src/mesa/drivers/dri/i965/brw_fs_live_variables.cpp

   1 /*
   2  * Copyright © 2012 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  * Authors:
  24  *    Eric Anholt <eric@anholt.net>
  25  *
  26  */
  27
  28 #include "brw_cfg.h"
  29 #include "brw_fs_live_variables.h"
  30
  31 using namespace brw;
  32
  33 /** @file brw_fs_live_variables.cpp
  34  *
  35  * Support for computing at the basic block level which variables
  36  * (virtual GRFs in our case) are live at entry and exit.
  37  *
  38  * See Muchnik's Advanced Compiler Design and Implementation, section
  39  * 14.1 (p444).
  40  */
  41
  42 /**
  43  * Sets up the use[] and def[] arrays.
  44  *
  45  * The basic-block-level live variable analysis needs to know which
  46  * variables get used before they're completely defined, and which
  47  * variables are completely defined before they're used.
  48  */
  49 void
  50 fs_live_variables::setup_def_use()
  51 {
  52    int ip = 0;
  53
  54    for (int b = 0; b < cfg->num_blocks; b++) {
  55       bblock_t *block = cfg->blocks[b];
  56
  57       assert(ip == block->start_ip);
  58       if (b > 0)
  59          assert(cfg->blocks[b - 1]->end_ip == ip - 1);
  60
  61       for (fs_inst *inst = (fs_inst *)block->start;
  62            inst != block->end->next;
  63            inst = (fs_inst *)inst->next) {
  64
  65          /* Set use[] for this instruction */
  66          for (unsigned int i = 0; i < 3; i++) {
  67             if (inst->src[i].file == GRF) {
  68                int reg = inst->src[i].reg;
  69
  70                if (!bd[b].def[reg])
  71                   bd[b].use[reg] = true;
  72             }
  73          }
  74
  75          /* Check for unconditional writes to whole registers. These
  76           * are the things that screen off preceding definitions of a
  77           * variable, and thus qualify for being in def[].
  78           */
  79          if (inst->dst.file == GRF &&
  80              inst->regs_written() == v->virtual_grf_sizes[inst->dst.reg] &&
  81              !inst->predicate &&
  82              !inst->force_uncompressed &&
  83              !inst->force_sechalf) {
  84             int reg = inst->dst.reg;
  85             if (!bd[b].use[reg])
  86                bd[b].def[reg] = true;
  87          }
  88
  89          ip++;
  90       }
  91    }
  92 }
  93
  94 /**
  95  * The algorithm incrementally sets bits in liveout and livein,
  96  * propagating it through control flow.  It will eventually terminate
  97  * because it only ever adds bits, and stops when no bits are added in
  98  * a pass.
  99  */
 100 void
 101 fs_live_variables::compute_live_variables()
 102 {
 103    bool cont = true;
 104
 105    while (cont) {
 106       cont = false;
 107
 108       for (int b = 0; b < cfg->num_blocks; b++) {
 109          /* Update livein */
 110          for (int i = 0; i < num_vars; i++) {
 111             if (bd[b].use[i] || (bd[b].liveout[i] && !bd[b].def[i])) {
 112                if (!bd[b].livein[i]) {
 113                   bd[b].livein[i] = true;
 114                   cont = true;
 115                }
 116             }
 117          }
 118
 119          /* Update liveout */
 120          foreach_list(block_node, &cfg->blocks[b]->children) {
 121             bblock_link *link = (bblock_link *)block_node;
 122             bblock_t *block = link->block;
 123
 124             for (int i = 0; i < num_vars; i++) {
 125                if (bd[block->block_num].livein[i] && !bd[b].liveout[i]) {
 126                   bd[b].liveout[i] = true;
 127                   cont = true;
 128                }
 129             }
 130          }
 131       }
 132    }
 133 }
 134
 135 fs_live_variables::fs_live_variables(fs_visitor *v, cfg_t *cfg)
 136    : v(v), cfg(cfg)
 137 {
 138    mem_ctx = ralloc_context(cfg->mem_ctx);
 139
 140    num_vars = v->virtual_grf_count;
 141    bd = rzalloc_array(mem_ctx, struct block_data, cfg->num_blocks);
 142
 143    for (int i = 0; i < cfg->num_blocks; i++) {
 144       bd[i].def = rzalloc_array(mem_ctx, bool, num_vars);
 145       bd[i].use = rzalloc_array(mem_ctx, bool, num_vars);
 146       bd[i].livein = rzalloc_array(mem_ctx, bool, num_vars);
 147       bd[i].liveout = rzalloc_array(mem_ctx, bool, num_vars);
 148    }
 149
 150    setup_def_use();
 151    compute_live_variables();
 152 }
 153
 154 fs_live_variables::~fs_live_variables()
 155 {
 156    ralloc_free(mem_ctx);
 157 }
 158
 159 #define MAX_INSTRUCTION (1 << 30)
 160
 161 void
 162 fs_visitor::calculate_live_intervals()
 163 {
 164    int num_vars = this->virtual_grf_count;
 165
 166    if (this->live_intervals_valid)
 167       return;
 168
 169    int *def = ralloc_array(mem_ctx, int, num_vars);
 170    int *use = ralloc_array(mem_ctx, int, num_vars);
 171    ralloc_free(this->virtual_grf_def);
 172    ralloc_free(this->virtual_grf_use);
 173    this->virtual_grf_def = def;
 174    this->virtual_grf_use = use;
 175
 176    for (int i = 0; i < num_vars; i++) {
 177       def[i] = MAX_INSTRUCTION;
 178       use[i] = -1;
 179    }
 180
 181    /* Start by setting up the intervals with no knowledge of control
 182     * flow.
 183     */
 184    int ip = 0;
 185    foreach_list(node, &this->instructions) {
 186       fs_inst *inst = (fs_inst *)node;
 187
 188       for (unsigned int i = 0; i < 3; i++) {
 189          if (inst->src[i].file == GRF) {
 190             int reg = inst->src[i].reg;
 191
 192             use[reg] = ip;
 193
 194             /* In most cases, a register can be written over safely by the
 195              * same instruction that is its last use.  For a single
 196              * instruction, the sources are dereferenced before writing of the
 197              * destination starts (naturally).  This gets more complicated for
 198              * simd16, because the instruction:
 199              *
 200              * mov(16)      g4<1>F      g4<8,8,1>F   g6<8,8,1>F
 201              *
 202              * is actually decoded in hardware as:
 203              *
 204              * mov(8)       g4<1>F      g4<8,8,1>F   g6<8,8,1>F
 205              * mov(8)       g5<1>F      g5<8,8,1>F   g7<8,8,1>F
 206              *
 207              * Which is safe.  However, if we have uniform accesses
 208              * happening, we get into trouble:
 209              *
 210              * mov(8)       g4<1>F      g4<0,1,0>F   g6<8,8,1>F
 211              * mov(8)       g5<1>F      g4<0,1,0>F   g7<8,8,1>F
 212              *
 213              * Now our destination for the first instruction overwrote the
 214              * second instruction's src0, and we get garbage for those 8
 215              * pixels.  There's a similar issue for the pre-gen6
 216              * pixel_x/pixel_y, which are registers of 16-bit values and thus
 217              * would get stomped by the first decode as well.
 218              */
 219             if (dispatch_width == 16 && (inst->src[i].smear ||
 220                                          (this->pixel_x.reg == reg ||
 221                                           this->pixel_y.reg == reg))) {
 222                use[reg]++;
 223             }
 224          }
 225       }
 226
 227       if (inst->dst.file == GRF) {
 228          int reg = inst->dst.reg;
 229
 230          def[reg] = MIN2(def[reg], ip);
 231       }
 232
 233       ip++;
 234    }
 235
 236    /* Now, extend those intervals using our analysis of control flow. */
 237    cfg_t cfg(this);
 238    fs_live_variables livevars(this, &cfg);
 239
 240    for (int b = 0; b < cfg.num_blocks; b++) {
 241       for (int i = 0; i < num_vars; i++) {
 242          if (livevars.bd[b].livein[i]) {
 243             def[i] = MIN2(def[i], cfg.blocks[b]->start_ip);
 244             use[i] = MAX2(use[i], cfg.blocks[b]->start_ip);
 245          }
 246
 247          if (livevars.bd[b].liveout[i]) {
 248             def[i] = MIN2(def[i], cfg.blocks[b]->end_ip);
 249             use[i] = MAX2(use[i], cfg.blocks[b]->end_ip);
 250          }
 251       }
 252    }
 253
 254    this->live_intervals_valid = true;
 255
 256    /* Note in the non-control-flow code above, that we only take def[] as the
 257     * first store, and use[] as the last use.  We use this in dead code
 258     * elimination, to determine when a store never gets used.  However, we
 259     * also use these arrays to answer the virtual_grf_interferes() question
 260     * (live interval analysis), which is used for register coalescing and
 261     * register allocation.
 262     *
 263     * So, there's a conflict over what the array should mean: if use[]
 264     * considers a def after the last use, then the dead code elimination pass
 265     * never does anything (and it's an important pass!).  But if we don't
 266     * include dead code, then virtual_grf_interferes() lies and we'll do
 267     * horrible things like coalesce the register that is dead-code-written
 268     * into another register that was live across the dead write (causing the
 269     * use of the second register to take the dead write's source value instead
 270     * of the coalesced MOV's source value).
 271     *
 272     * To resolve the conflict, immediately after calculating live intervals,
 273     * detect dead code, nuke it, and if we changed anything, calculate again
 274     * before returning to the caller.  Now we happen to produce def[] and
 275     * use[] arrays that will work for virtual_grf_interferes().
 276     */
 277    if (dead_code_eliminate())
 278       calculate_live_intervals();
 279 }
 280
 281 bool
 282 fs_visitor::virtual_grf_interferes(int a, int b)
 283 {
 284    int a_def = this->virtual_grf_def[a], a_use = this->virtual_grf_use[a];
 285    int b_def = this->virtual_grf_def[b], b_use = this->virtual_grf_use[b];
 286
 287    /* If there's dead code (def but not use), it would break our test
 288     * unless we consider it used.
 289     */
 290    if ((a_use == -1 && a_def != MAX_INSTRUCTION) ||
 291        (b_use == -1 && b_def != MAX_INSTRUCTION)) {
 292       return true;
 293    }
 294
 295    int start = MAX2(a_def, b_def);
 296    int end = MIN2(a_use, b_use);
 297
 298    return start < end;
 299 }