Even numbered register pairs restriction on some instructions

[Matthew Malcomson]

> I think you can use pdp11 as an example, it does two things that are similar to what you're describing.
>
> One is that it requires SImode to go into an even regno, and indicates that it uses two registers.  See TARGET_HARD_REGNO_MODE_OK and TARGET_HARD_REGNO_NREGS.
>
> The other is that it has one instruction that wants an odd (!) register: 16 bit multiply.  Multiply is weird: if you give it an even destination register it produces a 32 bit result in that register pair, with an odd register number it produces a 16 bit result.  So pdp11.md defines both "mulhi3" and "mulsihi3" insns.  The latter has an SImode output so that uses the even numbered register, as I already described.  The former uses a machine-specific constraint "d".  That is defined in constraints.md to mean a register in class MUL_REGS.  pdp11.h defines that name and what registers it refers to, and pdp11.h does the reverse mapping (REGNO_REG_CLASS).
>
> If you want even register numbers for some instructions but that's not tied to a specific type size (like SImode in my case), I think you'd want to use something analogous to the MUL_REGS thing I described.  Say, "EVEN_REGS".  REGNO_REG_CLASS would report even regnum to be in EVEN_REGS, odd in GENERAL_REGS.  The bitmaps for REG_CLASS_CONTENTS would show that EVEN_REGS contains only even numbered registers while GENERAL_REGS contains both odd and even.  And you'd defined a register constraint which matches EVEN_REGS.  Then the instructions where you want them would use that constraint.
>
> 	paul
>

> Yes, it's possible.  You can look at TDmode (128-bit decimal floating point)
> on powerpc64*-linux, which is only allowed in even-odd register pairs.
> It's in *all* cases though, not some of the time.
>
> Peter
>

Thanks for the suggestions,
I've had a look into these, and unfortunately it seems they have the 
same problem I've been hitting before.

The use of the TARGET_HARD_REGNO_MODE_OK macro limits all uses of 
registers in a given mode (so that we wouldn't be able to use register 
pairs beginning with odd registers for other instructions), while using 
an EVEN_REGS register class won't work in modes that span more than one 
hard register (as all hard registers covered by a pseudo register must 
be in the same class).

Thanks again,
Matthew