACH_PHI_SOURCE_END();
651 }
652
653 /* Now collect the actually dead phis */
654 FOREACH_PHI(phi) {
655 if (zend_bitset_in(ctx.phi_dead, phi->ssa_var)) {
656 zend_ssa_remove_uses_of_var(ssa, phi->ssa_var);
657 zend_ssa_remove_phi(ssa, phi);
658 } else {
659 /* Remove trivial phis (phis with identical source operands) */
660 try_remove_trivial_phi(&ctx, phi);
661 }
662 } FOREACH_PHI_END();
663
664 zend_arena_release(&optimizer_ctx->arena, checkpoint);
665
666 return removed_ops;
667 }
668
