Posted on August 01, 2012 at 17:58You just have to worry about what the tools actually do, and if that behaviour is consistent. I have a trust but verify philosophy. I use assembler in these circumstances because it provides strong containment, and can have negligible RAM or STACK usage. They take less time for me to write and validate. Your circumstances could of course be different.
In C/C++ you need to be very careful with statics, external code usage, and the local/automatic footprint. Fixed data should be 'const static' and within the scope of the routine, not outside. Watch for preprocessor macros, be sure they inline code, and not call external routines. Generally you'd want to confirm code placement by reviewing the map and listing files. You could also put the specific code in a single object, and check for external linkage requirements, and test it in a stand-alone fashion.
Consider also if the code can return, or if you destroyed/changed the flash code that originally called in.
How any given version of GCC handles literals, or how optimization settings change that, I really don't know. Typically you'd park them at the end of the function, or behind an non-conditional branch. I believe Thumb2 can load high and low order half-word with separate immediate instructions, and certain constants can be loaded immediately by ARM chips. Also select address independent code generation.
