midgard_helper_invocations.c

/*
 * Copyright (C) 2019 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.
 *
 * Authors (Collabora):
 *    Alyssa Rosenzweig <[email protected]>
 */

#include "compiler.h"

/* Midgard texture/derivative operations have a pair of bits controlling the
 * behaviour of helper invocations:
 *
 *  - Should a helper invocation terminate after executing this instruction?
 *  - Should a helper invocation actually execute this instruction?
 *
 * The terminate bit should be set on the last instruction requiring helper
 * invocations. Without control flow, that's literally the last instruction;
 * with control flow, there may be multiple such instructions (with ifs) or no
 * such instruction (with loops).
 *
 * The execute bit should be set if the value of this instruction is required
 * by a future instruction requiring helper invocations. Consider:
 *
 *      0 = texture ...
 *      1 = fmul 0, #10
 *      2 = dfdx 1
 *      store 2
 *
 * Since the derivative calculation 2 requires helper invocations, the value 1
 * must be calculated by helper invocations, and since it depends on 0, 0 must
 * be calculated by helpers. Hence the texture op has the execute bit set, and
 * the derivative op has the terminate bit set.
 *
 * Calculating the terminate bit occurs by forward dataflow analysis to
 * determine which blocks require helper invocations. A block requires
 * invocations in if any of its instructions use helper invocations, or if it
 * depends on a block that requires invocation. With that analysis, the
 * terminate bit is set on the last instruction using invocations within any
 * block that does *not* require invocations out.
 *
 * Likewise, calculating the execute bit requires backward dataflow analysis
 * with union as the join operation and the generating set being the union of
 * sources of instructions writing executed values.
 */

/* Does a block use helpers directly */
static bool
mir_block_uses_helpers(gl_shader_stage stage, midgard_block *block)
{
        mir_foreach_instr_in_block(block, ins) {
                if (ins->type != TAG_TEXTURE_4) continue;
                if (mir_op_computes_derivatives(stage, ins->op))
                        return true;
        }

        return false;
}

static bool
mir_block_terminates_helpers(midgard_block *block)
{
        /* Can't terminate if there are no helpers */
        if (!block->helpers_in)
                return false;

        /* Can't terminate if a successor needs helpers */
        pan_foreach_successor((&block->base), succ) {
                if (((midgard_block *) succ)->helpers_in)
                        return false;
        }

        /* Otherwise we terminate */
        return true;
}

void
mir_analyze_helper_terminate(compiler_context *ctx)
{
        /* Set blocks as directly requiring helpers, and if they do add them to
         * the worklist to propagate to their predecessors */

        struct set *worklist = _mesa_set_create(NULL,
                        _mesa_hash_pointer,
                        _mesa_key_pointer_equal);

        struct set *visited = _mesa_set_create(NULL,
                        _mesa_hash_pointer,
                        _mesa_key_pointer_equal);

        mir_foreach_block(ctx, _block) {
                midgard_block *block = (midgard_block *) _block;
                block->helpers_in |= mir_block_uses_helpers(ctx->stage, block);

                if (block->helpers_in)
                        _mesa_set_add(worklist, _block);
        }

        /* Next, propagate back. Since there are a finite number of blocks, the
         * worklist (a subset of all the blocks) is finite. Since a block can
         * only be added to the worklist if it is not on the visited list and
         * the visited list - also a subset of the blocks - grows every
         * iteration, the algorithm must terminate. */

        struct set_entry *cur;

        while((cur = _mesa_set_next_entry(worklist, NULL)) != NULL) {
                /* Pop off a block requiring helpers */
                pan_block *blk = (struct pan_block *) cur->key;
                _mesa_set_remove(worklist, cur);

                /* Its predecessors also require helpers */
                pan_foreach_predecessor(blk, pred) {
                        if (!_mesa_set_search(visited, pred)) {
                                ((midgard_block *) pred)->helpers_in = true;
                                _mesa_set_add(worklist, pred);
                        }
                }
 
                _mesa_set_add(visited, blk);
        }

        _mesa_set_destroy(visited, NULL);
        _mesa_set_destroy(worklist, NULL);

        /* Finally, set helper_terminate on the last derivative-calculating
         * instruction in a block that terminates helpers */
        mir_foreach_block(ctx, _block) {
                midgard_block *block = (midgard_block *) _block;

                if (!mir_block_terminates_helpers(block))
                        continue;

                mir_foreach_instr_in_block_rev(block, ins) {
                        if (ins->type != TAG_TEXTURE_4) continue;
                        if (!mir_op_computes_derivatives(ctx->stage, ins->op)) continue;

                        ins->helper_terminate = true;
                        break;
                }
        }
}

static bool
mir_helper_block_update(BITSET_WORD *deps, pan_block *_block, unsigned temp_count)
{
        bool progress = false;
        midgard_block *block = (midgard_block *) _block;

        mir_foreach_instr_in_block_rev(block, ins) {
                /* Ensure we write to a helper dependency */
                if (ins->dest >= temp_count || !BITSET_TEST(deps, ins->dest))
                        continue;

                /* Then add all of our dependencies */
                mir_foreach_src(ins, s) {
                        if (ins->src[s] >= temp_count)
                                continue;

                        /* Progress if the dependency set changes */
                        progress |= !BITSET_TEST(deps, ins->src[s]);
                        BITSET_SET(deps, ins->src[s]);
                }
        }

        return progress;
}

void
mir_analyze_helper_requirements(compiler_context *ctx)
{
        mir_compute_temp_count(ctx);
        unsigned temp_count = ctx->temp_count;
        BITSET_WORD *deps = calloc(sizeof(BITSET_WORD), BITSET_WORDS(temp_count));

        /* Initialize with the sources of instructions consuming
         * derivatives */

        mir_foreach_instr_global(ctx, ins) {
                if (ins->type != TAG_TEXTURE_4) continue;
                if (ins->dest >= ctx->temp_count) continue;
                if (!mir_op_computes_derivatives(ctx->stage, ins->op)) continue;

                mir_foreach_src(ins, s) {
                        if (ins->src[s] < temp_count)
                                BITSET_SET(deps, ins->src[s]);
                }
        }

        /* Propagate that up */

        struct set *work_list = _mesa_set_create(NULL,
                        _mesa_hash_pointer,
                        _mesa_key_pointer_equal);

        struct set *visited = _mesa_set_create(NULL,
                        _mesa_hash_pointer,
                        _mesa_key_pointer_equal);

        struct set_entry *cur = _mesa_set_add(work_list, pan_exit_block(&ctx->blocks));

        do {
                pan_block *blk = (struct pan_block *) cur->key;
                _mesa_set_remove(work_list, cur);

                bool progress = mir_helper_block_update(deps, blk, temp_count);

                if (progress || !_mesa_set_search(visited, blk)) {
                        pan_foreach_predecessor(blk, pred)
                                _mesa_set_add(work_list, pred);
                }

                _mesa_set_add(visited, blk);
        } while((cur = _mesa_set_next_entry(work_list, NULL)) != NULL);

        _mesa_set_destroy(visited, NULL);
        _mesa_set_destroy(work_list, NULL);

        /* Set the execute bits */

        mir_foreach_instr_global(ctx, ins) {
                if (ins->type != TAG_TEXTURE_4) continue;
                if (ins->dest >= ctx->temp_count) continue;

                ins->helper_execute = BITSET_TEST(deps, ins->dest);
        }

        free(deps);
}