Re: [Qemu-devel] [PULL 00/40] ppc patch queue 2015-06-03

[Peter Maydell]

On 5 June 2015 at 16:02, Paolo Bonzini <address@hidden> wrote:
>
>
> On 05/06/2015 16:40, Peter Maydell wrote:
>> > Speaking in general, I find that this makes code worse.  If you're using
>> > ~0 you probably want the value to extend with infinite ones.
>> >
>> > Using ~0u instead of ~0ull may cause problems down the line, and  ~0ul
>> > is even worse because it is not 64-bit safe.
>>
>> I agree that C's semantics are terrible here (ideally
>> left shift of negative values should Just Work in the 2s
>> complement style, and right shift of negative values should
>> be an arithmetic shift). Unfortunately we're stuck with
>> the standard, which says this is undefined behaviour :-(
>
> But this is not something that C compiler writers can reasonably change.

Right, which is why we need to change our code to not invoke
undefined behaviour. (More generally, C compiler writers can
agitate with the C standards bodies to get ideas like "friendly C"
dialects accepted, http://blog.regehr.org/archives/1180, and they
can provide them as vendor extensions too.)

> Can someone add a checkpatch rule that forbids shifting left U or UL
> constants (i.e. only ULL)?  That would alleviate my concerns with these
> ubsan warnings.

...but things like "(1U << 31)" are entirely valid. That's the
reason these warnings are runtime rather than compile time
in the first place...

thanks
-- PMM