approaches to carry-flag modelling in RTL

[Richard Henderson]

On 10/28/2011 06:49 AM, Peter Bigot wrote:
> I'm inclined to follow sparc's lead, but is one or another of the choices
> more likely to help combine/reload/etc do a better job?

I don't know.

In the case of RX, we don't model CC_REG until after reload, so combine really
doesn't get a shot at it.

Be careful here.  If you explicitly model the carry flag before reload, you need
to have an ADD instruction that can avoid any flags modification.  Reload needs
to generate such instructions in some cases, and needs to be able to insert them
between any two arbitrary insns.

If you're like sparc (separate add, addcc insns), or i386 (separate add, lea insns),
then you're fine.  If you're like m68k or RX and have only an add that clobbers
the flags, then you'll have to delay splitting flags-using patterns until after
reload is complete.


r~