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(!v3d_qpu_writes_flags(&inst->qpu));
  51         vir_remove_instruction(c, inst);
  52 }
  53
  54 static bool
  55 has_nonremovable_reads(struct v3d_compile *c, struct qinst *inst)
  56 {
  57         for (int i = 0; i < vir_get_nsrc(inst); i++) {
  58                 if (inst->src[i].file == QFILE_VPM)
  59                         return true;
  60         }
  61
  62         return false;
  63 }
  64
  65 static bool
  66 can_write_to_null(struct v3d_compile *c, struct qinst *inst)
  67 {
  68         /* The SFU instructions must write to a physical register. */
  69         if (c->devinfo->ver >= 41 && v3d_qpu_uses_sfu(&inst->qpu))
  70                 return false;
  71
  72         return true;
  73 }
  74
  75 static void
  76 vir_dce_flags(struct v3d_compile *c, struct qinst *inst)
  77 {
  78         if (debug) {
  79                 fprintf(stderr,
  80                         "Removing flags write from: ");
  81                 vir_dump_inst(c, inst);
  82                 fprintf(stderr, "\n");
  83         }
  84
  85         assert(inst->qpu.type == V3D_QPU_INSTR_TYPE_ALU);
  86
  87         inst->qpu.flags.apf = V3D_QPU_PF_NONE;
  88         inst->qpu.flags.mpf = V3D_QPU_PF_NONE;
  89         inst->qpu.flags.auf = V3D_QPU_UF_NONE;
  90         inst->qpu.flags.muf = V3D_QPU_UF_NONE;
  91 }
  92
  93 bool
  94 vir_opt_dead_code(struct v3d_compile *c)
  95 {
  96         bool progress = false;
  97         bool *used = calloc(c->num_temps, sizeof(bool));
  98
  99         /* Defuse the "are you removing the cursor?" assertion in the core.
 100          * You'll need to set up a new cursor for any new instructions after
 101          * doing DCE (which we would expect, anyway).
 102          */
 103         c->cursor.link = NULL;
 104
 105         vir_for_each_inst_inorder(inst, c) {
 106                 for (int i = 0; i < vir_get_nsrc(inst); i++) {
 107                         if (inst->src[i].file == QFILE_TEMP)
 108                                 used[inst->src[i].index] = true;
 109                 }
 110         }
 111
 112         vir_for_each_block(block, c) {
 113                 struct qinst *last_flags_write = NULL;
 114
 115                 vir_for_each_inst_safe(inst, block) {
 116                         /* If this instruction reads the flags, we can't
 117                          * remove the flags generation for it.
 118                          */
 119                         if (v3d_qpu_reads_flags(&inst->qpu))
 120                                 last_flags_write = NULL;
 121
 122                         if (inst->dst.file != QFILE_NULL &&
 123                             !(inst->dst.file == QFILE_TEMP &&
 124                               !used[inst->dst.index])) {
 125                                 continue;
 126                         }
 127
 128                         if (vir_has_side_effects(c, inst))
 129                                 continue;
 130
 131                         if (v3d_qpu_writes_flags(&inst->qpu)) {
 132                                 /* If we obscure a previous flags write,
 133                                  * drop it.
 134                                  */
 135                                 if (last_flags_write &&
 136                                     (inst->qpu.flags.apf != V3D_QPU_PF_NONE ||
 137                                      inst->qpu.flags.mpf != V3D_QPU_PF_NONE)) {
 138                                         vir_dce_flags(c, last_flags_write);
 139                                         progress = true;
 140                                 }
 141
 142                                 last_flags_write = inst;
 143                         }
 144
 145                         if (v3d_qpu_writes_flags(&inst->qpu) ||
 146                             has_nonremovable_reads(c, inst)) {
 147                                 /* If we can't remove the instruction, but we
 148                                  * don't need its destination value, just
 149                                  * remove the destination.  The register
 150                                  * allocator would trivially color it and it
 151                                  * wouldn't cause any register pressure, but
 152                                  * it's nicer to read the VIR code without
 153                                  * unused destination regs.
 154                                  */
 155                                 if (inst->dst.file == QFILE_TEMP &&
 156                                     can_write_to_null(c, inst)) {
 157                                         if (debug) {
 158                                                 fprintf(stderr,
 159                                                         "Removing dst from: ");
 160                                                 vir_dump_inst(c, inst);
 161                                                 fprintf(stderr, "\n");
 162                                         }
 163                                         c->defs[inst->dst.index] = NULL;
 164                                         inst->dst.file = QFILE_NULL;
 165                                         progress = true;
 166                                 }
 167                                 continue;
 168                         }
 169
 170                         assert(inst != last_flags_write);
 171                         dce(c, inst);
 172                         progress = true;
 173                         continue;
 174                 }
 175         }
 176
 177         free(used);
 178
 179         return progress;
 180 }