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Subject: Re: [tree-ssa] Out of SSA status and issues
From: Andrew MacLeod <amacleod@redhat.com>
To: Diego Novillo <dnovillo@redhat.com>
Cc: gcc mailing list <gcc@gcc.gnu.org>
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Date: Tue, 13 May 2003 12:49:00 -0000
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On Tue, 2003-05-13 at 08:33, Diego Novillo wrote:
> On Mon, 2003-05-12 at 14:45, Andrew MacLeod wrote:
> 
> > ie it would look something like this hacked up example:
> >  
> >       #   (*T.6)_13 = VDEF <(*T.6)_7>;
> >       T.6_12 = T.2_8 + T.5_11;
> > 
> >       #   VUSE <T.6_12>;
> >       i_14 = (*T.6)_13
> > 
> >       #   (*T.6)_22 = VDEF <(*T.6)_13>      
> >       #   VUSE <T.6_12>
> >       *T.6 = 30;
> >     };
> >   #   i_1 = PHI <i_6(0), (*T.6)_13(1)>;
> > 
> > Or is there a reason that copyprop would never happen?
> >
> Correctness wise?  No, there's no reason.  We can safely do the
> propagation in this case.
> 
> But I wouldn't think it's efficient.  Pointer dereferences are bound to
> be slower than a straight scalar reference.  The copy propagator in the
> SSA renamer blocks propagation of INDIRECT_REF nodes.  The stand-alone
> copy propagator should probably do the same.
> 
>
but this code is rewritten into:

 T.6 = T.2 + T.5
 i_14 = *T.6
*T.6 = 30
 i_1 = *T.6

Thats not the same meaning at all....

Andrew