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[ExecuTorch][WebGPU] Coalesce SDPA AV V-cache reads along contiguous head-dim #20459

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[ExecuTorch][WebGPU] Coalesce SDPA AV V-cache reads along contiguous head-dim #20459

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JulianCloudNTH Jun 23, 2026
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JulianCloudNTH Jun 24, 2026
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JulianCloudNTH Jun 25, 2026
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31 changes: 15 additions & 16 deletions backends/webgpu/runtime/ops/sdpa/sdpa_compute_out.wgsl
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Original file line number Diff line number Diff line change
Expand Up @@ -32,17 +32,16 @@ fn load_a_vec4(s: u32, h: u32, c4: u32) -> vec4<f32> {
return r;
}

fn load_v_vec4(d: u32, kvh: u32, c4: u32) -> vec4<f32> {
fn load_v_d4(c: u32, kvh: u32, d0: u32) -> vec4<f32> {
var r = vec4<f32>(0.0, 0.0, 0.0, 0.0);
if (d >= params.D) {
if (c >= params.context_len) {
return r;
}
let stride = params.Hkv * params.D;
let off = kvh * params.D + d;
if (c4 + 0u < params.context_len) { r.x = t_v_cache[(c4 + 0u) * stride + off]; }
if (c4 + 1u < params.context_len) { r.y = t_v_cache[(c4 + 1u) * stride + off]; }
if (c4 + 2u < params.context_len) { r.z = t_v_cache[(c4 + 2u) * stride + off]; }
if (c4 + 3u < params.context_len) { r.w = t_v_cache[(c4 + 3u) * stride + off]; }
let base = c * params.Hkv * params.D + kvh * params.D + d0;
if (d0 + 0u < params.D) { r.x = t_v_cache[base + 0u]; }
if (d0 + 1u < params.D) { r.y = t_v_cache[base + 1u]; }
if (d0 + 2u < params.D) { r.z = t_v_cache[base + 2u]; }
if (d0 + 3u < params.D) { r.w = t_v_cache[base + 3u]; }
return r;
}

Expand Down Expand Up @@ -87,14 +86,14 @@ fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
let a1 = load_a_vec4(s0 + 1u, h, c4);
let a2 = load_a_vec4(s0 + 2u, h, c4);
let a3 = load_a_vec4(s0 + 3u, h, c4);
let v0 = load_v_vec4(d0 + 0u, kvh, c4);
let v1 = load_v_vec4(d0 + 1u, kvh, c4);
let v2 = load_v_vec4(d0 + 2u, kvh, c4);
let v3 = load_v_vec4(d0 + 3u, kvh, c4);
acc[0] += vec4<f32>(dot(a0, v0), dot(a0, v1), dot(a0, v2), dot(a0, v3));
acc[1] += vec4<f32>(dot(a1, v0), dot(a1, v1), dot(a1, v2), dot(a1, v3));
acc[2] += vec4<f32>(dot(a2, v0), dot(a2, v1), dot(a2, v2), dot(a2, v3));
acc[3] += vec4<f32>(dot(a3, v0), dot(a3, v1), dot(a3, v2), dot(a3, v3));
let v0 = load_v_d4(c4 + 0u, kvh, d0);
let v1 = load_v_d4(c4 + 1u, kvh, d0);
let v2 = load_v_d4(c4 + 2u, kvh, d0);
let v3 = load_v_d4(c4 + 3u, kvh, d0);
acc[0] += a0.x * v0 + a0.y * v1 + a0.z * v2 + a0.w * v3;
acc[1] += a1.x * v0 + a1.y * v1 + a1.z * v2 + a1.w * v3;
acc[2] += a2.x * v0 + a2.y * v1 + a2.z * v2 + a2.w * v3;
acc[3] += a3.x * v0 + a3.y * v1 + a3.z * v2 + a3.w * v3;
c4 = c4 + 4u;
}

Expand Down
33 changes: 16 additions & 17 deletions backends/webgpu/runtime/ops/sdpa/sdpa_compute_out_wgsl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -13,7 +13,7 @@
namespace executorch::backends::webgpu {

// @generated from sdpa_compute_out.wgsl - DO NOT EDIT.
// wgsl-sha256: 4ffc13bad0bf56b87a57f75307f29e851dd2bd6bf0dba094488df5d262e910e3
// wgsl-sha256: 545f624567b08eba407954034df821010e49124fa6f8fd6b05c64ca4354ee4cc
inline constexpr const char* kSdpaComputeOutWGSL = R"(
@group(0) @binding(0) var<storage, read_write> t_out: array<f32>;
@group(0) @binding(1) var<storage, read> t_attn_weights_softmax: array<f32>;
Expand Down Expand Up @@ -49,17 +49,16 @@ fn load_a_vec4(s: u32, h: u32, c4: u32) -> vec4<f32> {
return r;
}

fn load_v_vec4(d: u32, kvh: u32, c4: u32) -> vec4<f32> {
fn load_v_d4(c: u32, kvh: u32, d0: u32) -> vec4<f32> {
var r = vec4<f32>(0.0, 0.0, 0.0, 0.0);
if (d >= params.D) {
if (c >= params.context_len) {
return r;
}
let stride = params.Hkv * params.D;
let off = kvh * params.D + d;
if (c4 + 0u < params.context_len) { r.x = t_v_cache[(c4 + 0u) * stride + off]; }
if (c4 + 1u < params.context_len) { r.y = t_v_cache[(c4 + 1u) * stride + off]; }
if (c4 + 2u < params.context_len) { r.z = t_v_cache[(c4 + 2u) * stride + off]; }
if (c4 + 3u < params.context_len) { r.w = t_v_cache[(c4 + 3u) * stride + off]; }
let base = c * params.Hkv * params.D + kvh * params.D + d0;
if (d0 + 0u < params.D) { r.x = t_v_cache[base + 0u]; }
if (d0 + 1u < params.D) { r.y = t_v_cache[base + 1u]; }
if (d0 + 2u < params.D) { r.z = t_v_cache[base + 2u]; }
if (d0 + 3u < params.D) { r.w = t_v_cache[base + 3u]; }
return r;
}

Expand Down Expand Up @@ -104,14 +103,14 @@ fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
let a1 = load_a_vec4(s0 + 1u, h, c4);
let a2 = load_a_vec4(s0 + 2u, h, c4);
let a3 = load_a_vec4(s0 + 3u, h, c4);
let v0 = load_v_vec4(d0 + 0u, kvh, c4);
let v1 = load_v_vec4(d0 + 1u, kvh, c4);
let v2 = load_v_vec4(d0 + 2u, kvh, c4);
let v3 = load_v_vec4(d0 + 3u, kvh, c4);
acc[0] += vec4<f32>(dot(a0, v0), dot(a0, v1), dot(a0, v2), dot(a0, v3));
acc[1] += vec4<f32>(dot(a1, v0), dot(a1, v1), dot(a1, v2), dot(a1, v3));
acc[2] += vec4<f32>(dot(a2, v0), dot(a2, v1), dot(a2, v2), dot(a2, v3));
acc[3] += vec4<f32>(dot(a3, v0), dot(a3, v1), dot(a3, v2), dot(a3, v3));
let v0 = load_v_d4(c4 + 0u, kvh, d0);
let v1 = load_v_d4(c4 + 1u, kvh, d0);
let v2 = load_v_d4(c4 + 2u, kvh, d0);
let v3 = load_v_d4(c4 + 3u, kvh, d0);
acc[0] += a0.x * v0 + a0.y * v1 + a0.z * v2 + a0.w * v3;
acc[1] += a1.x * v0 + a1.y * v1 + a1.z * v2 + a1.w * v3;
acc[2] += a2.x * v0 + a2.y * v1 + a2.z * v2 + a2.w * v3;
acc[3] += a3.x * v0 + a3.y * v1 + a3.z * v2 + a3.w * v3;
c4 = c4 + 4u;
}

Expand Down
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