Re: [RFC] Contributing tree-ssa to mainline

[Daniel Berlin <dberlin at dberlin dot org>]

On Jan 17, 2004, at 8:47 AM, Florian Weimer wrote:

> Diego Novillo wrote:
>
> > On Fri, 2004-01-16 at 22:24, Richard Kenner wrote:
> >
> > > Remember that the last time we had the discussion of the timing of 
> > > tree-ssa,
> > > people claimed it was "essential" for 3.5 since there was a 
> > > tremendous
> > > improvement on some C++ cases.  So let's see those cases.
> > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12747
>
> I've been toying with boost::tie() lately, and the temporary tuple is
> still *not* eleminated in the interesting cases, unfortunately.
>
> For an example, look at the code generated for bar(foo&) and baz(foo&)
> below, which should be quite similar once the template metaprogram has
> been expanded.  However, in the template case, indirection through
> references still clutters the generated machine code.
>
> We can do better; bar_neg(foo&) and baz_neg(foo&) show this.  The
> template version results in identical code in this case, except that
> unnecessary space is allocated on the stack.  (However, this
> optimization happens on mainline, too, not just on the branch.)

> (Probably this is a different issue, but I had hoped that the tree-ssa
> branch would somehow improve things in this area.)
As Diego stated when SRA was committed, the following limitation exists:
/* Maximum number of fields that a structure should have to be 
scalarized.
   FIXME  This limit has been arbitrarily set to 5.  Experiment to find 
a
    sensible setting.  */
#define MAX_NFIELDS_FOR_SRA 5

This was done, IIRC, to avoid severely increased register pressure.

If you want SRA to do something on your structure, you'll need to 
increase that limit