On Wed, Nov 23, 2016 at 05:53:43PM +0100, Georg-Johann Lay wrote: > >So why does the define_insn allow it? > > Because the insn predicate is register_operand:HI which should be fine > as it is non-strict RTL. Or are predicates supposed to reject such odd > operands the backend would never generate itself?
I am not sure. > Are you saying that register_operand:HI is not correct? No; I think register_operand:HI should not allow odd hard registers. > IMO, if combine is playing with hard regs, it should be conservative and > not create new hard regs out of thin air... like > > * Test HARD_REGNO_MODE_OK Yes, I'll have a patch for that in a minute. Combine is supposed to make code better, not worse ;-) > * Don't increase HARD_REGNO_NREGS because it's not a good idea to create > hard regs out of thin air. On avr, for example, one should strive to > get to smaller modes, not to blow mode sizes... Combine tries to make fewer RTL insns (not machine insns), and then rejects a combination if the rtx costs say it is more expensive than before. It sounds like your costs are not tuned well enough? Segher
