[Libguestfs] [PATCH nbdkit v2] common: isaligned: Use a macro instead of relying on implicit truncation.

[Eric Blake]

On 9/17/18 3:39 PM, Nir Soffer wrote:

>> +#define IS_ALIGNED(size, align) ({              \
>> +      assert (is_power_of_2 ((align)));         \
>> +      !((size) & ((align) - 1));                \
>> +})
>>
> 
> But this version will happily accept singed int, and I think this code
> behavior with signed int is undefined.

Well, sort of. Bit shifts are very likely to hit undefined behavior on 
ints. But & and | are not shifts.

> 
> See
> https://wiki.sei.cmu.edu/confluence/display/c/INT13-C.+Use+bitwise+operators+only+on+unsigned+operands
> 
> Why use a macro when we can use a function that is likely to be inlined
> anyway?
> This is not used in tight loops, so there is no reason to use a macro.
> 
> If you want to use a macro, see the kernel align macros:

Alas, the kernel has a more restrictive license (GPLv2 in general); 
which means we can't copy it into nbdkit.  (True, the definitions will 
look similar, but if we claim we copied from somewhere, that source had 
better be permissively licensed).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org