mir_promote_uniforms.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"
#include "util/u_math.h"
#include "util/u_memory.h"

/* This pass promotes reads from UBOs to register-mapped uniforms.  This saves
 * both instructions and work register pressure, but it reduces the work
 * registers available, requiring a balance.
 *
 * We use a heuristic to determine the ideal count, implemented by
 * mir_work_heuristic, which returns the ideal number of work registers.
 */

static bool
mir_is_ubo(midgard_instruction *ins)
{
        return (ins->type == TAG_LOAD_STORE_4) &&
                (OP_IS_UBO_READ(ins->op));
}

static bool
mir_is_direct_aligned_ubo(midgard_instruction *ins)
{
        return mir_is_ubo(ins) &&
                !(ins->constants.u32[0] & 0xF) &&
                (ins->src[1] == ~0) &&
                (ins->src[2] == ~0);
}

/* Represents use data for a single UBO */

#define MAX_UBO_QWORDS (65536 / 16)

struct mir_ubo_block {
        BITSET_DECLARE(uses, MAX_UBO_QWORDS);
        BITSET_DECLARE(pushed, MAX_UBO_QWORDS);
};

struct mir_ubo_analysis {
        /* Per block analysis */
        unsigned nr_blocks;
        struct mir_ubo_block *blocks;
};

static struct mir_ubo_analysis
mir_analyze_ranges(compiler_context *ctx)
{
        struct mir_ubo_analysis res = {
                .nr_blocks = ctx->nir->info.num_ubos + 1,
        };

        res.blocks = calloc(res.nr_blocks, sizeof(struct mir_ubo_block));

        mir_foreach_instr_global(ctx, ins) {
                if (!mir_is_direct_aligned_ubo(ins)) continue;

                unsigned ubo = midgard_unpack_ubo_index_imm(ins->load_store);
                unsigned offset = ins->constants.u32[0] / 16;

                assert(ubo < res.nr_blocks);

                if (offset < MAX_UBO_QWORDS)
                        BITSET_SET(res.blocks[ubo].uses, offset);
        }

        return res;
}

/* Select UBO words to push. A sophisticated implementation would consider the
 * number of uses and perhaps the control flow to estimate benefit. This is not
 * sophisticated. Select from the last UBO first to prioritize sysvals. */

static void
mir_pick_ubo(struct panfrost_ubo_push *push, struct mir_ubo_analysis *analysis, unsigned max_qwords)
{
        unsigned max_words = MIN2(PAN_MAX_PUSH, max_qwords * 4);

        for (signed ubo = analysis->nr_blocks - 1; ubo >= 0; --ubo) {
                struct mir_ubo_block *block = &analysis->blocks[ubo];

                unsigned vec4;
                BITSET_FOREACH_SET(vec4, block->uses, MAX_UBO_QWORDS) {
                        /* Don't push more than possible */
                        if (push->count > max_words - 4)
                                return;

                        for (unsigned offs = 0; offs < 4; ++offs) {
                                struct panfrost_ubo_word word = {
                                        .ubo = ubo,
                                        .offset = (vec4 * 16) + (offs * 4)
                                };

                                push->words[push->count++] = word;
                        }

                        /* Mark it as pushed so we can rewrite */
                        BITSET_SET(block->pushed, vec4);
                }
        }
}

#if 0
static void
mir_dump_ubo_analysis(struct mir_ubo_analysis *res)
{
        printf("%u blocks\n", res->nr_blocks);

        for (unsigned i = 0; i < res->nr_blocks; ++i) {
                BITSET_WORD *uses = res->blocks[i].uses;
                BITSET_WORD *push = res->blocks[i].pushed;

                unsigned last = BITSET_LAST_BIT_SIZED(uses, BITSET_WORDS(MAX_UBO_QWORDS));

                printf("\t");

                for (unsigned j = 0; j < last; ++j) {
                        bool used = BITSET_TEST(uses, j);
                        bool pushed = BITSET_TEST(push, j);
                        assert(used || !pushed);

                        putchar(pushed ? '*' : used ? '-' : '_');
                }

                printf("\n");
        }
}
#endif

static unsigned
mir_promoteable_uniform_count(struct mir_ubo_analysis *analysis)
{
        unsigned count = 0;

        for (unsigned i = 0; i < analysis->nr_blocks; ++i) {
                BITSET_WORD *uses = analysis->blocks[i].uses;

                for (unsigned w = 0; w < BITSET_WORDS(MAX_UBO_QWORDS); ++w)
                        count += util_bitcount(uses[w]);
        }

        return count;
}

static unsigned
mir_count_live(uint16_t *live, unsigned temp_count)
{
        unsigned count = 0;

        for (unsigned i = 0; i < temp_count; ++i)
                count += util_bitcount(live[i]);

        return count;
}

static unsigned
mir_estimate_pressure(compiler_context *ctx)
{
        mir_invalidate_liveness(ctx);
        mir_compute_liveness(ctx);

        unsigned max_live = 0;

        mir_foreach_block(ctx, _block) {
                midgard_block *block = (midgard_block *) _block;
                uint16_t *live = mem_dup(block->base.live_out, ctx->temp_count * sizeof(uint16_t));

                mir_foreach_instr_in_block_rev(block, ins) {
                        unsigned count = mir_count_live(live, ctx->temp_count);
                        max_live = MAX2(max_live, count);
                        mir_liveness_ins_update(live, ins, ctx->temp_count);
                }

                free(live);
        }

        return DIV_ROUND_UP(max_live, 16);
}

static unsigned
mir_work_heuristic(compiler_context *ctx, struct mir_ubo_analysis *analysis)
{
        unsigned uniform_count = mir_promoteable_uniform_count(analysis);

        /* If there are 8 or fewer uniforms, it doesn't matter what we do, so
         * allow as many work registers as needed */

        if (uniform_count <= 8)
                return 16;

        /* Otherwise, estimate the register pressure */

        unsigned pressure = mir_estimate_pressure(ctx);

        /* Prioritize not spilling above all else. The relation between the
         * pressure estimate and the actual register pressure is a little
         * murkier than we might like (due to scheduling, pipeline registers,
         * failure to pack vector registers, load/store registers, texture
         * registers...), hence why this is a heuristic parameter */

        if (pressure > 6)
                return 16;

        /* If there's no chance of spilling, prioritize UBOs and thread count */

        return 8;
}

/* Bitset of indices that will be used as a special register -- inputs to a
 * non-ALU op. We precompute this set so that testing is efficient, otherwise
 * we end up O(mn) behaviour for n instructions and m uniform reads */

static BITSET_WORD *
mir_special_indices(compiler_context *ctx)
{
        mir_compute_temp_count(ctx);
        BITSET_WORD *bset = calloc(BITSET_WORDS(ctx->temp_count), sizeof(BITSET_WORD));

        mir_foreach_instr_global(ctx, ins) {
                /* Look for special instructions */
                bool is_ldst = ins->type == TAG_LOAD_STORE_4;
                bool is_tex = ins->type == TAG_TEXTURE_4;
                bool is_writeout = ins->compact_branch && ins->writeout;

                if (!(is_ldst || is_tex || is_writeout))
                        continue;

                /* Anything read by a special instruction is itself special */
                mir_foreach_src(ins, i) {
                        unsigned idx = ins->src[i];

                        if (idx < ctx->temp_count)
                                BITSET_SET(bset, idx);
                }
        }

        return bset;
}

void
midgard_promote_uniforms(compiler_context *ctx)
{
        if (ctx->inputs->no_ubo_to_push) {
                /* If nothing is pushed, all UBOs need to be uploaded
                 * conventionally */
                ctx->ubo_mask = ~0;
                return;
        }

        struct mir_ubo_analysis analysis = mir_analyze_ranges(ctx);

        unsigned work_count = mir_work_heuristic(ctx, &analysis);
        unsigned promoted_count = 24 - work_count;

        /* Ensure we are 16 byte aligned to avoid underallocations */
        mir_pick_ubo(&ctx->info->push, &analysis, promoted_count);
        ctx->info->push.count = ALIGN_POT(ctx->info->push.count, 4);

        /* First, figure out special indices a priori so we don't recompute a lot */
        BITSET_WORD *special = mir_special_indices(ctx);

        ctx->ubo_mask = 0;

        mir_foreach_instr_global_safe(ctx, ins) {
                if (!mir_is_ubo(ins)) continue;

                unsigned ubo = midgard_unpack_ubo_index_imm(ins->load_store);
                unsigned qword = ins->constants.u32[0] / 16;

                if (!mir_is_direct_aligned_ubo(ins)) {
                        if (ins->src[1] == ~0)
                                ctx->ubo_mask |= BITSET_BIT(ubo);
                        else
                                ctx->ubo_mask = ~0;

                        continue;
                }

                /* Check if we decided to push this */
                assert(ubo < analysis.nr_blocks);
                if (!BITSET_TEST(analysis.blocks[ubo].pushed, qword)) {
                        ctx->ubo_mask |= BITSET_BIT(ubo);
                        continue;
                }

                /* Find where we pushed to, TODO: unaligned pushes to pack */
                unsigned base = pan_lookup_pushed_ubo(&ctx->info->push, ubo, qword * 16);
                assert((base & 0x3) == 0);

                unsigned address = base / 4;
                unsigned uniform_reg = 23 - address;

                /* Should've taken into account when pushing */
                assert(address < promoted_count);
                unsigned promoted = SSA_FIXED_REGISTER(uniform_reg);

                /* We do need the move for safety for a non-SSA dest, or if
                 * we're being fed into a special class */

                bool needs_move = ins->dest & PAN_IS_REG || ins->dest == ctx->blend_src1;

                if (ins->dest < ctx->temp_count)
                        needs_move |= BITSET_TEST(special, ins->dest);

                if (needs_move) {
                        unsigned type_size = nir_alu_type_get_type_size(ins->dest_type);
                        midgard_instruction mov = v_mov(promoted, ins->dest);
                        mov.dest_type = nir_type_uint | type_size;
                        mov.src_types[1] = mov.dest_type;

                        uint16_t rounded = mir_round_bytemask_up(mir_bytemask(ins), type_size);
                        mir_set_bytemask(&mov, rounded);
                        mir_insert_instruction_before(ctx, ins, mov);
                } else {
                        mir_rewrite_index_src(ctx, ins->dest, promoted);
                }

                mir_remove_instruction(ins);
        }

        free(special);
        free(analysis.blocks);
}