And both are wrong since the values would have to be in a linear color space for for the compositing math to make sense. But in some non-linear space to be useful when mapped to 0..255 (e.g non-linear sRGB).

Which happens right after the Porter-Duff Over operator above -- a smoking gun. Which one is it gonna be?

I.e. the display transform is omitted from this and the math involved with the latter makes your whole argument moot.

It can't be expressed well enough with bitshifts to keep your purported 10x speedup anyway (and which I strongly doubt btw).

And lastly: in a software renderer that stuff is usually <0.01% of the compute in the absolut worst case.

P.S.: I'm speaking from 30 years of experience with software rendering in the context of VFX.

If the latter is 10x faster, the issue is some kind of weird compilation failure for the above version. For one, it only cuts a third of the multiplies.

How is this supposed to be 10x faster if all you did was drop one out of three multiplications?

Because you are working in the cache.

Also, you should use SIMD.