On Thu, Oct 03, 2019 at 10:52:38AM +0100, Richard Sandiford wrote: > /* Certain fixed registers might be of the class NO_REGS. This means > that not only can they not be allocated by the compiler, but > they cannot be used in substitutions or canonicalizations > either. */ > > (from cse.c). This isn't actually documented AFAICT. But it is > useful for registers that are only allowed to appear in special > instructions that the port generates itself. (Or at least it was useful. > operand_reg_set was supposed to be a more flexible way of doing this.)
You can also put such regs in ALL_REGS but in no other register class. And it works fine if you do not handle non-allocatable registers at all in REGNO_REG_CLASS, btw., but it should really be ALL_REGS then. > There's also the problem that if you don't actually need a register > to be in a class for its own sake, but only because the documentation > says so, then it's less clear how individual registers should be > grouped into classes. E.g. is having separate SFP_REG and AFP_REG > classes right (as above), or should they really be members of a > single class? In practice it doesn't matter because nothing ever > uses these classes [ ... ] Yeah exactly. Register classes only matter for allocatable registers. Thanks for the exposition, Segher
