Catcrafts/Crafter.Graphics
2
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

WebGPU RT: enable TLAS spatial sort via bitonic network

Browse source 
Replace the disabled LSD radix sort in lbvhBuildMain with a data-oblivious
workgroup bitonic sorting network and enable it. The radix scatter was gated
behind `if (false)` because it produced count/distribution-dependent
corruption (TODO-lbvh-sort.md) — a memory-ordering bug in the Hillis-Steele
scan / parallel scatter that surfaced only for certain Morton distributions
(a small object beside a tight cluster), making geometry flicker.

A bitonic network's compare-exchange schedule depends only on N_PADDED, never
on key values, so it sidesteps that entire class of distribution-dependent
races (TODO strategy #5). 105 sub-stages over 2^14 keys, single workgroup of
1024 threads, 8 compare-exchanges/thread/sub-stage, operating in-place on
sortA with a storageBarrier between sub-stages. Sentinel keys (0xFFFFFFFF)
compare largest and settle at the tail, exactly where Phase 4 expects them.
Restores Morton (Z-order) spatial coherence to TLAS BVH leaves, which the
many-instance case needs. Removes the now-dead radix histogram/scan workgroup
memory and constants.

Verified on the Firefox/Dawn WebGPU stack: a GPU unit test diffs the kernel
output against a CPU oracle across all three required distributions
(all-uniform, all-one-bucket, small-object-next-to-cluster) plus random,
reverse, and empty inputs — all match bit-for-bit with a valid index
permutation. Sponza renders correctly with the sort live.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
catbot 2026-05-31 15:48:29 +00:00
commit 14091dcdca
2 changed files with 66 additions and 132 deletions
Download patch file Download diff file Expand all files Collapse all files

15
TODO-lbvh-sort.md
View file

@ -1,5 +1,20 @@
# LBVH parallel radix sort: count-dependent corruption
> **RESOLVED (strategy #5 — bitonic sort).** The LSD radix scatter was
> replaced with a data-oblivious workgroup **bitonic sorting network** in
> `lbvhBuildMain` (`additional/dom-webgpu.js`, Phase 2). Because a bitonic
> network's compare-exchange schedule depends only on N_PADDED — never on
> the key distribution — it cannot exhibit the count-dependent corruption
> documented below. The sort is now enabled (the old `if (false)` guard is
> gone) so TLAS leaves are Morton (Z-order) coherent again.
>
> Verified on the Firefox/Dawn WebGPU stack with a GPU unit test that diffs
> the kernel output against a CPU oracle across all three required
> distributions (all-uniform, all-one-bucket, and the "small object next to
> a tight cluster" repro) plus random/reverse/empty edge cases — all match
> bit-for-bit, with a valid index permutation. Sponza renders correctly with
> the sort live. The historical analysis below is retained for context.
## Summary
The parallel radix sort in `lbvhBuildMain` (additional/dom-webgpu.js) produces