1 /* Optimization of PHI nodes by converting them into straightline code.
2 Copyright (C) 2004 Free Software Foundation, Inc.
4 This file is part of GCC.
6 GCC is free software; you can redistribute it and/or modify it
7 under the terms of the GNU General Public License as published by the
8 Free Software Foundation; either version 2, or (at your option) any
11 GCC is distributed in the hope that it will be useful, but WITHOUT
12 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
16 You should have received a copy of the GNU General Public License
17 along with GCC; see the file COPYING. If not, write to the Free
18 Software Foundation, 59 Temple Place - Suite 330, Boston, MA
23 #include "coretypes.h"
31 #include "basic-block.h"
33 #include "diagnostic.h"
34 #include "tree-flow.h"
35 #include "tree-pass.h"
36 #include "tree-dump.h"
37 #include "langhooks.h"
39 static void tree_ssa_phiopt (void);
40 static bool conditional_replacement (basic_block, basic_block, basic_block,
41 edge, edge, tree, tree, tree);
42 static bool value_replacement (basic_block, basic_block, basic_block,
43 edge, edge, tree, tree, tree);
44 static bool abs_replacement (basic_block, basic_block, basic_block,
45 edge, edge, tree, tree, tree);
46 static void replace_phi_edge_with_variable (basic_block, basic_block, edge,
49 /* This pass eliminates PHI nodes which can be trivially implemented as
50 an assignment from a conditional expression. i.e. if we have something
54 if (cond) goto bb2; else goto bb1;
57 x = PHI (0 (bb1), 1 (bb0)
59 We can rewrite that as:
66 bb1 will become unreachable and bb0 and bb2 will almost always
67 be merged into a single block. This occurs often due to gimplification
70 Also done is the following optimization:
73 if (a != b) goto bb2; else goto bb1;
76 x = PHI (a (bb1), b (bb0))
78 We can rewrite that as:
85 This can sometimes occur as a result of other optimizations. A
86 similar transformation is done by the ifcvt RTL optimizer.
88 This pass also eliminates PHI nodes which are really absolute
89 values. i.e. if we have something like:
92 if (a >= 0) goto bb2; else goto bb1;
96 x = PHI (x (bb1), a (bb0));
98 We can rewrite that as:
105 bb1 will become unreachable and bb0 and bb2 will almost always be merged
106 into a single block. Similar transformations are done by the ifcvt
110 tree_ssa_phiopt (void)
113 bool removed_phis = false;
115 /* Search every basic block for COND_EXPR we may be able to optimize in reverse
116 order so we can find more. */
117 FOR_EACH_BB_REVERSE (bb)
121 basic_block bb1, bb2;
124 cond_expr = last_stmt (bb);
125 /* Check to see if the last statement is a COND_EXPR */
127 || TREE_CODE (cond_expr) != COND_EXPR)
130 e1 = EDGE_SUCC (bb, 0);
132 e2 = EDGE_SUCC (bb, 1);
135 /* We cannot do the optimization on abnormal edges. */
136 if ((e1->flags & EDGE_ABNORMAL) != 0
137 || (e2->flags & EDGE_ABNORMAL) != 0)
140 /* If either bb1's succ or bb2 or bb2's succ is non NULL. */
141 if (EDGE_COUNT (bb1->succs) < 1
143 || EDGE_COUNT (bb2->succs) < 1)
146 /* Find the bb which is the fall through to the other. */
147 if (EDGE_SUCC (bb1, 0)->dest == bb2)
149 else if (EDGE_SUCC (bb2, 0)->dest == bb1)
151 basic_block bb_tmp = bb1;
161 e1 = EDGE_SUCC (bb1, 0);
163 /* Make sure that bb1 is just a fall through. */
164 if (EDGE_COUNT (bb1->succs) > 1
165 || (e1->flags & EDGE_FALLTHRU) == 0)
168 /* Also make that bb1 only have one pred and it is bb. */
169 if (EDGE_COUNT (bb1->preds) > 1
170 || EDGE_PRED (bb1, 0)->src != bb)
173 phi = phi_nodes (bb2);
175 /* Check to make sure that there is only one PHI node.
176 TODO: we could do it with more than one iff the other PHI nodes
177 have the same elements for these two edges. */
178 if (phi && PHI_CHAIN (phi) == NULL)
180 tree arg0 = NULL, arg1 = NULL;
183 arg0 = PHI_ARG_DEF_TREE (phi, e1->dest_idx);
184 arg1 = PHI_ARG_DEF_TREE (phi, e2->dest_idx);
186 /* We know something is wrong if we cannot find the edges in the PHI
188 gcc_assert (arg0 != NULL && arg1 != NULL);
190 /* Do the replacement of conditional if it can be done. */
191 if (conditional_replacement (bb, bb1, bb2, e1, e2, phi, arg0, arg1)
192 || value_replacement (bb, bb1, bb2, e1, e2, phi, arg0, arg1)
193 || abs_replacement (bb, bb1, bb2, e1, e2, phi, arg0, arg1))
195 /* We have done the replacement so we need to rebuild the
196 cfg when this pass is complete. */
203 /* Return TRUE if block BB has no executable statements, otherwise return
206 empty_block_p (basic_block bb)
208 block_stmt_iterator bsi;
210 /* BB must have no executable statements. */
211 bsi = bsi_start (bb);
212 while (!bsi_end_p (bsi)
213 && (TREE_CODE (bsi_stmt (bsi)) == LABEL_EXPR
214 || IS_EMPTY_STMT (bsi_stmt (bsi))))
217 if (!bsi_end_p (bsi))
223 /* Replace PHI node element whoes edge is E in block BB with variable NEW.
224 Remove the edge from COND_BLOCK which does not lead to BB (COND_BLOCK
225 is known to have two edges, one of which must reach BB). */
228 replace_phi_edge_with_variable (basic_block cond_block, basic_block bb,
229 edge e, tree phi, tree new)
231 basic_block block_to_remove;
233 block_stmt_iterator bsi;
235 /* Change the PHI argument to new. */
236 PHI_ARG_DEF_TREE (phi, e->dest_idx) = new;
238 /* Remove the empty basic block. */
239 if (EDGE_SUCC (cond_block, 0)->dest == bb)
241 EDGE_SUCC (cond_block, 0)->flags |= EDGE_FALLTHRU;
242 EDGE_SUCC (cond_block, 0)->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
244 block_to_remove = EDGE_SUCC (cond_block, 1)->dest;
248 EDGE_SUCC (cond_block, 1)->flags |= EDGE_FALLTHRU;
249 EDGE_SUCC (cond_block, 1)->flags
250 &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
252 block_to_remove = EDGE_SUCC (cond_block, 0)->dest;
254 delete_basic_block (block_to_remove);
256 /* Eliminate the COND_EXPR at the end of COND_BLOCK. */
257 bsi = bsi_last (cond_block);
260 if (dump_file && (dump_flags & TDF_DETAILS))
262 "COND_EXPR in block %d and PHI in block %d converted to straightline code.\n",
267 /* The function conditional_replacement does the main work of doing the
268 conditional replacement. Return true if the replacement is done.
269 Otherwise return false.
270 BB is the basic block where the replacement is going to be done on. ARG0
271 is argument 0 from PHI. Likewise for ARG1. */
274 conditional_replacement (basic_block cond_bb, basic_block middle_bb,
275 basic_block phi_bb, edge e0, edge e1, tree phi,
276 tree arg0, tree arg1)
279 tree old_result = NULL;
281 block_stmt_iterator bsi;
282 edge true_edge, false_edge;
286 /* The PHI arguments have the constants 0 and 1, then convert
287 it to the conditional. */
288 if ((integer_zerop (arg0) && integer_onep (arg1))
289 || (integer_zerop (arg1) && integer_onep (arg0)))
294 if (!empty_block_p (middle_bb))
297 /* If the condition is not a naked SSA_NAME and its type does not
298 match the type of the result, then we have to create a new
299 variable to optimize this case as it would likely create
300 non-gimple code when the condition was converted to the
302 cond = COND_EXPR_COND (last_stmt (cond_bb));
303 result = PHI_RESULT (phi);
304 if (TREE_CODE (cond) != SSA_NAME
305 && !lang_hooks.types_compatible_p (TREE_TYPE (cond), TREE_TYPE (result)))
307 new_var = make_rename_temp (TREE_TYPE (cond), NULL);
312 /* If the condition was a naked SSA_NAME and the type is not the
313 same as the type of the result, then convert the type of the
315 if (!lang_hooks.types_compatible_p (TREE_TYPE (cond), TREE_TYPE (result)))
316 cond = fold_convert (TREE_TYPE (result), cond);
318 /* We need to know which is the true edge and which is the false
319 edge so that we know when to invert the condition below. */
320 extract_true_false_edges_from_block (cond_bb, &true_edge, &false_edge);
322 /* Insert our new statement at the end of condtional block before the
324 bsi = bsi_last (cond_bb);
325 bsi_insert_before (&bsi, build_empty_stmt (), BSI_NEW_STMT);
330 if (!COMPARISON_CLASS_P (old_result))
333 new1 = build (TREE_CODE (old_result), TREE_TYPE (old_result),
334 TREE_OPERAND (old_result, 0),
335 TREE_OPERAND (old_result, 1));
337 new1 = build (MODIFY_EXPR, TREE_TYPE (old_result), new_var, new1);
338 bsi_insert_after (&bsi, new1, BSI_NEW_STMT);
341 new_var1 = duplicate_ssa_name (PHI_RESULT (phi), NULL);
344 /* At this point we know we have a COND_EXPR with two successors.
345 One successor is BB, the other successor is an empty block which
346 falls through into BB.
348 There is a single PHI node at the join point (BB) and its arguments
349 are constants (0, 1).
351 So, given the condition COND, and the two PHI arguments, we can
352 rewrite this PHI into non-branching code:
354 dest = (COND) or dest = COND'
356 We use the condition as-is if the argument associated with the
357 true edge has the value one or the argument associated with the
358 false edge as the value zero. Note that those conditions are not
359 the same since only one of the outgoing edges from the COND_EXPR
360 will directly reach BB and thus be associated with an argument. */
361 if ((e0 == true_edge && integer_onep (arg0))
362 || (e0 == false_edge && integer_zerop (arg0))
363 || (e1 == true_edge && integer_onep (arg1))
364 || (e1 == false_edge && integer_zerop (arg1)))
366 new = build (MODIFY_EXPR, TREE_TYPE (new_var1), new_var1, cond);
370 tree cond1 = invert_truthvalue (cond);
373 /* If what we get back is a conditional expression, there is no
374 way that it can be gimple. */
375 if (TREE_CODE (cond) == COND_EXPR)
377 release_ssa_name (new_var1);
381 /* If what we get back is not gimple try to create it as gimple by
382 using a temporary variable. */
383 if (is_gimple_cast (cond)
384 && !is_gimple_val (TREE_OPERAND (cond, 0)))
386 tree temp = TREE_OPERAND (cond, 0);
387 tree new_var_1 = make_rename_temp (TREE_TYPE (temp), NULL);
388 new = build (MODIFY_EXPR, TREE_TYPE (new_var_1), new_var_1, temp);
389 bsi_insert_after (&bsi, new, BSI_NEW_STMT);
390 cond = fold_convert (TREE_TYPE (result), new_var_1);
393 if (TREE_CODE (cond) == TRUTH_NOT_EXPR
394 && !is_gimple_val (TREE_OPERAND (cond, 0)))
396 release_ssa_name (new_var1);
400 new = build (MODIFY_EXPR, TREE_TYPE (new_var1), new_var1, cond);
403 bsi_insert_after (&bsi, new, BSI_NEW_STMT);
405 SSA_NAME_DEF_STMT (new_var1) = new;
407 replace_phi_edge_with_variable (cond_bb, phi_bb, e1, phi, new_var1);
409 /* Note that we optimized this PHI. */
413 /* The function value_replacement does the main work of doing the value
414 replacement. Return true if the replacement is done. Otherwise return
416 BB is the basic block where the replacement is going to be done on. ARG0
417 is argument 0 from the PHI. Likewise for ARG1. */
420 value_replacement (basic_block cond_bb, basic_block middle_bb,
421 basic_block phi_bb, edge e0, edge e1, tree phi,
422 tree arg0, tree arg1)
426 edge true_edge, false_edge;
428 /* If the type says honor signed zeros we cannot do this
430 if (HONOR_SIGNED_ZEROS (TYPE_MODE (TREE_TYPE (arg1))))
433 if (!empty_block_p (middle_bb))
436 cond = COND_EXPR_COND (last_stmt (cond_bb));
437 result = PHI_RESULT (phi);
439 /* This transformation is only valid for equality comparisons. */
440 if (TREE_CODE (cond) != NE_EXPR && TREE_CODE (cond) != EQ_EXPR)
443 /* We need to know which is the true edge and which is the false
444 edge so that we know if have abs or negative abs. */
445 extract_true_false_edges_from_block (cond_bb, &true_edge, &false_edge);
447 /* At this point we know we have a COND_EXPR with two successors.
448 One successor is BB, the other successor is an empty block which
449 falls through into BB.
451 The condition for the COND_EXPR is known to be NE_EXPR or EQ_EXPR.
453 There is a single PHI node at the join point (BB) with two arguments.
455 We now need to verify that the two arguments in the PHI node match
456 the two arguments to the equality comparison. */
458 if ((operand_equal_for_phi_arg_p (arg0, TREE_OPERAND (cond, 0))
459 && operand_equal_for_phi_arg_p (arg1, TREE_OPERAND (cond, 1)))
460 || (operand_equal_for_phi_arg_p (arg1, TREE_OPERAND (cond, 0))
461 && operand_equal_for_phi_arg_p (arg0, TREE_OPERAND (cond, 1))))
466 /* For NE_EXPR, we want to build an assignment result = arg where
467 arg is the PHI argument associated with the true edge. For
468 EQ_EXPR we want the PHI argument associated with the false edge. */
469 e = (TREE_CODE (cond) == NE_EXPR ? true_edge : false_edge);
471 /* Unfortunately, E may not reach BB (it may instead have gone to
472 OTHER_BLOCK). If that is the case, then we want the single outgoing
473 edge from OTHER_BLOCK which reaches BB and represents the desired
474 path from COND_BLOCK. */
475 if (e->dest == middle_bb)
476 e = EDGE_SUCC (e->dest, 0);
478 /* Now we know the incoming edge to BB that has the argument for the
479 RHS of our new assignment statement. */
485 replace_phi_edge_with_variable (cond_bb, phi_bb, e1, phi, arg);
487 /* Note that we optimized this PHI. */
493 /* The function absolute_replacement does the main work of doing the absolute
494 replacement. Return true if the replacement is done. Otherwise return
496 bb is the basic block where the replacement is going to be done on. arg0
497 is argument 0 from the phi. Likewise for arg1. */
500 abs_replacement (basic_block cond_bb, basic_block middle_bb,
501 basic_block phi_bb, edge e0 ATTRIBUTE_UNUSED, edge e1, tree phi,
502 tree arg0, tree arg1)
506 block_stmt_iterator bsi;
507 edge true_edge, false_edge;
510 tree rhs = NULL, lhs = NULL;
512 enum tree_code cond_code;
514 /* If the type says honor signed zeros we cannot do this
516 if (HONOR_SIGNED_ZEROS (TYPE_MODE (TREE_TYPE (arg1))))
519 /* OTHER_BLOCK must have only one executable statement which must have the
520 form arg0 = -arg1 or arg1 = -arg0. */
521 bsi = bsi_start (middle_bb);
522 while (!bsi_end_p (bsi))
524 tree stmt = bsi_stmt (bsi);
526 /* Empty statements and labels are uninteresting. */
527 if (TREE_CODE (stmt) == LABEL_EXPR
528 || IS_EMPTY_STMT (stmt))
534 /* If we found the assignment, but it was not the only executable
535 statement in OTHER_BLOCK, then we can not optimize. */
539 /* If we got here, then we have found the first executable statement
540 in OTHER_BLOCK. If it is anything other than arg = -arg1 or
541 arg1 = -arg0, then we can not optimize. */
542 if (TREE_CODE (stmt) == MODIFY_EXPR)
544 lhs = TREE_OPERAND (stmt, 0);
545 rhs = TREE_OPERAND (stmt, 1);
547 if (TREE_CODE (rhs) == NEGATE_EXPR)
549 rhs = TREE_OPERAND (rhs, 0);
551 /* The assignment has to be arg0 = -arg1 or arg1 = -arg0. */
552 if ((lhs == arg0 && rhs == arg1)
553 || (lhs == arg1 && rhs == arg0))
568 /* If we did not find the proper negation assignment, then we can not
573 cond = COND_EXPR_COND (last_stmt (cond_bb));
574 result = PHI_RESULT (phi);
576 /* Only relationals comparing arg[01] against zero are interesting. */
577 cond_code = TREE_CODE (cond);
578 if (cond_code != GT_EXPR && cond_code != GE_EXPR
579 && cond_code != LT_EXPR && cond_code != LE_EXPR)
582 /* Make sure the conditional is arg[01] OP y. */
583 if (TREE_OPERAND (cond, 0) != rhs)
586 if (FLOAT_TYPE_P (TREE_TYPE (TREE_OPERAND (cond, 1)))
587 ? real_zerop (TREE_OPERAND (cond, 1))
588 : integer_zerop (TREE_OPERAND (cond, 1)))
593 /* We need to know which is the true edge and which is the false
594 edge so that we know if have abs or negative abs. */
595 extract_true_false_edges_from_block (cond_bb, &true_edge, &false_edge);
597 /* For GT_EXPR/GE_EXPR, if the true edge goes to OTHER_BLOCK, then we
598 will need to negate the result. Similarly for LT_EXPR/LE_EXPR if
599 the false edge goes to OTHER_BLOCK. */
600 if (cond_code == GT_EXPR || cond_code == GE_EXPR)
605 if (e->dest == middle_bb)
610 result = duplicate_ssa_name (result, NULL);
613 lhs = make_rename_temp (TREE_TYPE (result), NULL);
617 /* Build the modify expression with abs expression. */
618 new = build (MODIFY_EXPR, TREE_TYPE (lhs),
619 lhs, build1 (ABS_EXPR, TREE_TYPE (lhs), rhs));
621 bsi = bsi_last (cond_bb);
622 bsi_insert_before (&bsi, new, BSI_NEW_STMT);
627 /* Get the right BSI. We want to insert after the recently
628 added ABS_EXPR statement (which we know is the first statement
630 new = build (MODIFY_EXPR, TREE_TYPE (result),
631 result, build1 (NEGATE_EXPR, TREE_TYPE (lhs), lhs));
633 bsi_insert_after (&bsi, new, BSI_NEW_STMT);
637 SSA_NAME_DEF_STMT (result) = new;
638 replace_phi_edge_with_variable (cond_bb, phi_bb, e1, phi, result);
640 /* Note that we optimized this PHI. */
645 /* Always do these optimizations if we have SSA
653 struct tree_opt_pass pass_phiopt =
656 gate_phiopt, /* gate */
657 tree_ssa_phiopt, /* execute */
660 0, /* static_pass_number */
661 TV_TREE_PHIOPT, /* tv_id */
662 PROP_cfg | PROP_ssa | PROP_alias, /* properties_required */
663 0, /* properties_provided */
664 0, /* properties_destroyed */
665 0, /* todo_flags_start */
666 TODO_cleanup_cfg | TODO_dump_func | TODO_ggc_collect /* todo_flags_finish */
667 | TODO_verify_ssa | TODO_rename_vars
668 | TODO_verify_flow | TODO_verify_stmts,