search

AntonioND / ucity

The open-source city-building game for Game Boy Color.
423 stars 17 forks source link

Improve multiplication function #2

Closed lancekindle closed 6 years ago

lancekindle commented 7 years ago

I'm not sure if it's worth it (readability-wise), but for a minor speed boost you can set H=A & L=0 at the beginning of mul_u8u8u16, and use 'add hl,hl' to do both 'rla' and 'add hl,hl' at once. This shaves off a few cycles in the overall execution of the mul_u8u8u16 routine. It took me a while to wrap my head around this, but it helps to realize that 'add hl,bc' never overwrites any bits from A remaining in register H.

AntonioND commented 6 years ago

First of all, sorry for not answering before. I saw the pull request, but I was really busy that week. By the time I was about to answer I had forgotten what I was going to say, and then I simply forgot to answer at all.

This optimization has a small problem, it relies on the C flag being set correctly after an "add hl, hl". It makes me feel a bit uneasy because there are some crappy emulators out there that have trouble with the 16-bit arithmetic additions. I'm not worried about PC emulators, but about emulators for other platforms that may be the only way to play this game on them...

lancekindle commented 6 years ago

That's totally cool, man. I closed my pull request because I had been wondering if it was worthwhile implementing. I imagine it would take some effort (such as writing tests) to verify it worked--and not just "oh hey, the game doesn't crash". I hadn't considered emulator errors; that in itself is reason enough.

P.S. your code is excellent! I am enjoying reading through--and seeing a lot of cool tricks in--your code. Thanks for making it available :)

AntonioND commented 6 years ago

Oh, ok. Thanks for taking the time to check the code and do it, though!

AntonioND commented 6 years ago

For future reference, this has been merged as explained in https://github.com/AntonioND/ucity/pull/4