Den ons 7 sep. 2022 kl 09:48 skrev Richard Biener <
richard.guent...@gmail.com>:
> On Tue, Sep 6, 2022 at 5:19 PM Henrik Holst
> <henrik.ho...@millistream.com> wrote:
> >
> >
> >
> > Den tis 6 sep. 2022 kl 16:47 skrev Richard Biener <
> richard.guent...@gmail.com>:
> >>
> >>
> >>
> >> > Am 06.09.2022 um 16:23 schrieb Henrik Holst <
> henrik.ho...@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" ?
>
> Yes, there's currently nothing the user can do. Looking at the access
> attribute specification it could be used
> to initialize the middle-end used 'fn spec' specification - for
> example the Fortran Frontend uses that to ferry
> the guarantees by the 'INTENT' argument specification.
>
> Richard.
>
Ok, so patches to utilize the access attribute to inform the optimizer
might be accepted?
/HH
>
> >
> > /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
>
