[PATCH v3] aarch64: add support for hwcap3,4

[Yury Khrustalev]

Hi,

> From: enh <enh@google.com>
> Sent: 22 April 2025 04:58 PM
> To: Yury Khrustalev
> Cc: libc-alpha@sourceware.org; fweimer@redhat.com; adhemerval.zanella@linaro.org
> Subject: Re: [PATCH v3] aarch64: add support for hwcap3,4
> 
> On Tue, Apr 22, 2025 at 11:52 AM enh <enh@google.com> wrote:
> ...
> having updated some ifunc resolvers to see what this looks like, i
> find the "off by one" in the array versus the constants quite
> off-putting.
> 
> adding
> ```
> #define _hwcap3 _hwcap_array[2]
> ```
> etc seems quite tempting for better readability at the call sites.

To be honest, I would just do that:

  struct __ifunc_arg_t
  {
    unsigned long _size;
    unsigned long _hwcap_array[_IFUNC_HWCAP_MAX];
  };

And define these macros for the sake of existing code to compiler without errors:

  #define _hwcap _hwcap_array[0]
  #define _hwcap2 _hwcap_array[1]

No problem from union and allows to write a helper function to access HWCAP value
by its number (suggested by Chris Bazley <chris.bazley@arm.com>):

  static inline long int
  __ifunc_arg_get_hwcap(const struct __ifunc_arg_t *arg, size_t n)
  {
    /* Return 0 if n corresponds to beyond the runtime size in
       "arg->_size" and return "_hwcap_array[n]" otherwise.  */
  }

The only problem is that _hwcap and _hwcap2 are likely to be used as identifiers
elsewhere in existing code that includes sys/ifunc.h. I'm not sure if it's a big
issue.

Kind regards,
Yury