1 /* Perform doloop optimizations
2 Copyright (C) 1999, 2000 Free Software Foundation, Inc.
3 Contributed by Michael P. Hayes (m.hayes@elec.canterbury.ac.nz)
5 This file is part of GNU CC.
7 GNU CC is free software; you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation; either version 2, or (at your option)
12 GNU CC is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with GNU CC; see the file COPYING. If not, write to
19 the Free Software Foundation, 59 Temple Place - Suite 330,
20 Boston, MA 02111-1307, USA. */
25 #include "insn-flags.h"
29 #include "hard-reg-set.h"
30 #include "basic-block.h"
34 /* This module is used to modify loops with a determinable number of
35 iterations to use special low-overhead looping instructions.
37 It first validates whether the loop is well behaved and has a
38 determinable number of iterations (either at compile or run-time).
39 It then modifies the loop to use a low-overhead looping pattern as
42 1. A pseudo register is allocated as the loop iteration counter.
44 2. The number of loop iterations is calculated and is stored
47 3. At the end of the loop, the jump insn is replaced by the
48 doloop_end pattern. The compare must remain because it might be
49 used elsewhere. If the loop-variable or condition register are
50 used elsewhere, they will be eliminated by flow.
52 4. An optional doloop_begin pattern is inserted at the top of the
57 #ifdef HAVE_doloop_end
59 static rtx doloop_condition_get
61 static unsigned HOST_WIDE_INT doloop_iterations_max
62 PARAMS ((const struct loop_info *, enum machine_mode, int));
63 static int doloop_valid_p
64 PARAMS ((const struct loop *, rtx));
65 static int doloop_modify
66 PARAMS ((const struct loop *, rtx, rtx, rtx, rtx, rtx));
67 static int doloop_modify_runtime
68 PARAMS ((const struct loop *, rtx, rtx, rtx, enum machine_mode, rtx));
71 /* Return the loop termination condition for PATTERN or zero
72 if it is not a decrement and branch jump insn. */
74 doloop_condition_get (pattern)
82 /* The canonical doloop pattern we expect is:
84 (parallel [(set (pc) (if_then_else (condition)
87 (set (reg) (plus (reg) (const_int -1)))
88 (additional clobbers and uses)])
90 Some machines (IA-64) make the decrement conditional on
91 the condition as well, so we don't bother verifying the
92 actual decrement. In summary, the branch must be the
93 first entry of the parallel (also required by jump.c),
94 and the second entry of the parallel must be a set of
95 the loop counter register. */
97 if (GET_CODE (pattern) != PARALLEL)
100 cmp = XVECEXP (pattern, 0, 0);
101 inc = XVECEXP (pattern, 0, 1);
103 /* Check for (set (reg) (something)). */
104 if (GET_CODE (inc) != SET || ! REG_P (SET_DEST (inc)))
107 /* Extract loop counter register. */
108 reg = SET_DEST (inc);
110 /* Check for (set (pc) (if_then_else (condition)
113 if (GET_CODE (cmp) != SET
114 || SET_DEST (cmp) != pc_rtx
115 || GET_CODE (SET_SRC (cmp)) != IF_THEN_ELSE
116 || GET_CODE (XEXP (SET_SRC (cmp), 1)) != LABEL_REF
117 || XEXP (SET_SRC (cmp), 2) != pc_rtx)
120 /* Extract loop termination condition. */
121 condition = XEXP (SET_SRC (cmp), 0);
123 if ((GET_CODE (condition) != GE && GET_CODE (condition) != NE)
124 || GET_CODE (XEXP (condition, 1)) != CONST_INT)
127 if (XEXP (condition, 0) == reg)
130 if (GET_CODE (XEXP (condition, 0)) == PLUS
131 && XEXP (XEXP (condition, 0), 0) == reg)
134 /* ??? If a machine uses a funny comparison, we could return a
135 canonicalised form here. */
141 /* Return an estimate of the maximum number of loop iterations for the
142 loop specified by LOOP or zero if the loop is not normal.
143 MODE is the mode of the iteration count and NONNEG is non-zero if
144 the the iteration count has been proved to be non-negative. */
145 static unsigned HOST_WIDE_INT
146 doloop_iterations_max (loop_info, mode, nonneg)
147 const struct loop_info *loop_info;
148 enum machine_mode mode;
151 unsigned HOST_WIDE_INT n_iterations_max;
155 HOST_WIDE_INT abs_inc;
159 abs_inc = INTVAL (loop_info->increment);
168 code = swap_condition (loop_info->comparison_code);
169 min_value = loop_info->final_equiv_value;
170 max_value = loop_info->initial_equiv_value;
174 code = loop_info->comparison_code;
175 min_value = loop_info->initial_equiv_value;
176 max_value = loop_info->final_equiv_value;
179 /* Since the loop has a VTOP, we know that the initial test will be
180 true and thus the value of max_value should be greater than the
181 value of min_value. Thus the difference should always be positive
182 and the code must be LT, LE, LTU, LEU, or NE. Otherwise the loop is
183 not normal, e.g., `for (i = 0; i < 10; i--)'. */
189 unsigned HOST_WIDE_INT umax;
190 unsigned HOST_WIDE_INT umin;
192 if (GET_CODE (min_value) == CONST_INT)
193 umin = INTVAL (min_value);
197 if (GET_CODE (max_value) == CONST_INT)
198 umax = INTVAL (max_value);
200 umax = (2U << (GET_MODE_BITSIZE (mode) - 1)) - 1;
202 n_iterations_max = umax - umin;
212 if (GET_CODE (min_value) == CONST_INT)
213 smin = INTVAL (min_value);
215 smin = -(1U << (GET_MODE_BITSIZE (mode) - 1));
217 if (GET_CODE (max_value) == CONST_INT)
218 smax = INTVAL (max_value);
220 smax = (1U << (GET_MODE_BITSIZE (mode) - 1)) - 1;
222 n_iterations_max = smax - smin;
227 if (GET_CODE (min_value) == CONST_INT
228 && GET_CODE (max_value) == CONST_INT)
229 n_iterations_max = INTVAL (max_value) - INTVAL (min_value);
231 /* We need to conservatively assume that we might have the maximum
232 number of iterations without any additional knowledge. */
233 n_iterations_max = (2U << (GET_MODE_BITSIZE (mode) - 1)) - 1;
240 n_iterations_max /= abs_inc;
242 /* If we know that the iteration count is non-negative then adjust
243 n_iterations_max if it is so large that it appears negative. */
244 if (nonneg && n_iterations_max > (1U << (GET_MODE_BITSIZE (mode) - 1)))
245 n_iterations_max = (1U << (GET_MODE_BITSIZE (mode) - 1)) - 1;
247 return n_iterations_max;
251 /* Return non-zero if the loop specified by LOOP is suitable for
252 the use of special low-overhead looping instructions. */
254 doloop_valid_p (loop, jump_insn)
255 const struct loop *loop;
258 const struct loop_info *loop_info = LOOP_INFO (loop);
260 /* The loop must have a conditional jump at the end. */
261 if (! any_condjump_p (jump_insn)
262 || ! onlyjump_p (jump_insn))
264 if (loop_dump_stream)
265 fprintf (loop_dump_stream,
266 "Doloop: Invalid jump at loop end.\n");
270 /* Give up if a loop has been completely unrolled. */
271 if (loop_info->n_iterations == loop_info->unroll_number)
273 if (loop_dump_stream)
274 fprintf (loop_dump_stream,
275 "Doloop: Loop completely unrolled.\n");
279 /* The loop must have a single exit target. A break or return
280 statement within a loop will generate multiple loop exits.
281 Another example of a loop that currently generates multiple exit
282 targets is for (i = 0; i < (foo ? 8 : 4); i++) { }. */
283 if (loop_info->has_multiple_exit_targets)
285 if (loop_dump_stream)
286 fprintf (loop_dump_stream,
287 "Doloop: Loop has multiple exit targets.\n");
291 /* An indirect jump may jump out of the loop. */
292 if (loop_info->has_indirect_jump)
294 if (loop_dump_stream)
295 fprintf (loop_dump_stream,
296 "Doloop: Indirect jump in function.\n");
300 /* A called function may clobber any special registers required for
301 low-overhead looping. */
302 if (loop_info->has_call)
304 if (loop_dump_stream)
305 fprintf (loop_dump_stream,
306 "Doloop: Function call in loop.\n");
310 /* Some targets (eg, PPC) use the count register for branch on table
311 instructions. ??? This should be a target specific check. */
312 if (loop_info->has_tablejump)
314 if (loop_dump_stream)
315 fprintf (loop_dump_stream,
316 "Doloop: Computed branch in the loop.\n");
320 if (! loop_info->increment)
322 if (loop_dump_stream)
323 fprintf (loop_dump_stream,
324 "Doloop: Could not determine iteration info.\n");
328 if (GET_CODE (loop_info->increment) != CONST_INT)
330 if (loop_dump_stream)
331 fprintf (loop_dump_stream,
332 "Doloop: Increment not an integer constant.\n");
336 /* There is no guarantee that a NE loop will terminate if the
337 absolute increment is not unity. ??? We could compute this
338 condition at run-time and have a additional jump around the loop
339 to ensure an infinite loop. */
340 if (loop_info->comparison_code == NE
341 && INTVAL (loop_info->increment) != -1
342 && INTVAL (loop_info->increment) != 1)
344 if (loop_dump_stream)
345 fprintf (loop_dump_stream,
346 "Doloop: NE loop with non-unity increment.\n");
350 /* Check for loops that may not terminate under special conditions. */
351 if (! loop_info->n_iterations
352 && ((loop_info->comparison_code == LEU
353 && INTVAL (loop_info->increment) > 0)
354 || (loop_info->comparison_code == GEU
355 && INTVAL (loop_info->increment) < 0)))
357 /* If the comparison is LEU and the comparison value is UINT_MAX
358 then the loop will not terminate. Similarly, if the
359 comparison code is GEU and the initial value is 0, the loop
362 Note that with LE and GE, the loop behaviour can be
363 implementation dependent if an overflow occurs, say between
364 INT_MAX and INT_MAX + 1. We thus don't have to worry about
367 ??? We could compute these conditions at run-time and have a
368 additional jump around the loop to ensure an infinite loop.
369 However, it is very unlikely that this is the intended
370 behaviour of the loop and checking for these rare boundary
371 conditions would pessimize all other code. */
372 if (loop_dump_stream)
373 fprintf (loop_dump_stream,
374 "Doloop: Possible infinite iteration case ignored.\n");
381 /* Modify the loop to use the low-overhead looping insn where LOOP
382 describes the loop, ITERATIONS is an RTX containing the desired
383 number of loop iterations, ITERATIONS_MAX is a CONST_INT specifying
384 the maximum number of loop iterations, and DOLOOP_INSN is the
385 low-overhead looping insn to emit at the end of the loop. This
386 returns non-zero if it was successful. */
388 doloop_modify (loop, iterations, iterations_max,
389 doloop_seq, start_label, condition)
390 const struct loop *loop;
404 jump_insn = prev_nonnote_insn (loop->end);
406 if (loop_dump_stream)
408 fprintf (loop_dump_stream, "Doloop: Inserting doloop pattern (");
409 if (GET_CODE (iterations) == CONST_INT)
410 fprintf (loop_dump_stream, HOST_WIDE_INT_PRINT_DEC,
411 INTVAL (iterations));
413 fputs ("runtime", loop_dump_stream);
414 fputs (" iterations).", loop_dump_stream);
417 /* Discard original jump to continue loop. The original compare
418 result may still be live, so it cannot be discarded explicitly. */
419 delete_insn (jump_insn);
421 /* Emit the label that will delimit the start of the loop. */
422 emit_label_after (start_label, loop->start);
423 LABEL_NUSES (start_label)++;
425 counter_reg = XEXP (condition, 0);
426 if (GET_CODE (counter_reg) == PLUS)
427 counter_reg = XEXP (counter_reg, 0);
433 switch (GET_CODE (condition))
436 /* Currently only NE tests against zero and one are supported. */
437 if (XEXP (condition, 1) == const0_rtx)
439 else if (XEXP (condition, 1) != const1_rtx)
444 /* Currently only GE tests against zero are supported. */
445 if (XEXP (condition, 1) != const0_rtx)
448 /* The iteration count needs decrementing for a GE test. */
451 /* Determine if the iteration counter will be non-negative.
452 Note that the maximum value loaded is iterations_max - 1. */
453 if ((unsigned HOST_WIDE_INT) INTVAL (iterations_max)
454 <= (1U << (GET_MODE_BITSIZE (GET_MODE (counter_reg)) - 1)))
458 /* Abort if an invalid doloop pattern has been generated. */
465 if (GET_CODE (count) == CONST_INT)
466 count = GEN_INT (INTVAL (count) - 1);
468 count = expand_binop (GET_MODE (counter_reg), sub_optab,
470 0, 0, OPTAB_LIB_WIDEN);
473 /* Insert initialization of the count register into the loop header. */
474 convert_move (counter_reg, count, 1);
475 sequence = gen_sequence ();
477 emit_insn_before (sequence, loop->start);
479 /* Some targets (eg, C4x) need to initialize special looping
481 #ifdef HAVE_doloop_begin
485 init = gen_doloop_begin (counter_reg,
486 GET_CODE (iterations) == CONST_INT
487 ? iterations : const0_rtx, iterations_max,
488 GEN_INT (loop->level));
493 sequence = gen_sequence ();
495 emit_insn_after (sequence, loop->start);
500 /* Insert the new low-overhead looping insn. */
501 emit_jump_insn_before (doloop_seq, loop->end);
502 jump_insn = prev_nonnote_insn (loop->end);
503 JUMP_LABEL (jump_insn) = start_label;
505 /* Add a REG_NONNEG note if the actual or estimated maximum number
506 of iterations is non-negative. */
509 REG_NOTES (jump_insn)
510 = gen_rtx_EXPR_LIST (REG_NONNEG, NULL_RTX, REG_NOTES (jump_insn));
516 /* Handle the more complex case, where the bounds are not known at
517 compile time. In this case we generate a run_time calculation of
518 the number of iterations. We rely on the existence of a run-time
519 guard to ensure that the loop executes at least once, i.e.,
520 initial_value obeys the loop comparison condition. If a guard is
521 not present, we emit one. The loop to modify is described by LOOP.
522 ITERATIONS_MAX is a CONST_INT specifying the estimated maximum
523 number of loop iterations. DOLOOP_INSN is the low-overhead looping
524 insn to insert. Returns non-zero if loop successfully modified. */
526 doloop_modify_runtime (loop, iterations_max,
527 doloop_seq, start_label, mode, condition)
528 const struct loop *loop;
532 enum machine_mode mode;
535 const struct loop_info *loop_info = LOOP_INFO (loop);
536 HOST_WIDE_INT abs_inc;
545 enum rtx_code comparison_code;
547 increment = loop_info->increment;
548 initial_value = loop_info->initial_value;
549 final_value = loop_info->final_value;
552 abs_inc = INTVAL (increment);
559 comparison_code = loop_info->comparison_code;
560 unsigned_p = (comparison_code == LTU
561 || comparison_code == LEU
562 || comparison_code == GTU
563 || comparison_code == GEU
564 || comparison_code == NE);
566 /* The number of iterations (prior to any loop unrolling) is given by:
567 (abs (final - initial) + abs_inc - 1) / abs_inc.
569 However, it is possible for the summation to overflow, and a
572 abs (final - initial) / abs_inc + (abs (final - initial) % abs_inc) != 0
574 If the loop has been unrolled, then the loop body has been
575 preconditioned to iterate a multiple of unroll_number times.
576 The number of iterations of the loop body is simply:
577 abs (final - initial) / (abs_inc * unroll_number).
579 The division and modulo operations can be avoided by requiring
580 that the increment is a power of 2 (precondition_loop_p enforces
581 this requirement). Nevertheless, the RTX_COSTS should be checked
582 to see if a fast divmod is available. */
585 /* abs (final - initial) */
586 diff = expand_binop (mode, sub_optab,
587 copy_rtx (neg_inc ? initial_value : final_value),
588 copy_rtx (neg_inc ? final_value : initial_value),
589 NULL_RTX, unsigned_p, OPTAB_LIB_WIDEN);
591 if (loop_info->unroll_number == 1)
599 shift_count = exact_log2 (abs_inc);
603 /* abs (final - initial) / abs_inc */
604 iterations = expand_binop (GET_MODE (diff), lshr_optab,
605 diff, GEN_INT (shift_count),
609 /* abs (final - initial) % abs_inc */
610 extra = expand_binop (GET_MODE (iterations), and_optab,
611 diff, GEN_INT (abs_inc - 1),
615 /* If (abs (final - initial) % abs_inc == 0) jump past
616 following increment instruction. */
617 label = gen_label_rtx();
618 emit_cmp_and_jump_insns (extra, const0_rtx, EQ, NULL_RTX,
619 GET_MODE (extra), 0, 0, label);
620 JUMP_LABEL (get_last_insn ()) = label;
621 LABEL_NUSES (label)++;
623 /* Increment the iteration count by one. */
624 iterations = expand_binop (GET_MODE (iterations), add_optab,
625 iterations, GEN_INT (1),
638 /* precondition_loop_p has preconditioned the loop so that the
639 iteration count of the loop body is always a power of 2.
640 Since we won't get an overflow calculating the loop count,
641 the code we emit is simpler. */
642 shift_count = exact_log2 (loop_info->unroll_number * abs_inc);
646 iterations = expand_binop (GET_MODE (diff), lshr_optab,
647 diff, GEN_INT (shift_count),
653 /* If there is a NOTE_INSN_LOOP_VTOP, we have a `for' or `while'
654 style loop, with a loop exit test at the start. Thus, we can
655 assume that the loop condition was true when the loop was
658 `do-while' loops require special treatment since the exit test is
659 not executed before the start of the loop. We need to determine
660 if the loop will terminate after the first pass and to limit the
661 iteration count to one if necessary. */
666 if (loop_dump_stream)
667 fprintf (loop_dump_stream, "Doloop: Do-while loop.\n");
669 /* A `do-while' loop must iterate at least once. If the
670 iteration count is bogus, we set the iteration count to 1.
671 Note that if the loop has been unrolled, then the loop body
672 is guaranteed to execute at least once. */
673 if (loop_info->unroll_number == 1)
675 /* Emit insns to test if the loop will immediately
676 terminate and to set the iteration count to 1 if true. */
677 label = gen_label_rtx();
678 emit_cmp_and_jump_insns (copy_rtx (initial_value),
679 copy_rtx (loop_info->comparison_value),
680 comparison_code, NULL_RTX, mode, 0, 0,
682 JUMP_LABEL (get_last_insn ()) = label;
683 LABEL_NUSES (label)++;
684 emit_move_insn (iterations, const1_rtx);
689 sequence = gen_sequence ();
691 emit_insn_before (sequence, loop->start);
693 return doloop_modify (loop, iterations, iterations_max, doloop_seq,
694 start_label, condition);
698 /* This is the main entry point. Process loop described by LOOP
699 validating that the loop is suitable for conversion to use a low
700 overhead looping instruction, replacing the jump insn where
701 suitable. We distinguish between loops with compile-time bounds
702 and those with run-time bounds. Information from LOOP is used to
703 compute the number of iterations and to determine whether the loop
704 is a candidate for this optimization. Returns non-zero if loop
705 successfully modified. */
707 doloop_optimize (loop)
708 const struct loop *loop;
710 struct loop_info *loop_info = LOOP_INFO (loop);
715 enum machine_mode mode;
716 unsigned HOST_WIDE_INT n_iterations;
717 unsigned HOST_WIDE_INT n_iterations_max;
718 rtx doloop_seq, doloop_pat, doloop_reg;
724 if (loop_dump_stream)
725 fprintf (loop_dump_stream,
726 "Doloop: Processing loop %d, enclosed levels %d.\n",
727 loop->num, loop->level);
729 jump_insn = prev_nonnote_insn (loop->end);
731 /* Check that loop is a candidate for a low-overhead looping insn. */
732 if (! doloop_valid_p (loop, jump_insn))
735 /* Determine if the loop can be safely, and profitably,
736 preconditioned. While we don't precondition the loop in a loop
737 unrolling sense, this test ensures that the loop is well behaved
738 and that the increment is a constant integer. */
739 if (! precondition_loop_p (loop, &initial_value, &final_value,
742 if (loop_dump_stream)
743 fprintf (loop_dump_stream,
744 "Doloop: Cannot precondition loop.\n");
748 /* Determine or estimate the maximum number of loop iterations. */
749 n_iterations = loop_info->n_iterations;
752 /* This is the simple case where the initial and final loop
753 values are constants. */
754 n_iterations_max = n_iterations;
758 int nonneg = find_reg_note (jump_insn, REG_NONNEG, 0) != 0;
760 /* This is the harder case where the initial and final loop
761 values may not be constants. */
762 n_iterations_max = doloop_iterations_max (loop_info, mode, nonneg);
764 if (! n_iterations_max)
766 /* We have something like `for (i = 0; i < 10; i--)'. */
767 if (loop_dump_stream)
768 fprintf (loop_dump_stream,
769 "Doloop: Not normal loop.\n");
774 /* Account for loop unrolling in the iteration count. This will
775 have no effect if loop_iterations could not determine the number
777 n_iterations /= loop_info->unroll_number;
778 n_iterations_max /= loop_info->unroll_number;
780 if (n_iterations && n_iterations < 3)
782 if (loop_dump_stream)
783 fprintf (loop_dump_stream,
784 "Doloop: Too few iterations (%ld) to be profitable.\n",
785 (long int) n_iterations);
789 iterations = GEN_INT (n_iterations);
790 iterations_max = GEN_INT (n_iterations_max);
792 /* Generate looping insn. If the pattern FAILs then give up trying
793 to modify the loop since there is some aspect the back-end does
795 start_label = gen_label_rtx ();
796 doloop_reg = gen_reg_rtx (mode);
797 doloop_seq = gen_doloop_end (doloop_reg, iterations, iterations_max,
798 GEN_INT (loop->level), start_label);
799 if (! doloop_seq && mode != word_mode)
801 PUT_MODE (doloop_reg, word_mode);
802 doloop_seq = gen_doloop_end (doloop_reg, iterations, iterations_max,
803 GEN_INT (loop->level), start_label);
807 if (loop_dump_stream)
808 fprintf (loop_dump_stream,
809 "Doloop: Target unwilling to use doloop pattern!\n");
813 /* A raw define_insn may yield a plain pattern. If a sequence
814 was involved, the last must be the jump instruction. */
815 if (GET_CODE (doloop_seq) == SEQUENCE)
817 doloop_pat = XVECEXP (doloop_seq, 0, XVECLEN (doloop_seq, 0) - 1);
818 if (GET_CODE (doloop_pat) == JUMP_INSN)
819 doloop_pat = PATTERN (doloop_pat);
821 doloop_pat = NULL_RTX;
824 doloop_pat = doloop_seq;
827 || ! (condition = doloop_condition_get (doloop_pat)))
829 if (loop_dump_stream)
830 fprintf (loop_dump_stream,
831 "Doloop: Unrecognizable doloop pattern!\n");
835 if (n_iterations != 0)
836 /* Handle the simpler case, where we know the iteration count at
838 return doloop_modify (loop, iterations, iterations_max, doloop_seq,
839 start_label, condition);
841 /* Handle the harder case, where we must add additional runtime tests. */
842 return doloop_modify_runtime (loop, iterations_max, doloop_seq,
843 start_label, mode, condition);
846 #endif /* HAVE_doloop_end */