pan_lower_sample_position.c

/*
 * Copyright (C) 2021 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 "pan_ir.h"
#include "compiler/nir/nir_builder.h"

/* Sample positions are supplied in a packed 8:8 fixed-point vec2 format in GPU
 * memory indexed by the sample. We lower in NIR to take advantage of possible
 * ALU optimizations at the end. This is convenient for Bifrost, since the
 * sample positions are passed in this format and it saves the driver from any
 * system value handling. For Midgard, it's a bit suboptimal (fp16 positions
 * could be supplied directly), but this lets us unify the implementation, and
 * it's a pretty trivial difference */

static bool
pan_lower_sample_pos_impl(struct nir_builder *b,
                nir_instr *instr, UNUSED void *data)
{
        if (instr->type != nir_instr_type_intrinsic)
                return false;

        nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);
        if (intr->intrinsic != nir_intrinsic_load_sample_pos)
                return false;

        b->cursor = nir_before_instr(instr);

        /* Elements are 4 bytes */
        nir_ssa_def *addr = nir_iadd(b,
                        nir_load_sample_positions_pan(b),
                        nir_u2u64(b, nir_imul_imm(b, nir_load_sample_id(b), 4)));

        /* Decode 8:8 fixed-point */
        nir_ssa_def *raw = nir_load_global(b, addr, 2, 2, 16);
        nir_ssa_def *decoded = nir_fmul_imm(b, nir_i2f16(b, raw), 1.0 / 256.0);

        /* Make NIR validator happy */
        if (decoded->bit_size != nir_dest_bit_size(intr->dest))
                decoded = nir_f2fN(b, decoded, nir_dest_bit_size(intr->dest));

        nir_ssa_def_rewrite_uses(&intr->dest.ssa, decoded);
        return true;
}

bool
pan_lower_sample_pos(nir_shader *shader)
{
        if (shader->info.stage != MESA_SHADER_FRAGMENT)
                return false;

        return nir_shader_instructions_pass(shader,
                        pan_lower_sample_pos_impl,
                        nir_metadata_block_index | nir_metadata_dominance,
                        NULL);
}