[PATCH 6/6] malloc: Ensure mtag code path in checked_request2size is cold

[DJ Delorie]

Szabolcs Nagy <szabolcs.nagy@arm.com> writes:
> This is a workaround (hack) for a gcc optimization issue (PR 99551).
> Without this the generated code may evaluate the expression in the
> cold path which causes performance regression for small allocations
> in the memory tagging disabled (common) case.

>    if (__glibc_unlikely (mtag_enabled))
> -    req = (req + (__MTAG_GRANULE_SIZE - 1)) &
> -	  ~(size_t)(__MTAG_GRANULE_SIZE - 1);
> +    {
> +      /* Ensure this is not evaluated if !mtag_enabled, see gcc PR 99551.  */
> +      asm ("");
> +
> +      req = (req + (__MTAG_GRANULE_SIZE - 1)) &
> +	    ~(size_t)(__MTAG_GRANULE_SIZE - 1);
> +    }

Hack indeed, but cross-target-safe and shouldn't affect the unused code
removal on non-aarch64 targets.

LGTM.
Reviewed-by: DJ Delorie <dj@redhat.com>