vc4_opt_dead_code.c

/*
 * Copyright © 2014 Broadcom
 *
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 * copy of this software and associated documentation files (the "Software"),
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 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice (including the next
 * paragraph) shall be included in all copies or substantial portions of the
 * Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
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 * IN THE SOFTWARE.
 */

/**
 * @file vc4_opt_dead_code.c
 *
 * This is a simple dead code eliminator for SSA values in QIR.
 *
 * It walks all the instructions finding what temps are used, then walks again
 * to remove instructions writing unused temps.
 *
 * This is an inefficient implementation if you have long chains of
 * instructions where the entire chain is dead, but we expect those to have
 * been eliminated at the NIR level, and here we're just cleaning up small
 * problems produced by NIR->QIR.
 */

#include "vc4_qir.h"

static bool debug;

static void
dce(struct vc4_compile *c, struct qinst *inst)
{
        if (debug) {
                fprintf(stderr, "Removing: ");
                qir_dump_inst(c, inst);
                fprintf(stderr, "\n");
        }
        assert(!inst->sf);
        qir_remove_instruction(c, inst);
}

static bool
has_nonremovable_reads(struct vc4_compile *c, struct qinst *inst)
{
        for (int i = 0; i < qir_get_nsrc(inst); i++) {
                if (inst->src[i].file == QFILE_VPM) {
                        uint32_t attr = inst->src[i].index / 4;
                        uint32_t offset = (inst->src[i].index % 4) * 4;

                        if (c->vattr_sizes[attr] != offset + 4)
                                return true;

                        /* Can't get rid of the last VPM read, or the
                         * simulator (at least) throws an error.
                         */
                        uint32_t total_size = 0;
                        for (uint32_t i = 0; i < ARRAY_SIZE(c->vattr_sizes); i++)
                                total_size += c->vattr_sizes[i];
                        if (total_size == 4)
                                return true;
                }

                if (inst->src[i].file == QFILE_VARY &&
                    c->input_slots[inst->src[i].index].slot == 0xff) {
                        return true;
                }
        }

        return false;
}

bool
qir_opt_dead_code(struct vc4_compile *c)
{
        bool progress = false;
        bool *used = calloc(c->num_temps, sizeof(bool));

        qir_for_each_inst_inorder(inst, c) {
                for (int i = 0; i < qir_get_nsrc(inst); i++) {
                        if (inst->src[i].file == QFILE_TEMP)
                                used[inst->src[i].index] = true;
                }
        }

        qir_for_each_block(block, c) {
                qir_for_each_inst_safe(inst, block) {
                        if (inst->dst.file != QFILE_NULL &&
                            !(inst->dst.file == QFILE_TEMP &&
                              !used[inst->dst.index])) {
                                continue;
                        }

                        if (qir_has_side_effects(c, inst))
                                continue;

                        if (inst->sf ||
                            has_nonremovable_reads(c, inst)) {
                                /* If we can't remove the instruction, but we
                                 * don't need its destination value, just
                                 * remove the destination.  The register
                                 * allocator would trivially color it and it
                                 * wouldn't cause any register pressure, but
                                 * it's nicer to read the QIR code without
                                 * unused destination regs.
                                 */
                                if (inst->dst.file == QFILE_TEMP) {
                                        if (debug) {
                                                fprintf(stderr,
                                                        "Removing dst from: ");
                                                qir_dump_inst(c, inst);
                                                fprintf(stderr, "\n");
                                        }
                                        c->defs[inst->dst.index] = NULL;
                                        inst->dst.file = QFILE_NULL;
                                        progress = true;
                                }
                                continue;
                        }

                        for (int i = 0; i < qir_get_nsrc(inst); i++) {
                                if (inst->src[i].file != QFILE_VPM)
                                        continue;
                                uint32_t attr = inst->src[i].index / 4;
                                uint32_t offset = (inst->src[i].index % 4) * 4;

                                if (c->vattr_sizes[attr] == offset + 4) {
                                        c->num_inputs--;
                                        c->vattr_sizes[attr] -= 4;
                                }
                        }

                        dce(c, inst);
                        progress = true;
                        continue;
                }
        }

        free(used);

        return progress;
}