devkitPro

I've been fooling around seeing what I can get to compile from cmake's fetch content for the gba when I got an idea after trying this. It's a fixed precision math library and it compiles perfectly without modification which was very satisfying. Since it's math related I thought it would be a good idea to put it into iwram. However I realized I have no idea how to mark symbols from linked libraries to get put into the iwram section. Without adding IWRAM_DATA/IWRAM_CODE to the source of the library, is it possible to move some of the symbols into iwram at the time of linkage?

This isn't something we have any particular experience with. It may be possible using a custom linkscript but there are a number of caveats with this kind of process.
IWRAM on the GBA is only 32K and, with stock linkscripts anyway, this contains stack and uninitialised variables - space is very much at a premium here.

Code that runs from EWRAM and ROM is generally compiled as thumb - there's an access penalty for arm instructions in these areas since they only have a 16bit bus.

IWRAM has a full 32bit bus so arm instructions in here will run as fast as possible. Thumb will run a bit faster but I'm not sure it's worth the trouble.

Ultimately it would be best to pull functions out & compile them as arm rather than attempting to avoid modifying the original library.

Thanks I think this may be a little beyond me at the moment after all. I ended up just writing a git patch for the library and added the sections attribute to the ones I wanted in iwram.

Is code marked with the EWRAM_CODE define automatically compiled as thumb by the toolchain? Also IWRAM_CODE and IWRAM_DATA can't compile together in the same translation unit. Am I supposed to just keep such code separate?

Sorry this is unfamiliar for me.

Apologies. This is an issue that's appeared with newer gcc releases and we had a PR sitting there to address it that we never got round to merging. IWRAM_CODE and IWRAM_DATA should work fine together with latest libgba.

The compiler & the templates default to compiling code as thumb unless you tell it otherwise. The devkitPro compilation rules will compile source files named as *.iwram.c/cpp as arm and place resulting code in iwram. You can also use ARM_CODE to mark individual functions to be compiled as arm.

That's super helpful! Thank you so much!