[tree-ssa] Out of SSA status and issues

[Diego Novillo]

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.

> 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.


Diego.