In http://gcc.gnu.org/ml/gcc/2005-12/msg00642.html, Bernd Jendrissek wrote: > Which leads me to the subject. Would it be a win to have a macro > HARD_REGNO_MODE_OK_FOR_CLASS (REGNO, MODE, CLASS) which would be the > authoritative test for this loop in find_reg()? On my port, and I > imagine on many others too, I think a default > > #ifndef HARD_REGNO_MODE_OK_FOR_CLASS > #define HARD_REGNO_MODE_OK_FOR_CLASS(REGNO, MODE, CLASS) \ > HARD_REGNO_MODE_OK ((REGNO), (MODE)) > #endif > > would be okay.
It's not that simple. For example, consider multi-word integer arithmetic. If you want to allocate a 32 bit integer register on your 16 bit x86, all the integer registers are suitable as parts of the allocation. However, if you start with the last integer register, the second part will end up in a hard register which is not an integer register. So to make this work, you's have to say that the last integer register is not suitable for SImode, SFmode or CHImode, the last three ones are not suitable for DImode, DFmode, CSImode or SCmode etc.
