Monday, April 11, 2005

Assorted Hacking

Lately I've been dealing with the random bugs of close to release software. The 64-bit eabi stuff is giving UNPREDICTABLE results for a simple:

la reg,0x80000000

macro instruction. Now, I hate macro instructions. There's never an excuse for not saying exactly what you mean if you're going to bother to write assembly in the first place. Especially in cases like this - there are over 2 thousand lines of code in gas just to deal with the la and li macros - and have I mentioned that we don't document them anywhere?

I mean anywhere. One of the canonical bibles of the MIPS architecture is the Kane and Heinrich book. It describes this instruction as "an addiu and an lui if needed". This description is so woefully inadequate that it's hard to specify what the input to the macro should be. If it's an absolute expression like we have above it doesn't specify if we should sign extend the address before adding it. The method of least surprise says that we should. Then there's whether or not we should warn about this questionable behavior. Right now, we do - but only for n64.

The other question is whether or not you should just use the fully qualified address for both 32 and 64-bit cases. This would solve the problem, and is indeed allowed by gas, but it's a little iffy in my book.

I suppose I should start adding all of these things to the wiki page here: MIPS Assembly Programming Wikibook.

Just remember kids, assembler macros are a tool of the devil.