[cfe-users] Clang9 UBSan and GMP

[David Blaikie via cfe-users]

On Wed, Oct 30, 2019 at 2:29 PM Hans Åberg <haberg-1 at telia.com> wrote:

> Indeed, very hard to figure out. If it is some hidden undefined behavior
> causing it, the UBSan should have caught it, but it does not.


Right - but especially with numerics (especially floating point) there's
loads of room for valid but different behavior between different compilers
- behavior that isn't UB. How much precision a certain mathematical
equation maintains is really at the whim of the optimizers in a lot of ways.


> The link that Matthew gave says that the GMP developers experienced a
> number of such issues with Clang. One can though turn off the optimizer,
> and the tests pass.
>

Sure - most of the numeric effects would only appear with optimizations.
Without them every numeric operation's just done in registers, then written
right back to memory (so no chance of excess precision leaking in by
storing the value in an 80bit floating point register between multiple
operations, or any risk of fused operations that produces extra precision,
etc).

The only way to know is to trace down/reduce the point where the values
diverge & stare at the code to see who's right.

- Dave


>
>
> > On 30 Oct 2019, at 22:17, David Blaikie <dblaikie at gmail.com> wrote:
> >
> > I ran the test & understand it a bit better now - so the abort is part
> of the code, when the test fails, the test harness uses abort to fail.
> >
> > So this isn't "clang causes abort" (it didn't select a bad instruction,
> etc) this is "clang causes test failure" - this could be any number of
> things in terms of compiler optimizations due to overly dependent code (or
> due to miscompiles, to be sure). It's possible the test relies on specific
> numeric results that the C++ programming language doesn't guarantee (either
> overly high precision, or overly low precision - C++ allows extra precision
> in computations which can break numerical code that's relying on certain
> precision, for instance).
> >
> > So, yeah, really hard to say where the fault lies without further
> investigation.
> >
> > On Fri, Oct 25, 2019 at 4:13 PM Hans Åberg <haberg-1 at telia.com> wrote:
> > The GMP developers felt it was a compiler bug, so I think I will leave
> it at that. But thanks for the tips.
> >
> >
> > > On 26 Oct 2019, at 00:32, David Blaikie <dblaikie at gmail.com> wrote:
> > >
> > > UBSan doesn't catch everything - you could also try ASan and/or
> valgrind, etc. (MSan if you want, but that's a bit fussier/more work to use)
> >
>
>
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