RE: Tree-SSA and POST_INC address mode inompatible in GCC4?

["J.C. Pizarro" <jcpiza at gmail dot com>]

2007/11/3, Kenneth Zadeck wrote:
> I believe that this is something new and is most likely fallout from
> diego's reworking of the tree to rtl converter.
>
> To fix this will require a round of copy propagation, most likely in
> concert with some induction variable detection, since the most
> profitable place for this will be in loops.
>
> I wonder if any of this effects the rtl level induction variable
> discovery?
>
> > Hi, Ramana,
> > I tried the trunk version  with/without your patch. It still produces
> > the same code as gcc4.2.2 does. In auto-inc-dec.c, the comments say
> >
> >          *a
> >            ...
> >            a <- a + c
> >
> >         becomes
> >
> >            *(a += c) post
> >
> > But the problem is after Tree-SSA pass,  there is no
> >            a <- a + c
> > But something like
> >            a_1 <- a + c
> >
> > Unless the auto-inc-dec.c can reverse a_1 <- a + c to a <- a + c. I
> > don't see this transformation is applicable in most scenarios. Any
> > comments?
> >
> > Cheers,
> > Bingfeng

They need to add an algorithm post-SSA that the code reuse the variables
converting a_j <- phi(a_i,...) to a_k <- phi(a_k,...).

The algorithms of POST_INC and POST_DEC are very specific, so an above
general algorithm is sufficient.

   J.C. Pizarro