Re: [PATCH] Handle &__restrict parameters in tree-ssa-structalias.c like DECL_BY_REFERENCE parameters

[Jakub Jelinek <jakub@redhat.com>]

On Sat, Sep 24, 2011 at 01:26:36PM +0200, Richard Guenther wrote:
> > +int
> > +f3 (S &__restrict x, S &__restrict y)
> > +{
> > +  x.p[0] = 3;
> > +  y.p[0] = 0;
> > +// { dg-final { scan-tree-dump-times "return 3" 1 "optimized" } }
> > +  return x.p[0];
> > +}
> > +
> > +int
> > +f4 (S &x, S &y)
> > +{
> > +  x.p[0] = 4;
> > +  y.p[0] = 0;
> > +// { dg-final { scan-tree-dump-times "return 4" 0 "optimized" } }
> > +  return x.p[0];
> > +}

> I don't see why
> 
>   f4 (s, s)
> 
> would be invalid.  But you would miscompile it.

f3 (s, s) you mean?  I believe it is invalid.  For f4 it would be valid
and not optimized out.

> (I'm not sure that a restrict qualified component is properly defined
> by the C standard - we're just making this extension in a very constrained
> case to allow Fortran array descriptors to work).

Well, C standard doesn't have references, and C++ doesn't have restrict.
So it is all about extensions.
But what else would be & __restrict for than similar to *__restrict
to say that the pointed (resp. referenced) object must not be accessed
through other means than the reference or references/pointers derived from
it, in the spirit of ISO C99 6.7.3.1.
So, before jumping to __restrict fields, consider
int a[10], b[10];
int *
f8 (S &__restrict x, S &__restrict y)
{
  x.p = a;
  y.p = b;
  return x.p;
}
which we already optimize even before the patch.
It is certainly invalid to call f8 (s, s).

And the restricted fields, it is a straightforward extension to the restrict
definition of ISO C99.  We don't use it just for Fortran descriptors, but
e.g. std::valarray uses __restrict fields too and has that backed up by the
C++ standard requirements.  Two different std::valarray objects will have
different pointers inside of the structure.

My intent currently is to be able to vectorize:
#include <valarray>

std::valarray<int>
f9 (std::valarray<int> a, std::valarray<int> b, std::valarray<int> c, int z)
{
  int i;
  for (i = 0; i < z; i++)
    {
      a[i] = b[i] + c[i];
      a[i] += b[i] * c[i];
    }
  return a;
}

void
f10 (std::valarray<int> &__restrict a, std::valarray<int> &__restrict b, std::valarray<int> &__restrict c, int z)
{
  int i;
  for (i = 0; i < z; i++)
    {
      a[i] = b[i] + c[i];
      a[i] += b[i] * c[i];
    }
}

In f9 we currently handle it differently than in f10, while IMHO it should
be the same thing, a is guaranteed in both cases not to alias b nor c and b
is guaranteed not to alias c, furthermore, a._M_data[0] through a._M_data[z-1]
is guaranteed not to alias b._M_data[0] through b._M_data[z-1] and c._M_data[0]
through c._M_data[z-1] and similarly for b vs. c.  The __restrict on the
_M_data field in std::valarray is a hint that different std::valarray
objects will have different pointers.

In f9 we have:
  size_tD.1850 D.53593;
  intD.9 * restrict D.53592;
  intD.9 & D.53591;
...
  D.53592_7 = MEM[(struct valarrayD.50086 *)aD.50087_6(D) + 8B];
  D.53593_42 = D.53456_5 * 4;
  # PT = nonlocal escaped { D.53660 } (restr)
  D.53591_43 = D.53592_7 + D.53593_42;
...
  *D.53591_43 = D.53462_19;
and while PTA computes the restricted property here, we unfortunately still
don't use it, because D.53591 (which comes from all the inlined wrappers)
isn't TYPE_RESTRICT.  Shouldn't we propagate that property to either
SSA_NAMEs initialized from restricted pointers resp. POINTER_PLUS_EXPRs,
or if it is common to all VAR_DECLs underlying such SSA_NAMEs, to the
VAR_DECLs?

But in f10 we don't get even that far, the a._M_data (which is actually
a->_M_data, since a is a (restricted) reference) load is already itself
not considered as restricted by PTA.

It is nice that we optimize Fortran arrays well, but it would be nice if we
did the same for C++ too.

	Jakub