piet-gpu/shader/draw_scan.comp

// SPDX-License-Identifier: Apache-2.0 OR MIT OR Unlicense

// A scan pass for draw tag scan implemented as a tree reduction.

#version 450
#extension GL_GOOGLE_include_directive : enable

#include "setup.h"
#include "drawtag.h"

#define N_ROWS 8
#define LG_WG_SIZE (7 + LG_WG_FACTOR)
#define WG_SIZE (1 << LG_WG_SIZE)
#define PARTITION_SIZE (WG_SIZE * N_ROWS)

layout(local_size_x = WG_SIZE, local_size_y = 1) in;

#define Monoid DrawMonoid
#define combine_monoid combine_tag_monoid
#define monoid_identity tag_monoid_identity

layout(binding = 0) buffer DataBuf {
    Monoid[] data;
};

#ifndef ROOT
layout(binding = 1) readonly buffer ParentBuf {
    Monoid[] parent;
};
#endif

shared Monoid sh_scratch[WG_SIZE];

void main() {
    Monoid local[N_ROWS];

    uint ix = gl_GlobalInvocationID.x * N_ROWS;

    local[0] = data[ix];
    for (uint i = 1; i < N_ROWS; i++) {
        local[i] = combine_monoid(local[i - 1], data[ix + i]);
    }
    Monoid agg = local[N_ROWS - 1];
    sh_scratch[gl_LocalInvocationID.x] = agg;
    for (uint i = 0; i < LG_WG_SIZE; i++) {
        barrier();
        if (gl_LocalInvocationID.x >= (1u << i)) {
            Monoid other = sh_scratch[gl_LocalInvocationID.x - (1u << i)];
            agg = combine_monoid(other, agg);
        }
        barrier();
        sh_scratch[gl_LocalInvocationID.x] = agg;
    }

    barrier();
    // This could be a semigroup instead of a monoid if we reworked the
    // conditional logic, but that might impact performance.
    Monoid row = monoid_identity();
#ifdef ROOT
    if (gl_LocalInvocationID.x > 0) {
        row = sh_scratch[gl_LocalInvocationID.x - 1];
    }
#else
    if (gl_WorkGroupID.x > 0) {
        row = parent[gl_WorkGroupID.x - 1];
    }
    if (gl_LocalInvocationID.x > 0) {
        row = combine_monoid(row, sh_scratch[gl_LocalInvocationID.x - 1]);
    }
#endif
    for (uint i = 0; i < N_ROWS; i++) {
        Monoid m = combine_monoid(row, local[i]);
        data[ix + i] = m;
    }
}