Re: -fargument-noalias-global question

[Toon Moene <toon at moene dot org>]

[ Happy New Year first of all - hope we don't have too many more of them
  ... Hey, you can recognize a Jimi Hendrix fan, don't you ]

Richard Guenther wrote:

> Actually I have two questions.  First, what is the difference between
> -fargument-noalias-global and -fargument-noalias-anything (please
> provide an example, not cite documentation ;))

svn blame should point to me as the culprit.  I invented 
-fargument-noalias-anything because I ran out of specifics Fortran 
argument pointers *do not point at*.

The pointers used by the Fortran Front End to implement Fortran's 
argument association *cannot* point to anything else than the storage of 
those arguments, because they (those pointers) are generated by the 
compiler and cannot point to anything else (they cannot be modified).

Whether or not -fargument-noalias-anything is useful for languages other 
than Fortran is anybody's guess (I'm betting on "none").

> Second question.  Does "global memory" include memory that escaped?
> Thus, with -fargument-noalias-global:

The "global" in -fargument-noalias-global refers to data in COMMON. 
Again, it is hard to see how this option could be useful outside Fortran.

Note that COMMON data in Fortran escapes by default, because it can be 
accessible in any other compilation unit (just by including the COMMON 
declaration).

> int *p;
> void foo(int *q)
> {
>   may *q and *p alias?
> }

The canonical Fortran example is:

      SUBROUTINE AAP(P)
      COMMON /NOOT/ Q

P (which is a pointer in the backend sense of the word - it's not a 
Fortran pointer) cannot point to the memory indicated by Q.

> consider
>
> void bar()
> {
>   int i;
>   p = &i;
>   foo (&i);
> }
>
> does that call to foo invoke undefined behavior if built with
> -fargument-noalias-global?

It shouldn't, as 'int i' isn't global.  Now a more interesting question 
(one that I do not know the answer to, because these flags were targeted 
at Fortran code) is whether this would wreck havoc in case of 'static 
int i' :-)

> Is _that_ the difference vs.
> -fargument-noalias-anything (-global would not alias global
> decls, -anything would not alias any reachable memory (other than
> what the parameter reaches?  or just what it points to?)).

-anything means that it can't point to anything else but its arguments 
memory.

-global means it cannot point to global memory (but it assumes global 
memory works like Fortran's COMMON).

> Thanks for clarifications.
> Richard.

Hope this helps,

--
Toon Moene - e-mail: toon@moene.org (*NEW*) - phone: +31 346 214290
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