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Brian Degenhardt
0b9e9cdfcb
The per-lane colour offsets were packed with the signed saturating pack while both code generators used the unsigned one. The mask above the pack has already put every lane in 0..65535, which makes the unsigned pack the identity and makes the signed pack flatten everything from 32768 up to 32767. A descending gouraud gradient is how a lane gets there: its offset is negative, the mask turns it into a large positive, and the pack saturates it. Every pixel of the group then carries that instead of its own colour, for the whole scanline. The mask and the unsigned pack were introduced together to fix exactly this, in "GS/SW: Mask color gradients to prevent incorrect clamping"; a later refactor that rewrote the same lines to change how the shift table is loaded retyped the tail back to the signed pack. The generators were not part of that refactor, which is why only the C++ path regressed and why nothing noticed. Where the path is reachable, measured rather than argued: with the rasteriser JIT on, a probe at the top of the C++ setup never fires across corpus replays that generate tens of kilobytes of scanline code apiece. It is entered only when there is no code memory to compile into at all, and that same condition turns off the EE, IOP and VU recompilers, so it is not a configuration anyone plays in. What it is, is the path a measurement runs under -- the only way to ask what the renderer computes without a JIT in the way, and so the arbiter of a generated-code question. It was about to arbitrate one, and would have lied: the gs-shade console capture re-run under it differed from the generated arm in 42,240 bytes, concentrated in exactly the gouraud colour it was to be asked about. It is now byte-identical, and the generated arm is byte-identical to before the change, so nothing a shipping build renders moves. The new suite runs both paths over the same spans and compares the setup state and the stored pixels, so the next divergence anywhere in the scanline fails loudly instead of waiting for a capture to find it.