Re: Fortran aliasing in tree-ssa-alias.c (may_alias_p).

[Daniel Berlin <dberlin at dberlin dot org>]

On Sun, 2006-03-19 at 12:13 +0100, Toon Moene wrote:
> Diego Novillo wrote:
> 
> > On 03/11/06 07:51, Toon Moene wrote:
> 
> >>	* tree-ssa-alias.c (may_alias_p): if -fargument-noalias-global > 1,
> >>	a pointer argument may not point to anything else.
> 
> > Well, that's not what the documentation states.  The pointer could be
> > made to point to a local stack variable in the body of the function.  I
> > realize that this may not happen in Fortran, but we are in a
> > language-independent region of the code here.
> > 
> > I would propose adding flag_argument_noalias == 3 to convey what Fortran
> > guarantees: Pointer arguments that cannot point to globals nor locals.
> > Then you can do:
> > 
> > !   /* If -fargument-noalias-global is > 2, pointer arguments may
> > !      not point to anything else.  */
> > !   if (flag_argument_noalias > 2 && TREE_CODE (ptr) == PARM_DECL)
> > 
> > Not only here, but in other parts of alias.c.
> 
> OK, the attached patch is what I came up with.
> 
> There are two places where I left tests on flag_argument_noalias alone: 
> in alias.c and emit-rtl.c.  I believe they are still correct as coded.
> 
> I tested this patch by a bootstrap (C and Fortran only), make -k 
> check-fortran and a 9-cycle run of HIRLAM on x86_64-unknown-linux-gnu.
> 
> Of course, I'll do a complete bootstrap/make -k check on approval, but 
> AFAICS, no other front end sets flag_argument_noalias.
> 
> Is this OK for mainline ?

This is fine.

BTW, I *imagine* there are probably some places in the pruning we do in
tree-ssa-operands.c that you can use this information as well.

See access_can_touch_variable in tree-ssa-operands.c.

I know this seems like a lot of places to have to touch, but the other
places are really trying to figure out an exact points-to set for the
variable.  If they fail for some reason, we are going to fall back to
type based information, and this pruning will be useful.