Re: read_only access attribute as optimizer hint

[Henrik Holst <henrik.holst@millistream.com>]

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Den tis 6 sep. 2022 kl 16:47 skrev Richard Biener <
richard.guenther@gmail.com>:

>
>
> > Am 06.09.2022 um 16:23 schrieb Henrik Holst <
> henrik.holst@millistream.com>:
> >
> > Hi all,
> >
> >  is there any reason why the access attribute is not used as hints to the
> > optimizer?
> >
> > If we take this ancient example:
> >
> > void foo(const int *);
> >
> > int bar(void)
> > {
> >    int x = 0;
> >    int y = 0;
> >
> >    for (int i = 0; i < 10; i++) {
> >        foo(&x);
> >        y += x;  // this load not optimized out
> >    }
> >    return y;
> > }
> >
> > The load of X is not optimized out in the loop since the compiler does
> not
> > know if the external function foo() will cast away the const internally.
> > However changing the x variable to const as in:
> >
> > void foo(const int *);
> >
> > int bar(void)
> > {
> >    const int x = 0;
> >    int y = 0;
> >
> >    for (int i = 0; i < 10; i++) {
> >        foo(&x);
> >        y += x;  // this load is now optimized out
> >    }
> >    return y;
> > }
> >
> > The load of x is now optimized out since it is undefined behaviour if
> bar()
> > casts the const away when x is declared to be const.
> >
> > Now what strikes me as odd however is that declaring the function access
> > attribute to read_only does not hint the compiler to optimize out the
> load
> > of x even though read_only is defined as being stronger than const ("The
> > mode implies a stronger guarantee than the const qualifier which, when
> cast
> > away from a pointer, does not prevent the pointed-to object from being
> > modified."), so in the following code:
> >
> > __attribute__ ((access (read_only, 1))) void foo(const int *);
> >
> > int bar(void)
> > {
> >    int x = 0;
> >    int y = 0;
> >
> >    for (int i = 0; i < 10; i++) {
> >        foo(&x);
> >        y += x;  // this load not optimized out even though we have set
> the
> > access to read_only
> >    }
> >    return y;
> > }
> >
> > The load of x should really be optimized out but isn't. So is this an
> > oversight in gcc or is the access attribute completely ignored by the
> > optimizer for some good reason?
>
> It’s ignored because it is not thoroughly specified.  There’s an alternate
> representation the language Frontend can rewrite the attribute to to take
> advantage in optimization if it’s semantics matches.
>
> Richard
>
Ok, didn't really understand the bit about the language Frontend but I
guess that you are talking about internal GCC things here and thus there is
nothing that I as a GCC user can do to inform the optimizer that a variable
is read_only as a hint for external functions. And if so could it be
"thoroughly specified" to enable this type of optimization or is this just
"the way it is" ?

/HH

>
>
> > If there is no good reason for this then changing this to hint the
> > optimizer should enable some nice optimizations of external functions
> where
> > const in the declaration is not cast away.
> >
> > Regards,
> >  Henrik Holst
>