On Freitag, 22. Mai 2020 18:39:42 CEST Jonathan Wakely wrote:
> On 22/05/20 09:49 +0200, Matthias Kretz wrote:
> >On Donnerstag, 21. Mai 2020 17:46:01 CEST Marc Glisse wrote:
> >> On Thu, 21 May 2020, Jonathan Wakely wrote:
> >> > On 27/04/20 17:09 +0200, Matthias Kretz wrote:
> >> >> From: Matthias Kretz <kr...@kde.org>
> >> >>
> >> >> PR libstdc++/84949
> >> >> * include/std/limits: Let is_iec559 reflect whether
> >> >> __GCC_IEC_559 says float and double support IEEE 754-2008.
> >> >> * testsuite/18_support/numeric_limits/is_iec559.cc: Test IEC559
> >> >> mandated behavior if is_iec559 is true.
> >> >> * testsuite/18_support/numeric_limits/infinity.cc: Only test
> >> >> inf
> >> >> behavior if is_iec559 is true, otherwise there is no guarantee
> >> >> how arithmetic on inf behaves.
> >> >> * testsuite/18_support/numeric_limits/quiet_NaN.cc: ditto for
> >> >> NaN.
> >> >> * testsuite/18_support/numeric_limits/denorm_min-1.cc: Compile
> >> >> with -ffast-math.
> >> >> * testsuite/18_support/numeric_limits/epsilon-1.cc: ditto.
> >> >> * testsuite/18_support/numeric_limits/infinity-1.cc: ditto.
> >> >> * testsuite/18_support/numeric_limits/is_iec559-1.cc: ditto.
> >> >> * testsuite/18_support/numeric_limits/quiet_NaN-1.cc: ditto.
> >> >
> >> > I'm inclined to go ahead and commit this (to master only, obviously).
> >> > It certainly seems more correct to me, and we'll probably never find
> >> > out if it's "safe" to do unless we actually change it and see what
> >> > happens.
> >> >
> >> > Marc, do you have an opinion?
> >>
> >> I don't have a strong opinion on this. I thought we were refraining from
> >> changing numeric_limits based on flags (like -fwrapv for modulo) because
> >> that would lead to ODR violations when people link objects compiled with
> >> different flags. There is a value in libstdc++.so, which may have been
> >> compiled with different flags than the application.
> >
> >But these ODR violations happen in any case: The floating-point types are
> >different types with or without -ffast-math (and related) flags. They
> >behave differently. Compiling a function in multiple TUs with different
> >flags produces observably different results. Choosing a single one of them
> >is obviously fragile and broken. That's the spirit of an ODR violation...
> >
> >It would sometimes be useful to have different types:
> >float, float_no_nan, float_no_nan_no_signed_zero, ...
>
> Sure. There are ODR violations like that, and then there are ones
> like:
>
> template<typename T>
> struct X
> {
> conditional_t<numeric_limits<T>::is_iec559, T, BigNum> val;
> };
Nice. ;-) If only the mangling of a struct could include the type of its
members (recursively)... But at least val has a different type now. And
correctly so. Yes, the ABI breaks possible via this change is real, though
I'd guess there are zero or close-to-zero ABI dependencies on is_iec559 out in
the wild (at this point - because it didn't work anyway).
> I'm generally not concerned about ODR violations where one TU behaves
> as requested by the flags used to compile that TU and another behaves
> as requested by the flats used to compile that second TU. That happens
> all the time with -fno-exceptions and -fno-rtti and such like. That
> causes ODR violations too, but of the kind where each definition does
> what was requested.
I am concerned. Showcase: https://godbolt.org/z/KzM3si. If you link those TUs,
you get one of the two behaviors for both TUs. This can result in very hard to
find Heisenbugs.
> Constants defined by the library changing value is a bit more
> concerning. But I don't know if it's really a problem in this case.
template <typename T, bool = numeric_limits<T>::is_iec559>
struct Float
{
T val
};
Finally, the standard mechanism that can help resolve those silent ODR
violations works. I.e. one can build float_559 and float_non559 types
(overloading all operators is still rather tedious)
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Dr. Matthias Kretz https://mattkretz.github.io
GSI Helmholtz Centre for Heavy Ion Research https://gsi.de
std::experimental::simd https://github.com/VcDevel/std-simd
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