From: Andrew MacLeod <amacleod@redhat.com>
To: Diego Novillo <dnovillo@redhat.com>
Cc: Michael Matz <matz@suse.de>, gcc mailing list <gcc@gcc.gnu.org>
Subject: Re: [tree-ssa] Out of SSA status and issues
Date: Tue, 13 May 2003 13:18:00 -0000 [thread overview]
Message-ID: <1052831908.8201.96.camel@p4> (raw)
In-Reply-To: <1052831496.27232.247.camel@frodo.toronto.redhat.com>
On Tue, 2003-05-13 at 09:11, Diego Novillo wrote:
> On Tue, 2003-05-13 at 08:56, Michael Matz wrote:
> > Hi,
> >
> > On 13 May 2003, Diego Novillo wrote:
> >
> > > Why?
> > >
> > > 1. foo()
> > > 2. {
> > > 3. int i, *p;
> > > 4.
> > > 5. p = malloc();
> > > 6. i = *p;
> > > 7. return i + 9;
> > > 8. }
> > >
> > > I see nothing wrong in replacing 'i + 9' with '*p + 9'.
> >
> > I do. *p could have been changed in between 6 and 7.
> >
> The above is a complete program, *p has not changed between 6 and 7 nor
> it is volatile.
>
but you are missing the point. There are lots of trivial examples which
dont show problems.
The problem occurs when yoiu add a new stmt, 6.5 *p = 10
copyprop still thinks it can propagate the stmt, but it can't.
*p = 10 is not a co[py. It is a store to memory, and you have to go
looking at aliasing and such in order to move around loads. copy prop
doesnt do that.
> > I.e. *p is not a register but an expression, and a copy instruction copies
> > registers to registers.
> >
> Not in tree-ssa. INDIRECT_REFs are first-class variables. Since in
> GIMPLE pointers can only be one level deep, we can maintain SSA version
> numbers for the pointer and the pointed-to memory location. Granted, it
> is somewhat painful (see my other message in this thread) and I'm
> starting to think that it may not be sufficiently worth the pain.
>
> > > Not that tree-ssa will do anything with this code, the default
> > > type-based aliasing is too conservative, but PTA may disambiguate this.
> >
> > Of course you _can_ optimize such cases, but it's not a subset of copy
> > propagation but, hmm, un-PRE or something like that.
> >
> It is if INDIRECT_REFs are treated as regular variables, like we
> currently do. I agree that otherwise you need to treat them as
> expressions. If we stopped dealing with INDIRECT_REFs as first-class
> variables, we would probably move this in to the hands of SSAPRE.
>
I think the only issue is that the copy prop pass is incorrectly
assuming that a load from memory through a pointer is the same thing as
a copy between variables, and it isn't.
Andrew
next prev parent reply other threads:[~2003-05-13 13:18 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-05-12 14:42 Andrew MacLeod
2003-05-12 15:38 ` Diego Novillo
2003-05-12 15:57 ` Andrew MacLeod
2003-05-12 16:05 ` Michael Matz
2003-05-12 16:10 ` Andrew MacLeod
2003-05-12 16:16 ` law
2003-05-12 17:08 ` law
2003-05-12 17:12 ` Andrew MacLeod
2003-05-12 17:26 ` law
2003-05-12 18:57 ` Andrew MacLeod
2003-05-13 9:07 ` Michael Matz
2003-05-13 12:42 ` Diego Novillo
2003-05-13 12:50 ` Andrew MacLeod
2003-05-13 13:05 ` Diego Novillo
2003-05-13 13:29 ` Andrew MacLeod
2003-05-13 13:57 ` Diego Novillo
2003-05-13 12:57 ` Michael Matz
2003-05-13 13:11 ` Diego Novillo
2003-05-13 13:18 ` Andrew MacLeod [this message]
2003-05-14 17:19 ` Jan Vroonhof
2003-05-14 18:05 ` Andrew MacLeod
2003-05-14 18:33 ` Diego Novillo
2003-05-14 19:11 ` Daniel Berlin
2003-05-13 15:01 ` Daniel Berlin
2003-05-13 12:33 ` Diego Novillo
2003-05-13 12:49 ` Andrew MacLeod
2003-05-13 12:58 ` Diego Novillo
2003-05-13 13:17 Richard Kenner
2003-05-13 13:27 ` Diego Novillo
2003-05-13 13:40 ` Michael Matz
2003-05-13 15:08 ` Michael S. Zick
2003-05-13 13:42 Richard Kenner
2003-05-13 15:23 Richard Kenner
2003-05-13 18:50 ` Geoff Keating
2003-05-13 23:28 ` Michael S. Zick
2003-05-17 17:19 ` Michael S. Zick
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