From: Richard Henderson <rth@redhat.com>
To: Peter Bigot <bigotp@acm.org>
Cc: gcc@gcc.gnu.org
Subject: Re: approaches to carry-flag modelling in RTL
Date: Sat, 29 Oct 2011 14:17:00 -0000 [thread overview]
Message-ID: <4EAADF8F.2090002@redhat.com> (raw)
In-Reply-To: <CAPOJ94N-6Ek4-s6RJXDLHUCTN3EssVuYnh8xTZaeQR-0_cE3Og@mail.gmail.com>
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~
next prev parent reply other threads:[~2011-10-28 17:00 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-10-28 17:00 Peter Bigot
2011-10-29 14:17 ` Richard Henderson [this message]
2011-10-29 17:34 ` Peter Bigot
2011-10-29 21:30 ` Richard Henderson
2011-10-29 22:22 ` Peter Bigot
2011-10-31 12:01 ` Paulo J. Matos
2011-10-31 9:39 ` Hans-Peter Nilsson
2011-10-31 12:35 ` Paulo J. Matos
2011-11-01 2:43 ` Hans-Peter Nilsson
2011-11-01 13:51 ` Paulo J. Matos
2011-11-01 21:59 ` Hans-Peter Nilsson
2011-11-02 20:03 ` Richard Henderson
2012-02-07 22:42 ` Hans-Peter Nilsson
2011-10-31 14:58 ` Joern Rennecke
2011-10-31 9:40 ` Paulo J. Matos
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