hubert.reinterpretcast added inline comments.
================
Comment at: clang/lib/Parse/ParseExprCXX.cpp:2245
+ case tok::kw___ibm128:
+ DS.SetTypeSpecType(DeclSpec::TST_ibm128, Loc, PrevSpec, DiagID, Policy);
+ break;
----------------
jwakely wrote:
> hubert.reinterpretcast wrote:
> > jwakely wrote:
> > > hubert.reinterpretcast wrote:
> > > > qiucf wrote:
> > > > > hubert.reinterpretcast wrote:
> > > > > > qiucf wrote:
> > > > > > > hubert.reinterpretcast wrote:
> > > > > > > > Not sure what the best method is to implement this, but `long
> > > > > > > > double` and `__ibm128` are the same type for GCC when
> > > > > > > > `-mabi=ibmlongdouble` is in effect.
> > > > > > > Seems clang is also different from GCC under
> > > > > > > `-mabi=ieeelongdouble`? I saw `__float128` and `long double` are
> > > > > > > the same for GCC but not for clang.
> > > > > > Have you checked whether the new libstdc++ for which this support
> > > > > > is being added needs the GCC behaviour to work properly?
> > > > > >
> > > > > > The GCC behaviour allows the following to be compiled without
> > > > > > introducing novel overload resolution tiebreakers:
> > > > > > ```
> > > > > > void f(__float128);
> > > > > > void f(__ibm128);
> > > > > > void f(int);
> > > > > >
> > > > > > long double ld;
> > > > > >
> > > > > > int main() { f(ld); }
> > > > > > ```
> > > > > As I saw both GCC and clang have error for ambiguous `operator<<` for:
> > > > >
> > > > > ```
> > > > > std::cout << "long double:\n";
> > > > > std::cout << std::numeric_limits<long double>::max() << std::endl;
> > > > > std::cout << std::numeric_limits<long double>::min() << std::endl;
> > > > >
> > > > > std::cout << "__float128:\n";
> > > > > std::cout << std::numeric_limits<__float128>::max() << std::endl;
> > > > > std::cout << std::numeric_limits<__float128>::min() << std::endl;
> > > > >
> > > > > std::cout << "__ibm128:\n";
> > > > > std::cout << std::numeric_limits<__ibm128>::max() << std::endl;
> > > > > std::cout << std::numeric_limits<__ibm128>::min() << std::endl;
> > > > > ```
> > > > @jwakely, are the overload resolution errors expected? @qiucf, are you
> > > > sure you have a sufficiently new libstdc++?
> > > > @jwakely, are the overload resolution errors expected?
> > >
> > > Yes. Iostreams support `long double` but not `__float128`, unless that
> > > happens to be the same type as `long double` (due to a
> > > `-mabi=ieeelongdouble` option).
> > Meaning that Clang's `__float128` iosteams support (with libstdc++) would
> > diverge from GCC.
> >
> > For example, Clang reports the call below as ambiguous even with
> > `-mabi=ieeelongdouble`:
> > ```
> > void f(double);
> > void f(long double);
> >
> > void g(__float128 f128) { f(f128); }
> > ```
> >
> > https://godbolt.org/z/dhYEKa
> >
> > Insofar as the user experience goes, is this lack of iostreams support for
> > `__float128` (even with `-mabi=ieeelongdouble`) within the realm of the
> > intended design of libstdc++?
> The lack of iostreams support for `__float128` is the intended design.
>
> On power we support `float`, `double` and three types of `long double`. If
> `__float128` is a distinct type from all those `long double` types it won't
> work.
>
> GCC on power defines `__float128` as a macro expanding to `__ieee128`, so it
> is the same as one of the `long double` types.
Okay, it sounds like the different treatment Clang has does not really
interfere with recommended usage insofar as libstdc++ iostreams (and hopefully
this extends to the rest of the library).
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D93377/new/
https://reviews.llvm.org/D93377
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