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#include "brw_fs.h"
25#include "brw_cfg.h"
26
27/** @file brw_fs_sel_peephole.cpp
28 *
29 * This file contains the opt_peephole_sel() optimization pass that replaces
30 * MOV instructions to the same destination in the "then" and "else" bodies of
31 * an if statement with SEL instructions.
32 */
33
34/* Four MOVs seems to be pretty typical, so I picked the next power of two in
35 * the hopes that it would handle almost anything possible in a single
36 * pass.
37 */
38#define MAX_MOVS 8 /**< The maximum number of MOVs to attempt to match. */
39
40using namespace brw;
41
42/**
43 * Scans forwards from an IF counting consecutive MOV instructions in the
44 * "then" and "else" blocks of the if statement.
45 *
46 * A pointer to the bblock_t following the IF is passed as the <then_block>
47 * argument. The function stores pointers to the MOV instructions in the
48 * <then_mov> and <else_mov> arrays.
49 *
50 * \return the minimum number of MOVs found in the two branches or zero if
51 *         an error occurred.
52 *
53 * E.g.:
54 *                  IF ...
55 *    then_mov[0] = MOV g4, ...
56 *    then_mov[1] = MOV g5, ...
57 *    then_mov[2] = MOV g6, ...
58 *                  ELSE ...
59 *    else_mov[0] = MOV g4, ...
60 *    else_mov[1] = MOV g5, ...
61 *    else_mov[2] = MOV g7, ...
62 *                  ENDIF
63 *    returns 3.
64 */
65static int
66count_movs_from_if(const intel_device_info *devinfo,
67                   fs_inst *then_mov[MAX_MOVS], fs_inst *else_mov[MAX_MOVS],
68                   bblock_t *then_block, bblock_t *else_block)
69{
70   int then_movs = 0;
71   foreach_inst_in_block(fs_inst, inst, then_block) {
72      if (then_movs == MAX_MOVS || inst->opcode != BRW_OPCODE_MOV ||
73          inst->flags_written(devinfo))
74         break;
75
76      then_mov[then_movs] = inst;
77      then_movs++;
78   }
79
80   int else_movs = 0;
81   foreach_inst_in_block(fs_inst, inst, else_block) {
82      if (else_movs == MAX_MOVS || inst->opcode != BRW_OPCODE_MOV ||
83          inst->flags_written(devinfo))
84         break;
85
86      else_mov[else_movs] = inst;
87      else_movs++;
88   }
89
90   return MIN2(then_movs, else_movs);
91}
92
93/**
94 * Try to replace IF/MOV+/ELSE/MOV+/ENDIF with SEL.
95 *
96 * Many GLSL shaders contain the following pattern:
97 *
98 *    x = condition ? foo : bar
99 *
100 * or
101 *
102 *    if (...) a.xyzw = foo.xyzw;
103 *    else     a.xyzw = bar.xyzw;
104 *
105 * The compiler emits an ir_if tree for this, since each subexpression might be
106 * a complex tree that could have side-effects or short-circuit logic.
107 *
108 * However, the common case is to simply select one of two constants or
109 * variable values---which is exactly what SEL is for.  In this case, the
110 * assembly looks like:
111 *
112 *    (+f0) IF
113 *    MOV dst src0
114 *    ...
115 *    ELSE
116 *    MOV dst src1
117 *    ...
118 *    ENDIF
119 *
120 * where each pair of MOVs to a common destination and can be easily translated
121 * into
122 *
123 *    (+f0) SEL dst src0 src1
124 *
125 * If src0 is an immediate value, we promote it to a temporary GRF.
126 */
127bool
128fs_visitor::opt_peephole_sel()
129{
130   bool progress = false;
131
132   foreach_block (block, cfg) {
133      /* IF instructions, by definition, can only be found at the ends of
134       * basic blocks.
135       */
136      fs_inst *if_inst = (fs_inst *)block->end();
137      if (if_inst->opcode != BRW_OPCODE_IF)
138         continue;
139
140      fs_inst *else_mov[MAX_MOVS] = { NULL };
141      fs_inst *then_mov[MAX_MOVS] = { NULL };
142
143      bblock_t *then_block = block->next();
144      bblock_t *else_block = NULL;
145      foreach_list_typed(bblock_link, child, link, &block->children) {
146         if (child->block != then_block) {
147            if (child->block->prev()->end()->opcode == BRW_OPCODE_ELSE) {
148               else_block = child->block;
149            }
150            break;
151         }
152      }
153      if (else_block == NULL)
154         continue;
155
156      int movs = count_movs_from_if(devinfo, then_mov, else_mov, then_block, else_block);
157
158      if (movs == 0)
159         continue;
160
161      /* Generate SEL instructions for pairs of MOVs to a common destination. */
162      for (int i = 0; i < movs; i++) {
163         if (!then_mov[i] || !else_mov[i])
164            break;
165
166         /* Check that the MOVs are the right form. */
167         if (!then_mov[i]->dst.equals(else_mov[i]->dst) ||
168             then_mov[i]->exec_size != else_mov[i]->exec_size ||
169             then_mov[i]->group != else_mov[i]->group ||
170             then_mov[i]->force_writemask_all != else_mov[i]->force_writemask_all ||
171             then_mov[i]->is_partial_write() ||
172             else_mov[i]->is_partial_write() ||
173             then_mov[i]->conditional_mod != BRW_CONDITIONAL_NONE ||
174             else_mov[i]->conditional_mod != BRW_CONDITIONAL_NONE) {
175            movs = i;
176            break;
177         }
178
179         /* Check that source types for mov operations match. */
180         if (then_mov[i]->src[0].type != else_mov[i]->src[0].type) {
181            movs = i;
182            break;
183         }
184      }
185
186      if (movs == 0)
187         continue;
188
189      for (int i = 0; i < movs; i++) {
190         const fs_builder ibld = fs_builder(this, then_block, then_mov[i])
191                                 .at(block, if_inst);
192
193         if (then_mov[i]->src[0].equals(else_mov[i]->src[0])) {
194            ibld.MOV(then_mov[i]->dst, then_mov[i]->src[0]);
195         } else {
196            /* Only the last source register can be a constant, so if the MOV
197             * in the "then" clause uses a constant, we need to put it in a
198             * temporary.
199             */
200            fs_reg src0(then_mov[i]->src[0]);
201            if (src0.file == IMM) {
202               src0 = ibld.vgrf(then_mov[i]->src[0].type);
203               ibld.MOV(src0, then_mov[i]->src[0]);
204            }
205
206            /* 64-bit immediates can't be placed in src1. */
207            fs_reg src1(else_mov[i]->src[0]);
208            if (src1.file == IMM && type_sz(src1.type) == 8) {
209               src1 = ibld.vgrf(else_mov[i]->src[0].type);
210               ibld.MOV(src1, else_mov[i]->src[0]);
211            }
212
213            set_predicate_inv(if_inst->predicate, if_inst->predicate_inverse,
214                              ibld.SEL(then_mov[i]->dst, src0, src1));
215         }
216
217         then_mov[i]->remove(then_block);
218         else_mov[i]->remove(else_block);
219      }
220
221      progress = true;
222   }
223
224   if (progress)
225      invalidate_analysis(DEPENDENCY_INSTRUCTIONS | DEPENDENCY_VARIABLES);
226
227   return progress;
228}
229