Re: question about register pairs

[DJ Delorie <dj at redhat dot com>]

> > Some notes: I lie to gcc and tell it that $fp (reg 22) is two bytes
> > when it's really one.
> 
> Well, it's not really a lie if you map hardware registers 22 and 23 to
> a single register for the purposes of gcc internals.

Yeah, I'm basically making those two registers into a permanent bigger
register.

> Although it does make some other things more awkward, e.g. when you
> copy fp, and this gets split so you have an insn that copies the
> highpart of fp to another register.

Not a problem, the chip can copy the whole $fp in one insn.  Registers
are only 8 bits on this chip.

> Seeing the patched code in its entirety like this, I notice that we
> would use HARD_REGNO_NREGS for a regno that's not ok for the mode.

The core problem is that gcc has no way of dealing with register pairs
as real registers - the second halves are still registers, still need
to be in the reg class, still need _NREGS() values, etc.  There's no
way to tell gcc that a "register" is a set of (for example) N
registers starting at 3, 5, 9, 12, or 14.

> That can be avoided if we put a break into the if.  And then the
> !bad term in the loop condition becomes redundant.

The problem I had to solve was to re-synchronize the counter with the
even-numbered register.  The naive patch counted even registers up to
$fp, then checked odd (invalid) registers after that.

The only other way I can think to "solve" this problem is to add a
VALID_STARTING_REG(reg,mode) macro, but that would require changes all
over the core code.