Re: PING: PR/17236, improve long long multiply on x86 (middle-end)

[Paolo Bonzini <bonzini at gnu dot org>]

> This change is going to give a big bump to registers whose preferred
> class has only one or two registers, such as the HI/LO registers on
> MIPS.

I don't really think so.  For MIPS, it is going to favor registers whose 
 preferred class is MD*_REG, over those whose preferred class is 
LO_AND_GR_REGS.

But on MIPS in particular, I doubt regclass will give a preferred 
register class of MD*_REG to anything, because moves from and to LO/HI 
use starred constraints to avoid this case.  My regalloc-fu is low, but 
IIUC this means that only reload can create moves from/to the 
multiplication result registers.

That said, I wonder if it might actually make sense (no, I didn't think 
it that way, but I ask) if we do this change only in local-alloc.c?

> What happens if you only use size of register class as a tie-breaker
> for quantities which are otherwise equal?

I'm worried that in general it won't work, because priorities are rarely 
exactly equal (mostly because of the live range).  I'm almost sure this 
happens for the PR testcase, but I don't have data at hand.

Paolo