Re: Clang UndefinedBehaviorSanitize (Postgres14) Detected undefined-behavior

Em seg., 31 de ago. de 2020 às 17:05, Andres Freund <andres(at)anarazel(dot)de>
escreveu:

> Hi,
>
> On 2020-08-31 12:38:51 -0700, Peter Geoghegan wrote:
> > On Mon, Aug 31, 2020 at 11:42 AM Andres Freund <andres(at)anarazel(dot)de>
> wrote:
> > > Unsigned integer overflow is well defined in the standard. So I don't
> understand what this is purporting to warn about.
> >
> > Presumably it's simply warning that the value -4294901760 (i.e. the
> > result of 3 - 4294901763) cannot be faithfully represented as an
> > unsigned int. This is true, of course. It's just not relevant.
> >
> > I'm pretty sure that UBSan does not actually state that this is
> > undefined behavior. At least Ranier's sample output didn't seem to
> > indicate it.
>
> Well, my point is that there's no point in discussing unsigned integer
> overflow, since it's precisely specified. And hence I don't understand
> what we're discussing in this sub-thread.
>
> https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html says:
>
> > -fsanitize=unsigned-integer-overflow: Unsigned integer overflow, where
> > the result of an unsigned integer computation cannot be represented in
> > its type. Unlike signed integer overflow, this is not undefined
> > behavior, but it is often unintentional. This sanitizer does not check
> > for lossy implicit conversions performed before such a computation
> > (see -fsanitize=implicit-conversion).
>
> So it seems Rainier needs to turn this test off, because it actually is
> intentional.
>
No problem.
If intentional, the code at TransactionIdPrecedes, already knows that
overflow can occur
and trusts that the compiler will save it.

Ranier Vilela