src/broadcom/compiler/vir_opt_dead_code.c

   1 /*
   2  * Copyright © 2014 Broadcom
   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 /**
  25  * @file v3d_opt_dead_code.c
  26  *
  27  * This is a simple dead code eliminator for SSA values in VIR.
  28  *
  29  * It walks all the instructions finding what temps are used, then walks again
  30  * to remove instructions writing unused temps.
  31  *
  32  * This is an inefficient implementation if you have long chains of
  33  * instructions where the entire chain is dead, but we expect those to have
  34  * been eliminated at the NIR level, and here we're just cleaning up small
  35  * problems produced by NIR->VIR.
  36  */
  37
  38 #include "v3d_compiler.h"
  39
  40 static bool debug;
  41
  42 static void
  43 dce(struct v3d_compile *c, struct qinst *inst)
  44 {
  45         if (debug) {
  46                 fprintf(stderr, "Removing: ");
  47                 vir_dump_inst(c, inst);
  48                 fprintf(stderr, "\n");
  49         }
  50         assert(inst->qpu.flags.apf == V3D_QPU_PF_NONE);
  51         assert(inst->qpu.flags.mpf == V3D_QPU_PF_NONE);
  52         assert(inst->qpu.flags.auf == V3D_QPU_UF_NONE);
  53         assert(inst->qpu.flags.muf == V3D_QPU_UF_NONE);
  54         vir_remove_instruction(c, inst);
  55 }
  56
  57 static bool
  58 has_nonremovable_reads(struct v3d_compile *c, struct qinst *inst)
  59 {
  60         for (int i = 0; i < vir_get_nsrc(inst); i++) {
  61                 if (inst->src[i].file == QFILE_VPM) {
  62                         /* Instance ID, Vertex ID: Should have been removed at
  63                          * the NIR level
  64                          */
  65                         if (inst->src[i].index == ~0)
  66                                 return true;
  67
  68                         uint32_t attr = inst->src[i].index / 4;
  69                         uint32_t offset = inst->src[i].index % 4;
  70
  71                         if (c->vattr_sizes[attr] != offset)
  72                                 return true;
  73
  74                         /* Can't get rid of the last VPM read, or the
  75                          * simulator (at least) throws an error.
  76                          */
  77                         uint32_t total_size = 0;
  78                         for (uint32_t i = 0; i < ARRAY_SIZE(c->vattr_sizes); i++)
  79                                 total_size += c->vattr_sizes[i];
  80                         if (total_size == 1)
  81                                 return true;
  82                 }
  83         }
  84
  85         return false;
  86 }
  87
  88 static bool
  89 can_write_to_null(struct v3d_compile *c, struct qinst *inst)
  90 {
  91         /* The SFU instructions must write to a physical register. */
  92         if (c->devinfo->ver >= 41 && v3d_qpu_uses_sfu(&inst->qpu))
  93                 return false;
  94
  95         return true;
  96 }
  97
  98 bool
  99 vir_opt_dead_code(struct v3d_compile *c)
 100 {
 101         bool progress = false;
 102         bool *used = calloc(c->num_temps, sizeof(bool));
 103
 104         /* Defuse the "are you removing the cursor?" assertion in the core.
 105          * You'll need to set up a new cursor for any new instructions after
 106          * doing DCE (which we would expect, anyway).
 107          */
 108         c->cursor.link = NULL;
 109
 110         vir_for_each_inst_inorder(inst, c) {
 111                 for (int i = 0; i < vir_get_nsrc(inst); i++) {
 112                         if (inst->src[i].file == QFILE_TEMP)
 113                                 used[inst->src[i].index] = true;
 114                 }
 115         }
 116
 117         vir_for_each_block(block, c) {
 118                 vir_for_each_inst_safe(inst, block) {
 119                         if (inst->dst.file != QFILE_NULL &&
 120                             !(inst->dst.file == QFILE_TEMP &&
 121                               !used[inst->dst.index])) {
 122                                 continue;
 123                         }
 124
 125                         if (vir_has_side_effects(c, inst))
 126                                 continue;
 127
 128                         if (inst->qpu.flags.apf != V3D_QPU_PF_NONE ||
 129                             inst->qpu.flags.mpf != V3D_QPU_PF_NONE ||
 130                             inst->qpu.flags.auf != V3D_QPU_UF_NONE ||
 131                             inst->qpu.flags.muf != V3D_QPU_UF_NONE ||
 132                             has_nonremovable_reads(c, inst)) {
 133                                 /* If we can't remove the instruction, but we
 134                                  * don't need its destination value, just
 135                                  * remove the destination.  The register
 136                                  * allocator would trivially color it and it
 137                                  * wouldn't cause any register pressure, but
 138                                  * it's nicer to read the VIR code without
 139                                  * unused destination regs.
 140                                  */
 141                                 if (inst->dst.file == QFILE_TEMP &&
 142                                     can_write_to_null(c, inst)) {
 143                                         if (debug) {
 144                                                 fprintf(stderr,
 145                                                         "Removing dst from: ");
 146                                                 vir_dump_inst(c, inst);
 147                                                 fprintf(stderr, "\n");
 148                                         }
 149                                         c->defs[inst->dst.index] = NULL;
 150                                         inst->dst.file = QFILE_NULL;
 151                                         progress = true;
 152                                 }
 153                                 continue;
 154                         }
 155
 156                         for (int i = 0; i < vir_get_nsrc(inst); i++) {
 157                                 if (inst->src[i].file != QFILE_VPM)
 158                                         continue;
 159                                 uint32_t attr = inst->src[i].index / 4;
 160                                 uint32_t offset = (inst->src[i].index % 4);
 161
 162                                 if (c->vattr_sizes[attr] == offset) {
 163                                         c->num_inputs--;
 164                                         c->vattr_sizes[attr]--;
 165                                 }
 166                         }
 167
 168                         dce(c, inst);
 169                         progress = true;
 170                         continue;
 171                 }
 172         }
 173
 174         free(used);
 175
 176         return progress;
 177 }