Optimizing additive blending

I’m making a game where I use additive blending for some effects.
Now, I’ve got simple additive blending functions which work fine, but
they’re probably not very optimized. At the moment they perform
well enough, but when I add some explosion sprites later, all extra
speed would be useful.

I’m attaching the functions, and I’m hoping that you could take a peek,
and possibly give me some tips to increase their speed.
At the moment they support all high/true-color modes (12,15,16,24,32)
and I’d like to have them support at least 15/16 and 32 after any
possible improvements.

Thanks for any tips/code.
PS. I’d like to mention the names of those who’ve helped me in the
game’s credits section, so be sure to mention if you don’t want
yourself credited.–
Mika Halttunen
-------------- next part --------------
A non-text attachment was scrubbed…
Name: blend.cpp
Type: text/x-c++
Size: 6438 bytes
Desc: not available
URL: http://lists.libsdl.org/pipermail/sdl-libsdl.org/attachments/20021214/ad6ee6a0/attachment.bin