A Stale Safety Comment Parked a 10 ms Win for Three Days

The guard against GPU skinning was correct when written; nobody updated it after the two bugs it guarded against were fixed.

Core Finding:A code comment barred switching character skinning to the GPU until an A/B showed the body rendered correctly. It was written when that was true, then outlived two fixes by three days. Re-running the A/B it demanded showed 1 differing pixel in 3,686,400 and a frame-time drop from 15.520 ms to 5.064 ms.

Frame time, scalar vs GPU skinning

-67.4%

Frame time: scalar 15.520 ms, GPU 5.064 ms (-10.456 ms, -67.4%); no-skinned-body floor 3.587 ms; SIMD path 25.856 ms.

Scalar vs GPU image diff

1 differing pixel

Scalar vs scalar: 0 differing pixels of 3,686,400. Scalar vs GPU: 1 differing pixel, max channel delta 1.

Per-body-pair skin cost

11.933 ms to 1.477 ms

Per-body-pair skin cost: 11.933 ms to 1.477 ms.

Wrong records of this default

four were wrong

Records of the default for this one switch: five; four were wrong.

Empirical Findings Table

Re-running the A/B a stale comment demanded

Telemetry Metric	Measured Value	Context & Observability
Frame time, scalar vs GPU skinning	-67.4%	Frame time: scalar 15.520 ms, GPU 5.064 ms (-10.456 ms, -67.4%); no-skinned-body floor 3.587 ms; SIMD path 25.856 ms.
Scalar vs GPU image diff	1 differing pixel	Scalar vs scalar: 0 differing pixels of 3,686,400. Scalar vs GPU: 1 differing pixel, max channel delta 1.
Per-body-pair skin cost	11.933 ms to 1.477 ms	Per-body-pair skin cost: 11.933 ms to 1.477 ms.
Wrong records of this default	four were wrong	Records of the default for this one switch: five; four were wrong.
Body drawn	38,240 px	Body drawn: 38,240 px on both arms with identical bounding box.

Engineering Action Directives

A blocking comment is a claim about the world, and it needs the same expiry discipline as any other claim. This one encoded a valid precondition but not a way to notice when the precondition stopped being true.
Confirming that the new path actually ran mattered as much as the pixel diff: a fail-soft fallback would have measured the old path twice and reported a perfect match.

Methodology Boundaries & Limits

One scene, one character setup, one machine; the 10 ms figure depends on how much skinned geometry is on screen.
Four paired legs is a small sample, though the effect is large relative to the differences between legs we observed.
Hardware is not recorded in the source.