1 /*
2 +----------------------------------------------------------------------+
3 | Zend Engine, Sparse Conditional Data Flow Propagation Framework |
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 +----------------------------------------------------------------------+
17 */
18
19 #include "Optimizer/zend_optimizer_internal.h"
20 #include "Optimizer/scdf.h"
21
22 /* This defines a generic framework for sparse conditional dataflow propagation. The algorithm is
23 * based on "Sparse conditional constant propagation" by Wegman and Zadeck. We're using a
24 * generalized implementation as described in chapter 8.3 of the SSA book.
25 *
26 * Every SSA variable is associated with an element on a finite-height lattice, those value can only
27 * ever be lowered during the operation of the algorithm. If a value is lowered all instructions and
28 * phis using that value need to be reconsidered (this is done by adding the variable to a
29 * worklist). For phi functions the result is computed by applying the meet operation to the
30 * operands. This continues until a fixed point is reached.
31 *
32 * The algorithm is control-flow sensitive: All blocks except the start block are initially assumed
33 * to be unreachable. When considering a branch instruction, we determine the feasible successors
34 * based on the current state of the variable lattice. If a new edge becomes feasible we either have
35 * to mark the successor block executable and consider all instructions in it, or, if the target is
36 * already executable, we only have to reconsider the phi functions (as we only consider phi
37 * operands which are associated with a feasible edge).
38 *
39 * The generic framework requires the definition of three functions:
40 * * visit_instr() should recompute the lattice values of all SSA variables defined by an
41 * instruction.
42 * * visit_phi() should recompute the lattice value of the SSA variable defined by the phi. While
43 * doing this it should only consider operands for which scfg_is_edge_feasible() returns true.
44 * * get_feasible_successors() should determine the feasible successors for a branch instruction.
45 * Note that this callback only needs to handle conditional branches (with two successors).
46 */
47
48 #if 0
49 #define DEBUG_PRINT(...) fprintf(stderr, __VA_ARGS__)
50 #else
51 #define DEBUG_PRINT(...)
52 #endif
53
scdf_mark_edge_feasible(scdf_ctx * scdf,int from,int to)54 void scdf_mark_edge_feasible(scdf_ctx *scdf, int from, int to) {
55 uint32_t edge = scdf_edge(&scdf->ssa->cfg, from, to);
56
57 if (zend_bitset_in(scdf->feasible_edges, edge)) {
58 /* We already handled this edge */
59 return;
60 }
61
62 DEBUG_PRINT("Marking edge %d->%d feasible\n", from, to);
63 zend_bitset_incl(scdf->feasible_edges, edge);
64
65 if (!zend_bitset_in(scdf->executable_blocks, to)) {
66 if (!zend_bitset_in(scdf->block_worklist, to)) {
67 DEBUG_PRINT("Adding block %d to worklist\n", to);
68 }
69 zend_bitset_incl(scdf->block_worklist, to);
70 } else {
71 /* Block is already executable, only a new edge became feasible.
72 * Reevaluate phi nodes to account for changed source operands. */
73 zend_ssa_block *ssa_block = &scdf->ssa->blocks[to];
74 zend_ssa_phi *phi;
75 for (phi = ssa_block->phis; phi; phi = phi->next) {
76 zend_bitset_excl(scdf->phi_var_worklist, phi->ssa_var);
77 scdf->handlers.visit_phi(scdf, phi);
78 }
79 }
80 }
81
scdf_init(zend_optimizer_ctx * ctx,scdf_ctx * scdf,zend_op_array * op_array,zend_ssa * ssa)82 void scdf_init(zend_optimizer_ctx *ctx, scdf_ctx *scdf, zend_op_array *op_array, zend_ssa *ssa) {
83 scdf->op_array = op_array;
84 scdf->ssa = ssa;
85
86 scdf->instr_worklist_len = zend_bitset_len(op_array->last);
87 scdf->phi_var_worklist_len = zend_bitset_len(ssa->vars_count);
88 scdf->block_worklist_len = zend_bitset_len(ssa->cfg.blocks_count);
89
90 scdf->instr_worklist = zend_arena_calloc(&ctx->arena,
91 scdf->instr_worklist_len + scdf->phi_var_worklist_len + 2 * scdf->block_worklist_len + zend_bitset_len(ssa->cfg.edges_count),
92 sizeof(zend_ulong));
93
94 scdf->phi_var_worklist = scdf->instr_worklist + scdf->instr_worklist_len;
95 scdf->block_worklist = scdf->phi_var_worklist + scdf->phi_var_worklist_len;
96 scdf->executable_blocks = scdf->block_worklist + scdf->block_worklist_len;
97 scdf->feasible_edges = scdf->executable_blocks + scdf->block_worklist_len;
98
99 zend_bitset_incl(scdf->block_worklist, 0);
100 zend_bitset_incl(scdf->executable_blocks, 0);
101 }
102
scdf_solve(scdf_ctx * scdf,const char * name)103 void scdf_solve(scdf_ctx *scdf, const char *name) {
104 zend_ssa *ssa = scdf->ssa;
105 DEBUG_PRINT("Start SCDF solve (%s)\n", name);
106 while (!zend_bitset_empty(scdf->instr_worklist, scdf->instr_worklist_len)
107 || !zend_bitset_empty(scdf->phi_var_worklist, scdf->phi_var_worklist_len)
108 || !zend_bitset_empty(scdf->block_worklist, scdf->block_worklist_len)
109 ) {
110 int i;
111 while ((i = zend_bitset_pop_first(scdf->phi_var_worklist, scdf->phi_var_worklist_len)) >= 0) {
112 zend_ssa_phi *phi = ssa->vars[i].definition_phi;
113 ZEND_ASSERT(phi);
114 if (zend_bitset_in(scdf->executable_blocks, phi->block)) {
115 scdf->handlers.visit_phi(scdf, phi);
116 }
117 }
118
119 while ((i = zend_bitset_pop_first(scdf->instr_worklist, scdf->instr_worklist_len)) >= 0) {
120 int block_num = ssa->cfg.map[i];
121 if (zend_bitset_in(scdf->executable_blocks, block_num)) {
122 zend_basic_block *block = &ssa->cfg.blocks[block_num];
123 zend_op *opline = &scdf->op_array->opcodes[i];
124 zend_ssa_op *ssa_op = &ssa->ops[i];
125 if (opline->opcode == ZEND_OP_DATA) {
126 opline--;
127 ssa_op--;
128 }
129 scdf->handlers.visit_instr(scdf, opline, ssa_op);
130 if (i == block->start + block->len - 1) {
131 if (block->successors_count == 1) {
132 scdf_mark_edge_feasible(scdf, block_num, block->successors[0]);
133 } else if (block->successors_count > 1) {
134 scdf->handlers.mark_feasible_successors(scdf, block_num, block, opline, ssa_op);
135 }
136 }
137 }
138 }
139
140 while ((i = zend_bitset_pop_first(scdf->block_worklist, scdf->block_worklist_len)) >= 0) {
141 /* This block is now live. Interpret phis and instructions in it. */
142 zend_basic_block *block = &ssa->cfg.blocks[i];
143 zend_ssa_block *ssa_block = &ssa->blocks[i];
144
145 DEBUG_PRINT("Pop block %d from worklist\n", i);
146 zend_bitset_incl(scdf->executable_blocks, i);
147
148 {
149 zend_ssa_phi *phi;
150 for (phi = ssa_block->phis; phi; phi = phi->next) {
151 zend_bitset_excl(scdf->phi_var_worklist, phi->ssa_var);
152 scdf->handlers.visit_phi(scdf, phi);
153 }
154 }
155
156 if (block->len == 0) {
157 /* Zero length blocks don't have a last instruction that would normally do this */
158 scdf_mark_edge_feasible(scdf, i, block->successors[0]);
159 } else {
160 zend_op *opline = NULL;
161 int j, end = block->start + block->len;
162 for (j = block->start; j < end; j++) {
163 opline = &scdf->op_array->opcodes[j];
164 zend_bitset_excl(scdf->instr_worklist, j);
165 if (opline->opcode != ZEND_OP_DATA) {
166 scdf->handlers.visit_instr(scdf, opline, &ssa->ops[j]);
167 }
168 }
169 if (block->successors_count == 1) {
170 scdf_mark_edge_feasible(scdf, i, block->successors[0]);
171 } else if (block->successors_count > 1) {
172 ZEND_ASSERT(opline && "Should have opline in non-empty block");
173 if (opline->opcode == ZEND_OP_DATA) {
174 opline--;
175 j--;
176 }
177 scdf->handlers.mark_feasible_successors(scdf, i, block, opline, &ssa->ops[j-1]);
178 }
179 }
180 }
181 }
182 }
183
184 /* If a live range starts in a reachable block and ends in an unreachable block, we should
185 * not eliminate the latter. While it cannot be reached, the FREE opcode of the loop var
186 * is necessary for the correctness of temporary compaction. */
is_live_loop_var_free(scdf_ctx * scdf,const zend_op * opline,const zend_ssa_op * ssa_op)187 static bool is_live_loop_var_free(
188 scdf_ctx *scdf, const zend_op *opline, const zend_ssa_op *ssa_op) {
189 if (!zend_optimizer_is_loop_var_free(opline)) {
190 return false;
191 }
192
193 int var = ssa_op->op1_use;
194 if (var < 0) {
195 return false;
196 }
197
198 zend_ssa_var *ssa_var = &scdf->ssa->vars[var];
199 uint32_t def_block;
200 if (ssa_var->definition >= 0) {
201 def_block = scdf->ssa->cfg.map[ssa_var->definition];
202 } else {
203 def_block = ssa_var->definition_phi->block;
204 }
205 return zend_bitset_in(scdf->executable_blocks, def_block);
206 }
207
kept_alive_by_loop_var_free(scdf_ctx * scdf,const zend_basic_block * block)208 static bool kept_alive_by_loop_var_free(scdf_ctx *scdf, const zend_basic_block *block) {
209 const zend_op_array *op_array = scdf->op_array;
210 const zend_cfg *cfg = &scdf->ssa->cfg;
211 if (!(cfg->flags & ZEND_FUNC_FREE_LOOP_VAR)) {
212 return false;
213 }
214
215 for (uint32_t i = block->start; i < block->start + block->len; i++) {
216 if (is_live_loop_var_free(scdf, &op_array->opcodes[i], &scdf->ssa->ops[i])) {
217 return true;
218 }
219 }
220 return false;
221 }
222
cleanup_loop_var_free_block(scdf_ctx * scdf,zend_basic_block * block)223 static uint32_t cleanup_loop_var_free_block(scdf_ctx *scdf, zend_basic_block *block) {
224 zend_ssa *ssa = scdf->ssa;
225 const zend_op_array *op_array = scdf->op_array;
226 const zend_cfg *cfg = &ssa->cfg;
227 int block_num = block - cfg->blocks;
228 uint32_t removed_ops = 0;
229
230 /* Removes phi nodes */
231 for (zend_ssa_phi *phi = ssa->blocks[block_num].phis; phi; phi = phi->next) {
232 zend_ssa_remove_uses_of_var(ssa, phi->ssa_var);
233 zend_ssa_remove_phi(ssa, phi);
234 }
235
236 for (uint32_t i = block->start; i < block->start + block->len; i++) {
237 zend_op *opline = &op_array->opcodes[i];
238 zend_ssa_op *ssa_op = &scdf->ssa->ops[i];
239 if (opline->opcode == ZEND_NOP
240 || is_live_loop_var_free(scdf, opline, ssa_op)) {
241 continue;
242 }
243
244 /* While we have to preserve the loop var free, we can still remove other instructions
245 * in the block. */
246 zend_ssa_remove_defs_of_instr(ssa, ssa_op);
247 zend_ssa_remove_instr(ssa, opline, ssa_op);
248 removed_ops++;
249 }
250
251 zend_ssa_remove_block_from_cfg(ssa, block_num);
252
253 return removed_ops;
254 }
255
256 /* Removes unreachable blocks. This will remove both the instructions (and phis) in the
257 * blocks, as well as remove them from the successor / predecessor lists and mark them
258 * unreachable. Blocks already marked unreachable are not removed. */
scdf_remove_unreachable_blocks(scdf_ctx * scdf)259 uint32_t scdf_remove_unreachable_blocks(scdf_ctx *scdf) {
260 zend_ssa *ssa = scdf->ssa;
261 int i;
262 uint32_t removed_ops = 0;
263 for (i = 0; i < ssa->cfg.blocks_count; i++) {
264 zend_basic_block *block = &ssa->cfg.blocks[i];
265 if (!zend_bitset_in(scdf->executable_blocks, i) && (block->flags & ZEND_BB_REACHABLE)) {
266 if (!kept_alive_by_loop_var_free(scdf, block)) {
267 removed_ops += block->len;
268 zend_ssa_remove_block(scdf->op_array, ssa, i);
269 } else {
270 removed_ops += cleanup_loop_var_free_block(scdf, block);
271 }
272 }
273 }
274 return removed_ops;
275 }
276
