[PATCH v2] support: add support_address_diff function

[Yury Khrustalev]

On Thu, Mar 26, 2026 at 01:50:49PM +0000, Yury Khrustalev wrote:
> Some malloc tests compare pointers meaning to compare addresses.
> On AArch64, the 64-bit value of the pointer may contain metadata
> along with the values of the address.
> 
> In order to correctly compare addresses, we add new function for
> AArch64 target that will use the AArch64 64 SUBP (subtract pointer)
> instruction when it is available. This instruction uses the 56-bit
> addresses ignoring top-byte metadata.
> 
> Best implementation is selected using ifunc resolver.
> 
> On other targets and also on AArch64 when MTE is not available this
> function defaults to PTR_DIFF defined in libc-pointer-arith.h.
> 
> Three malloc tests are modified accordingly:
>  - tst-memalign-2.c
>  - tst-memalign-3.c
>  - tst-realloc.c
> 
> ---
> base-commit: 4a97c12302
> passes regression on aarch64 and x86-64
> 
> Changes in v2:
>  - Use sysdeps files rather than arch-specific macro for conditional
>    compilation of code.
>  - Renamed the support_address_diff and added comment explaining what
>    it does.
>  - v1: https://inbox.sourceware.org/libc-alpha/20260325105900.828958-1-yury.khrustalev@arm.com/
> 

Polite ping :)