Re: read_only access attribute as optimizer hint

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Sep 6, 2022 at 5:19 PM Henrik Holst
<henrik.holst@millistream.com> wrote:
>
>
>
> 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" ?

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.

>
> /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