src/panfrost/bifrost/bi_opt_dce.c

/*
 * Copyright (C) 2018 Alyssa Rosenzweig
 * Copyright (C) 2019-2020 Collabora, Ltd.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * 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
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#include "compiler.h"
#include "util/u_memory.h"

/* A simple SSA-based mark-and-sweep dead code elimination pass. */

void
bi_opt_dead_code_eliminate(bi_context *ctx)
{
        /* Mark live values */
        BITSET_WORD *mark = calloc(sizeof(BITSET_WORD), BITSET_WORDS(ctx->ssa_alloc));

        u_worklist worklist;
        u_worklist_init(&worklist, ctx->num_blocks, NULL);

        bi_foreach_block(ctx, block) {
                bi_worklist_push_head(&worklist, block);
        }

        while(!u_worklist_is_empty(&worklist)) {
                /* Pop in reverse order for backwards pass */
                bi_block *blk = bi_worklist_pop_head(&worklist);

                bool progress = false;

                bi_foreach_instr_in_block_rev(blk, I) {
                        bool needed = bi_side_effects(I);

                        bi_foreach_dest(I, d)
                                needed |= BITSET_TEST(mark, I->dest[d].value);

                        if (!needed)
                                continue;

                        bi_foreach_ssa_src(I, s) {
                                progress |= !BITSET_TEST(mark, I->src[s].value);
                                BITSET_SET(mark, I->src[s].value);
                        }
                }

                /* XXX: slow */
                if (progress) {
                        bi_foreach_block(ctx, block)
                                bi_worklist_push_head(&worklist, block);
                }
        }

        u_worklist_fini(&worklist);

        /* Sweep */
        bi_foreach_instr_global_safe(ctx, I) {
                bool needed = bi_side_effects(I);

                bi_foreach_dest(I, d)
                        needed |= BITSET_TEST(mark, I->dest[d].value);

                if (!needed)
                        bi_remove_instruction(I);
        }

        free(mark);
}

/* Post-RA liveness-based dead code analysis to clean up results of bundling */

uint64_t MUST_CHECK
bi_postra_liveness_ins(uint64_t live, bi_instr *ins)
{
        bi_foreach_dest(ins, d) {
                if (ins->dest[d].type == BI_INDEX_REGISTER) {
                        unsigned nr = bi_count_write_registers(ins, d);
                        unsigned reg = ins->dest[d].value;
                        live &= ~(BITFIELD64_MASK(nr) << reg);
                }
        }

        bi_foreach_src(ins, s) {
                if (ins->src[s].type == BI_INDEX_REGISTER) {
                        unsigned nr = bi_count_read_registers(ins, s);
                        unsigned reg = ins->src[s].value;
                        live |= (BITFIELD64_MASK(nr) << reg);
                }
        }

        return live;
}

static bool
bi_postra_liveness_block(bi_block *blk)
{
        bi_foreach_successor(blk, succ)
                blk->reg_live_out |= succ->reg_live_in;

        uint64_t live = blk->reg_live_out;

        bi_foreach_instr_in_block_rev(blk, ins)
                live = bi_postra_liveness_ins(live, ins);

        bool progress = blk->reg_live_in != live;
        blk->reg_live_in = live;
        return progress;
}

/* Globally, liveness analysis uses a fixed-point algorithm based on a
 * worklist. We initialize a work list with the exit block. We iterate the work
 * list to compute live_in from live_out for each block on the work list,
 * adding the predecessors of the block to the work list if we made progress.
 */

void
bi_postra_liveness(bi_context *ctx)
{
        u_worklist worklist;
        bi_worklist_init(ctx, &worklist);

        bi_foreach_block(ctx, block) {
                block->reg_live_out = block->reg_live_in = 0;

                bi_worklist_push_tail(&worklist, block);
        }

        while (!u_worklist_is_empty(&worklist)) {
                /* Pop off in reverse order since liveness is backwards */
                bi_block *blk = bi_worklist_pop_tail(&worklist);

                /* Update liveness information. If we made progress, we need to
                 * reprocess the predecessors
                 */
                if (bi_postra_liveness_block(blk)) {
                        bi_foreach_predecessor(blk, pred)
                                bi_worklist_push_head(&worklist, *pred);
                }
        }

        u_worklist_fini(&worklist);
}

void
bi_opt_dce_post_ra(bi_context *ctx)
{
        bi_postra_liveness(ctx);

        bi_foreach_block_rev(ctx, block) {
                uint64_t live = block->reg_live_out;

                bi_foreach_instr_in_block_rev(block, ins) {
                        if (ins->op == BI_OPCODE_DTSEL_IMM)
                                ins->dest[0] = bi_null();

                        bi_foreach_dest(ins, d) {
                                if (ins->dest[d].type != BI_INDEX_REGISTER)
                                        continue;

                                unsigned nr = bi_count_write_registers(ins, d);
                                unsigned reg = ins->dest[d].value;
                                uint64_t mask = (BITFIELD64_MASK(nr) << reg);
                                bool cullable = (ins->op != BI_OPCODE_BLEND);
                                cullable &= !bi_opcode_props[ins->op].sr_write;

                                if (!(live & mask) && cullable)
                                        ins->dest[d] = bi_null();
                        }

                        live = bi_postra_liveness_ins(live, ins);
                }
        }
}