[cfe-users] Clang9 UBSan and GMP

[David Blaikie via cfe-users]

On Wed, Oct 30, 2019 at 5:36 PM Hans Åberg <haberg-1 at telia.com> wrote:

>
>
> > On 31 Oct 2019, at 00:28, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >
> >
> > On Wed, Oct 30, 2019 at 4:25 PM Hans Åberg <haberg-1 at telia.com> wrote:
> >
> >> I believe that GMP is just using integer types, and then uses that to
> make multiprecision integers, rational numbers, and floating point numbers.
> At least MPFR uses only the integer and rational number part of GMP, and
> builds multiprecision floating point numbers on top of that, which is
> necessary because of special requirements of the standards they adhere to.
> >
> > Ah, fair enough - that narrows down the points of failure a little.
>
> In addition, they use assembly code, but that can be turned off with
> configure --disable-assembly, though I did not 'make check' with that.
>
> >> GMP has been used in three years in a sequenced operation that must be
> exact and without errors to solve the problem [1], and I would think it
> used GCC with optimizations. So that puts Clang in a tough spot. :-)
> >
> > Not as much as it would seem - again, the spec allows for a fair bit of
> flexibility in a bunch of ways. (admittedly, within integer arithmetic
> without invoking UB (but, again, that's not proven - UBSan isn't guaranteed
> to catch everything)) Different compilers optimize code in different ways -
> that the code "works"/produces the desired behavior on one compiler/under
> some optimizations and not others doesn't give us much idea about whether
> the code or the compiler is correct. Different behavior is acceptable in
> C++ in a bunch of ways & compilers rely on that flexibility.
>
> Yes, but assuming that the GMP adheres to the C standard, there should be
> no difference in the arithmetical values produced.
>

Not necessarily - C (well, I don't know the C standard as well as I know
the C++ standard, admittedly) does allow various variations (implementation
and unspecified behavior, for instance). eg: order of evaluation of
function arguments (not that this is likely to vary due to optimizations -
and doesn't with clang to the best of my knowledge, but as an example of
the /sort/ of thing):

  int f() {
    static int i = 0;
    return i++;
  }
  int f2(int i, int j) {
    return i;
  }
  int main() {
    return f2(f(), f());
  }

This program could exit with 0 or 1 - both results are valid
interpretations of the program per the standard. (again, I don't know the C
spec quite as well, so I'm not sure exactly what it says about this code)

And the assumption that GMP adheres to the C standard isn't one, as a
compiler developer, I'd be willing to make without more data - (heck, even
the compiler itself has some "intentional" undefined behavior in it... )

- Dave
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