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Re: Why can't CSE recognize const_int's in HIGH LO_SUM pair?
- To: "John David Anglin" <dave at hiauly1 dot hia dot nrc dot ca>
- Subject: Re: Why can't CSE recognize const_int's in HIGH LO_SUM pair?
- From: law at redhat dot com
- Date: Mon, 24 Sep 2001 10:39:29 -0600
- cc: rth at redhat dot com, bernds at redhat dot com, gcc at gcc dot gnu dot org
- Reply-To: law at redhat dot com
In message <200109110311.f8B3BrYm014073@hiauly1.hia.nrc.ca>you write:
> > > (set (reg T) (const_int 0x10000))
> > > (set (reg A) (plus (reg T) (const_int 1)))
> > > (set (reg B) (plus (reg T) (const_int 2)))
> > >
> > > Now consider A and B loaded on two different paths. Code motion can
> > > then pull T up into the dominator.
> > But recall that we don't do this right now on the PA due to implementatio
> > issues.
> I was planning on trying to implement this later this week. At the moment,
> I am pounding nails helping with a new house for my sister.
Sounds like fun :-) I've spent most of my spare time over the last few
months renovating our old house. Richard -- you probably wouldn't recognize
the old house anymore :-)
> It is clear that either the above, or just
> (set (reg A) (const_int))
> is better than the current high/lo_sum. However, I don't see a consensis
> from the discussion as to which approach is better.
I'm not sure either.
(set (reg a) (const_int )) /* High part */
(set (reg b) (plus (reg a) (const_int)) /* Low part */
Has the advantage of being able to share the high part across multiple
constant loads, much like we do for symbolic references with offsets.
The only downside I see offhand is CSE knows high/lo_sum is somewhat special,
but I would expect it to be able to grok the above sequence just as well to
determine the constant value for (reg b).