Re: [PATCH] domain: use unsigned loop induction variable in complete_domain_destroy()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Feb 26, 2026 at 10:01:45AM +0100, Jan Beulich wrote:
> Using plain (signed) int variables as array indexes can be unhelpful on at
> least x86, where the compiler may see the need to insert sign-extension
> insns (strictly speaking it should be able to avoid that when the loop
> continuation condition says >= 0, but that's not generally the case even
> with gcc15).
> 
> Observed effects with gcc15 (will of course vary with compiler version and
> level of optimization):
> - on x86, one less preserved register in use, yet due to sub-optimal
>   choice of register variables still a small code size increase (%r12
>   isn't a good choice when it's used for base-without-index addressing, as
>   it requires a SIB byte which other registers wouldn't require),
> - on Arm64 code size decreases, albeit that's eaten up by padding which is
>   being inserted ahead of a few labels,
> - on Arm32 code size increases for a reason I didn't fully understand (my
>   ability to read Arm assembly is still somewhat limited).
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- a/xen/common/domain.c
> +++ b/xen/common/domain.c
> @@ -1475,7 +1475,7 @@ static void cf_check complete_domain_des
>  {
>      struct domain *d = container_of(head, struct domain, rcu);
>      struct vcpu *v;
> -    int i;
> +    unsigned int i;
>  
>      /*
>       * Flush all state for the vCPU previously having run on the current CPU.
> @@ -1485,7 +1485,7 @@ static void cf_check complete_domain_des
>       */
>      sync_local_execstate();
>  
> -    for ( i = d->max_vcpus - 1; i >= 0; i-- )
> +    for ( i = d->max_vcpus; i-- > 0; )

Is there any reason we need to do those loops backwards?

I would rather do:

for ( i = 0; i < d->max_vcpus; i++ )

?

Thanks, Roger.