uiCompactChunk ran the survivor scan entirely on gl_LocalInvocationIndex
== 0: ~64 serial iterations with 63/64 lanes idle, between two barriers,
4x per chunk in the fused kernel. It must emit a stable in-order
(buffer-order) exclusive prefix of the survivors so the per-pixel inner
loop still sees items in draw order.
Replace it with a per-subgroup subgroupExclusiveAdd of the keep bits plus
a carry across subgroups: each subgroup publishes its survivor total to
shared memory, then every lane sums the totals of all lower-id subgroups
for its base. A survivor's slot in s_order[] therefore equals the number
of survivors with a smaller local index — buffer (draw) order preserved
exactly, unlike an atomicAdd which would scramble it.
The carry makes the result correct for any subgroup width (2 subgroups on
the 32-wide descriptor_heap target, up to 8/16 on narrower parts), relying
only on the gl_LocalInvocationIndex <-> (gl_SubgroupID,
gl_SubgroupInvocationID) linear mapping every Vulkan compute
implementation provides. The feature is free at the device baseline
(apiVersion 1.4 + VK_EXT_descriptor_heap implies Vulkan 1.1 subgroup
arithmetic), so no correctness fallback is needed; WebGPU is unaffected
(separate embedded WGSL).
Applied to ui-fused.comp.glsl and the same pattern inlined in
ui-quads/circles/images/text.
Verified: all 23 tests pass (UIFusedShader recompiles + spirv-val +
pins push-constant offsets); a 140k-trial CPU simulation of the algorithm
matches the serial reference for subgroup sizes 1..64; HelloUI renders
correctly on an RTX 4090 (DispatchFused quads+circles+text) with draw
order intact.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
The standalone per-category UI shaders (quads/circles/images/text)
unconditionally imageLoad the destination pixel up front and imageStore
it at the end, paying a full read-modify-write even on tiles no item
touches — the common case for a sparse UI. The fused uber-kernel already
amortizes a single load/store across all four categories; the standalone
Dispatch* path has no such umbrella.
Defer the imageLoad to just before the first surviving blend (`loaded`
flag) and skip the imageStore unless something was actually blended.
Output is bit-identical: before the first blend `dst` is unused, and a
skipped store leaves the pixel exactly as a no-op load+store would have
rewritten it.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
The cooperative-load section already streams each item into shared memory
once per workgroup, but the per-pixel inner loop then recomputed values
that are constant for the whole item/glyph. The compiler can't hoist them
itself — they read shared memory at a varying index.
Precompute them once per item at load time into new shared slots:
- ui-images / ui-text / ui-fused (images+text phases): invRectSize =
1.0/rect.zw, turning the per-pixel `(sp-rect.xy)/rect.zw` vec2 divide
into a multiply.
- ui-text / ui-fused (text phase): the SDF AA `band` (a vec2 divide +
two maxes depending only on the glyph's uv span and rect size).
In ui-fused the new slots (s_inv, s_band) are reused across phases like
the existing s_v* scratch, so the LDS bump is per-category, not summed.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Each 8×8-tile workgroup previously had all 64 threads walk the entire
item list, re-reading every item from the SSBO 64× and running the
per-pixel reject for items that never touch the tile — O(tiles × N)
item loads dominated by SSBO traffic.
The workgroup now streams the item list in chunks of 64: each thread
loads one item, tests its AABB against the whole tile, and the survivors
are compacted — in original buffer order — into shared memory. Every
thread then runs the unchanged per-pixel accumulate over only those
survivors. This drops SSBO traffic ~64× (each item read once per
workgroup instead of once per pixel) and shrinks the inner loop to the
items that actually overlap the tile.
Draw order is preserved exactly: chunks run in array order and the
in-chunk compaction is a stable in-order scan, so later items still
overdraw earlier ones (no atomic-append nondeterminism). The inner loop
body is byte-for-byte the original per-pixel logic, so output is
pixel-identical — the cull only decides which items reach it. Threads
no longer early-return so every thread reaches the workgroup barriers.
Applied to ui-quads, ui-circles, ui-images and ui-text; shared helpers
(uiTileBounds / uiAabbOverlapsTile) live in ui-shared.glsl.
Resolves#46
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
