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      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 vc4_opt_dead_code.c
     26  *
     27  * This is a simple dead code eliminator for SSA values in QIR.
     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->QIR.
     36  */
     37 
     38 #include "vc4_qir.h"
     39 
     40 static bool debug;
     41 
     42 static void
     43 dce(struct vc4_compile *c, struct qinst *inst)
     44 {
     45         if (debug) {
     46                 fprintf(stderr, "Removing: ");
     47                 qir_dump_inst(c, inst);
     48                 fprintf(stderr, "\n");
     49         }
     50         assert(!inst->sf);
     51         qir_remove_instruction(c, inst);
     52 }
     53 
     54 static bool
     55 has_nonremovable_reads(struct vc4_compile *c, struct qinst *inst)
     56 {
     57         for (int i = 0; i < qir_get_nsrc(inst); i++) {
     58                 if (inst->src[i].file == QFILE_VPM) {
     59                         uint32_t attr = inst->src[i].index / 4;
     60                         uint32_t offset = (inst->src[i].index % 4) * 4;
     61 
     62                         if (c->vattr_sizes[attr] != offset + 4)
     63                                 return true;
     64 
     65                         /* Can't get rid of the last VPM read, or the
     66                          * simulator (at least) throws an error.
     67                          */
     68                         uint32_t total_size = 0;
     69                         for (uint32_t i = 0; i < ARRAY_SIZE(c->vattr_sizes); i++)
     70                                 total_size += c->vattr_sizes[i];
     71                         if (total_size == 4)
     72                                 return true;
     73                 }
     74 
     75                 if (inst->src[i].file == QFILE_VARY &&
     76                     c->input_slots[inst->src[i].index].slot == 0xff) {
     77                         return true;
     78                 }
     79         }
     80 
     81         return false;
     82 }
     83 
     84 bool
     85 qir_opt_dead_code(struct vc4_compile *c)
     86 {
     87         bool progress = false;
     88         bool *used = calloc(c->num_temps, sizeof(bool));
     89 
     90         qir_for_each_inst_inorder(inst, c) {
     91                 for (int i = 0; i < qir_get_nsrc(inst); i++) {
     92                         if (inst->src[i].file == QFILE_TEMP)
     93                                 used[inst->src[i].index] = true;
     94                 }
     95         }
     96 
     97         qir_for_each_block(block, c) {
     98                 qir_for_each_inst_safe(inst, block) {
     99                         if (inst->dst.file != QFILE_NULL &&
    100                             !(inst->dst.file == QFILE_TEMP &&
    101                               !used[inst->dst.index])) {
    102                                 continue;
    103                         }
    104 
    105                         if (qir_has_side_effects(c, inst))
    106                                 continue;
    107 
    108                         if (inst->sf ||
    109                             has_nonremovable_reads(c, inst)) {
    110                                 /* If we can't remove the instruction, but we
    111                                  * don't need its destination value, just
    112                                  * remove the destination.  The register
    113                                  * allocator would trivially color it and it
    114                                  * wouldn't cause any register pressure, but
    115                                  * it's nicer to read the QIR code without
    116                                  * unused destination regs.
    117                                  */
    118                                 if (inst->dst.file == QFILE_TEMP) {
    119                                         if (debug) {
    120                                                 fprintf(stderr,
    121                                                         "Removing dst from: ");
    122                                                 qir_dump_inst(c, inst);
    123                                                 fprintf(stderr, "\n");
    124                                         }
    125                                         c->defs[inst->dst.index] = NULL;
    126                                         inst->dst.file = QFILE_NULL;
    127                                         progress = true;
    128                                 }
    129                                 continue;
    130                         }
    131 
    132                         for (int i = 0; i < qir_get_nsrc(inst); i++) {
    133                                 if (inst->src[i].file != QFILE_VPM)
    134                                         continue;
    135                                 uint32_t attr = inst->src[i].index / 4;
    136                                 uint32_t offset = (inst->src[i].index % 4) * 4;
    137 
    138                                 if (c->vattr_sizes[attr] == offset + 4) {
    139                                         c->num_inputs--;
    140                                         c->vattr_sizes[attr] -= 4;
    141                                 }
    142                         }
    143 
    144                         dce(c, inst);
    145                         progress = true;
    146                         continue;
    147                 }
    148         }
    149 
    150         free(used);
    151 
    152         return progress;
    153 }
    154