vello/piet-wgsl/shader/draw_reduce.wgsl

// Copyright 2022 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// Also licensed under MIT license, at your choice.

#import config
#import drawtag

@group(0) @binding(0)
var<storage> config: Config;

@group(0) @binding(1)
var<storage> scene: array<u32>;

@group(0) @binding(2)
var<storage, read_write> reduced: array<DrawMonoid>;

let WG_SIZE = 256;

var<workgroup> sh_scratch: array<DrawMonoid, WG_SIZE>;

@compute @workgroup_size(256)
fn main(
    @builtin(global_invocation_id) global_id: vec3<u32>,
    @builtin(local_invocation_id) local_id: vec3<u32>,
) {
    let ix = global_id.x;
    let tag_word = scene[config.drawtag_base + ix];
    let agg = map_draw_tag(tag_word);
    sh_scratch[local_id.x] = agg;
    for (var i = 0u; i < firstTrailingBit(WG_SIZE); i += 1u) {
        workgroupBarrier();
        if local_id.x + (1u << i) < WG_SIZE {
            let other = sh_scratch[local_id.x + (1u << i)];
            agg = combine_draw_monoid(agg, other);
        }
        workgroupBarrier();
        sh_scratch[local_id.x] = agg;
    }
    if local_id.x == 0u {
        reduced[ix >> LG_WG_SIZE] = agg;
    }
}