Re: [PATCH 2/2][4.15?] x86: fix build when NR_CPUS == 1

[Ian Jackson <iwj@xenproject.org>]

Jan Beulich writes ("[PATCH 2/2][4.15?] x86: fix build when NR_CPUS == 1"):
> In this case the compiler is recognizing that no valid array indexes
> remain (in x2apic_cluster()'s access to per_cpu(cpu_2_logical_apicid,
> ...)), but oddly enough isn't really consistent about the checking it
> does (see the code comment).
...
> -        if (this_cpu == cpu || x2apic_cluster(this_cpu) != x2apic_cluster(cpu))
> +        if ( this_cpu == cpu )
> +            continue;
> +        /*
> +         * Guard in particular against the compiler suspecting out-of-bounds
> +         * array accesses below when NR_CPUS=1 (oddly enough with gcc 10 it
> +         * is the 1st of these alone which actually helps, not the 2nd, nor
> +         * are both required together there).
> +         */
> +        BUG_ON(this_cpu >= NR_CPUS);
> +        BUG_ON(cpu >= NR_CPUS);
> +        if ( x2apic_cluster(this_cpu) != x2apic_cluster(cpu) )
>              continue;

Is there some particular reason for not putting the BUG_ON before the
if test ?  That would avoid the refactoring.

Of course putting it in next to the array indexing would address that
too.

Ian.