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.
