1/*
2 * Copyright © 2013 Intel Corporation
3 *
4 * Permission is hereby granted, free of charge, to any person obtaining a
5 * copy of this software and associated documentation files (the "Software"),
6 * to deal in the Software without restriction, including without limitation
7 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
8 * and/or sell copies of the Software, and to permit persons to whom the
9 * Software is furnished to do so, subject to the following conditions:
10 *
11 * The above copyright notice and this permission notice (including the next
12 * paragraph) shall be included in all copies or substantial portions of the
13 * Software.
14 *
15 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
16 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
17 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
18 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
19 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
20 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
21 * IN THE SOFTWARE.
22 */
23
24/** @file brw_dead_control_flow.cpp
25 *
26 * This file implements the dead control flow elimination optimization pass.
27 */
28
29#include "brw_shader.h"
30#include "brw_cfg.h"
31
32/* Look for and eliminate dead control flow:
33 *
34 *   - if/endif
35 *   - else in else/endif
36 *   - then in if/else/endif
37 */
38bool
39dead_control_flow_eliminate(backend_shader *s)
40{
41   bool progress = false;
42
43   foreach_block_safe (block, s->cfg) {
44      bblock_t *prev_block = block->prev();
45
46      if (!prev_block)
47         continue;
48
49      backend_instruction *const inst = block->start();
50      backend_instruction *const prev_inst = prev_block->end();
51
52      /* ENDIF instructions, by definition, can only be found at the start of
53       * basic blocks.
54       */
55      if (inst->opcode == BRW_OPCODE_ENDIF &&
56          prev_inst->opcode == BRW_OPCODE_ELSE) {
57         bblock_t *const else_block = prev_block;
58         backend_instruction *const else_inst = prev_inst;
59
60         else_inst->remove(else_block);
61         progress = true;
62      } else if (inst->opcode == BRW_OPCODE_ENDIF &&
63                 prev_inst->opcode == BRW_OPCODE_IF) {
64         bblock_t *const endif_block = block;
65         bblock_t *const if_block = prev_block;
66         backend_instruction *const endif_inst = inst;
67         backend_instruction *const if_inst = prev_inst;
68
69         bblock_t *earlier_block = NULL, *later_block = NULL;
70
71         if (if_block->start_ip == if_block->end_ip) {
72            earlier_block = if_block->prev();
73         } else {
74            earlier_block = if_block;
75         }
76         if_inst->remove(if_block);
77
78         if (endif_block->start_ip == endif_block->end_ip) {
79            later_block = endif_block->next();
80         } else {
81            later_block = endif_block;
82         }
83         endif_inst->remove(endif_block);
84
85         assert((earlier_block == NULL) == (later_block == NULL));
86         if (earlier_block && earlier_block->can_combine_with(later_block)) {
87            earlier_block->combine_with(later_block);
88
89            /* If ENDIF was in its own block, then we've now deleted it and
90             * merged the two surrounding blocks, the latter of which the
91             * __next block pointer was pointing to.
92             */
93            if (endif_block != later_block) {
94               __next = earlier_block->next();
95            }
96         }
97
98         progress = true;
99      } else if (inst->opcode == BRW_OPCODE_ELSE &&
100                 prev_inst->opcode == BRW_OPCODE_IF) {
101         bblock_t *const else_block = block;
102         backend_instruction *const if_inst = prev_inst;
103         backend_instruction *const else_inst = inst;
104
105         /* Since the else-branch is becoming the new then-branch, the
106          * condition has to be inverted.
107          */
108         if_inst->predicate_inverse = !if_inst->predicate_inverse;
109         else_inst->remove(else_block);
110
111         progress = true;
112      }
113   }
114
115   if (progress)
116      s->invalidate_live_intervals();
117
118   return progress;
119}
120