[PATCH v4 3/3] aarch64: helper function to access hwcap elements in ifunc resolver

[enh]

On Fri, Apr 25, 2025 at 11:25 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> > On Fri, Apr 25, 2025 at 7:35 AM Yury Khrustalev <yury.khrustalev@arm.com> wrote:
> >>
> >> Add a portable way to access HWCAP elements via the parameter(s)
> >> passed to an ifunc resolver checking the _IFUNC_ARG_HWCAP in the
> >> first parameter and size of the buffer in the second parameter.
> >>
> >> Note that 0 is return when the requested element is not available.
> >>
> >> Also add an enum for HWCAP element IDs to facilitate writing more
> >> readable code.
> >> ---
> >>  sysdeps/aarch64/sys/ifunc.h       | 25 +++++++++++++++++++++++++
> >>  sysdeps/aarch64/tst-ifunc-arg-1.c | 12 ++++++++++++
> >>  sysdeps/aarch64/tst-ifunc-arg-2.c | 11 +++++++++++
> >>  3 files changed, 48 insertions(+)
> >>
> >> diff --git a/sysdeps/aarch64/sys/ifunc.h b/sysdeps/aarch64/sys/ifunc.h
> >> index fd4d49f00b..d74eb560e5 100644
> >> --- a/sysdeps/aarch64/sys/ifunc.h
> >> +++ b/sysdeps/aarch64/sys/ifunc.h
> >> @@ -19,6 +19,8 @@
> >>  #ifndef _SYS_IFUNC_H
> >>  #define _SYS_IFUNC_H
> >>
> >> +#include <sys/cdefs.h>
> >> +
> >>  /* A second argument is passed to the ifunc resolver.  */
> >>  #define _IFUNC_ARG_HWCAP       (1ULL << 62)
> >>
> >> @@ -64,4 +66,27 @@ struct __ifunc_arg_t
> >>
> >>  typedef struct __ifunc_arg_t __ifunc_arg_t;
> >>
> >> +/* ID values for the helper function below.  */
> >> +enum
> >> +{
> >> +  __IFUNC_HWCAP = 1,
> >> +  __IFUNC_HWCAP2,
> >> +  __IFUNC_HWCAP3,
> >> +  __IFUNC_HWCAP4,
> >> +};
> >
> > i get why you're adding this enum -- so you can say __IFUNC_HWCAP
> > rather than 1, which isn't as clear as 2 and later -- but this seems
> > to partly defeat the purpose of switching to the function. one of the
> > beauties of the function is that you never need to touch this header
> > again. personally i think i'd rather live with 1 than undo that?
>
> HWCAP is already passed as the first argument of the IFUNC resolver, so
> I don't think people would write
>
>   __ifunc_hwcap (1, hwcap, args)
>
> to get this value.  Wouldn't they just use hwcap directly?

/me checks the code i have indexed...

llvm/compiler-rt and partition alloc do. libgcc/libatomic do. the only
exception i found was bionic, funnily enough :-)

> Thanks,
> Florian
>