On Mon, 20 Jul 2020, Jonathan Wakely wrote:
> On 19/07/20 23:37 -0400, Patrick Palka via Libstdc++ wrote:
> > On Fri, 17 Jul 2020, Patrick Palka wrote:
> >
> > > On Fri, 17 Jul 2020, Patrick Palka wrote:
> > >
> > > > On Wed, 15 Jul 2020, Patrick Palka wrote:
> > > >
> > > > > On Tue, 14 Jul 2020, Patrick Palka wrote:
> > > > >
> > > > > > This implements the floating-point std::to_chars overloads for
> > > float,
> > > > > > double and long double. We use the Ryu library to compute the
> > > shortest
> > > > > > round-trippable fixed and scientific forms of a number for float,
> > > double
> > > > > > and long double. We also use Ryu for performing fixed and
> > > scientific
> > > > > > formatting of float and double. For formatting long double with an
> > > > > > explicit precision argument we use a printf fallback. Hexadecimal
> > > > > > formatting for float, double and long double is implemented from
> > > > > > scratch.
> > > > > >
> > > > > > The supported long double binary formats are float64 (same as
> > > double),
> > > > > > float80 (x86 extended precision), float128 and ibm128.
> > > > > >
> > > > > > Much of the complexity of the implementation is in computing the
> > > exact
> > > > > > output length before handing it off to Ryu (which doesn't do bounds
> > > > > > checking). In some cases it's hard to compute the output length
> > > before
> > > > > > the fact, so in these cases we instead compute an upper bound on the
> > > > > > output length and use a sufficiently-sized intermediate buffer (if
> > > the
> > > > > > output range is smaller than the upper bound).
> > > > > >
> > > > > > Another source of complexity is in the general-with-precision
> > > formatting
> > > > > > mode, where we need to do zero-trimming of the string returned by
> > > Ryu, and
> > > > > > where we also take care to avoid having to format the string a
> > > second
> > > > > > time when the general formatting mode resolves to fixed.
> > > > > >
> > > > > > Tested on x86_64-pc-linux-gnu, aarch64-unknown-linux-gnu,
> > > > > > s390x-ibm-linux-gnu, and powerpc64-unknown-linux-gnu.
> > > > > >
> > > > > > libstdc++-v3/ChangeLog:
> > > > > >
> > > > > > * acinclude.m4 (libtool_VERSION): Bump to 6:29:0.
> > > > > > * config/abi/pre/gnu.ver: Add new exports.
> > > > > > * configure: Regenerate.
> > > > > > * include/std/charconv (to_chars): Declare the floating-point
> > > > > > overloads for float, double and long double.
> > > > > > * src/c++17/Makefile.am (sources): Add floating_to_chars.cc.
> > > > > > * src/c++17/Makefile.in: Regenerate.
> > > > > > * src/c++17/floating_to_chars.cc: New file.
> > > > > > * testsuite/20_util/to_chars/long_double.cc: New test.
> > > > > > * testsuite/util/testsuite_abi.cc: Add new symbol version.
> > > > >
> > > > > Here is v2 of this patch, which fixes a build failure on i386 due to
> > > > > __int128 being unavailable, by refactoring the long double binary
> > > format
> > > > > selection to avoid referring to __int128 when it doesn't exist. The
> > > > > patch also makes the hex formatting for 80-bit long double use
> > > uint64_t
> > > > > instead of __int128 since the mantissa has exactly 64 bits in this
> > > case.
> > > >
> > > > Here's v3 which just makes some minor stylistic adjustments, and most
> > > > notably replaces the use of _GLIBCXX_DEBUG with _GLIBCXX_ASSERTIONS
> > > > since we just want to enable __glibcxx_assert and not all of debug mode.
> > >
> > > Here's v4, which should now correctly support using <charconv> with
> > > -mlong-double-64 on targets with a large default long double type.
> > > This is done by defining the long double to_chars overloads as inline
> > > wrappers around the double overloads within <charconv> whenever
> > > __DBL_MANT_DIG__ equals __LDBL_MANT_DIG__.
> >
> > >
> > > -- >8 --
> > >
> > > Subject: [PATCH 3/4] libstdc++: Add floating-point std::to_chars
> > > implementation
> > >
> > > This implements the floating-point std::to_chars overloads for float,
> > > double and long double. We use the Ryu library to compute the shortest
> > > round-trippable fixed and scientific forms of a number for float, double
> > > and long double. We also use Ryu for performing explicit-precision
> > > fixed and scientific formatting of float and double. For
> > > explicit-precision formatting of long double we fall back to using
> > > printf. Hexadecimal formatting for float, double and long double is
> > > implemented from scratch.
> > >
> > > The supported long double binary formats are binary64, binary80 (x86
> > > 80-bit extended precision), binary128 and ibm128.
> > >
> > > Much of the complexity of the implementation is in computing the exact
> > > output length before handing it off to Ryu (which doesn't do bounds
> > > checking). In some cases it's hard to compute the output length
> > > beforehand, so in these cases we instead compute an upper bound on the
> > > output length and use a sufficiently-sized intermediate buffer if
> > > necessary.
> > >
> > > Another source of complexity is in the general-with-precision formatting
> > > mode, where we need to do zero-trimming of the string returned by Ryu,
> > > and where we also take care to avoid having to format the string a
> > > second time when the general formatting mode resolves to fixed.
> > >
> > > This implementation is non-conforming in a couple of ways:
> > >
> > > 1. For the shortest hexadecimal formatting, we currently follow the
> > > Microsoft implementation's approach of being consistent with the
> > > output of printf's '%a' specifier at the expense of sometimes not
> > > printing the shortest representation. For example, the shortest hex
> > > form of 1.08p+0 is 2.1p-1, but we output the former instead of the
> > > latter, as does printf.
> > >
> > > 2. The Ryu routines for doing shortest formatting on types larger than
> > > binary64 use the __int128 type, and some targets (e.g. i386) have a
> > > large long double type but lack __int128. For such targets we make
> > > the long double to_chars overloads go through the double overloads,
> > > which means we lose precision in the output. (The mantissa of long
> > > double is 64 bits on i386, so I think we could potentially fix this
> > > by writing a specialized version of the generic Ryu formatting
> > > routine which works with uint64_t instead of __int128.)
> > >
> > > 3. The __ibm128 shortest formatting routines don't guarantee
> > > round-trippability if the difference between the high- and low-order
> > > exponent is too large. This is because we treat the type as if it
> > > has a contiguous 105-bit mantissa by merging the high- and low-order
> > > mantissas, so we potentially lose precision from the low-order part.
> > > Although this precision-dropping behavior is non-conforming, it seems
> > > consistent with how printf formats __ibm128.
> > >
> > > libstdc++-v3/ChangeLog:
> > >
> > > * acinclude.m4 (libtool_VERSION): Bump to 6:29:0.
> > > * config/abi/pre/gnu.ver: Add new exports.
> > > * configure: Regenerate.
> > > * include/std/charconv (to_chars): Declare the floating-point
> > > overloads for float, double and long double.
> > > * src/c++17/Makefile.am (sources): Add floating_to_chars.cc.
> > > * src/c++17/Makefile.in: Regenerate.
> > > * src/c++17/floating_to_chars.cc: New file.
> > > * testsuite/20_util/to_chars/long_double.cc: New test.
> > > * testsuite/util/testsuite_abi.cc: Add new symbol version.
> > > ---
> > > libstdc++-v3/acinclude.m4 | 2 +-
> > > libstdc++-v3/config/abi/pre/gnu.ver | 12 +
> > > libstdc++-v3/configure | 2 +-
> > > libstdc++-v3/include/std/charconv | 43 +
> > > libstdc++-v3/src/c++17/Makefile.am | 1 +
> > > libstdc++-v3/src/c++17/Makefile.in | 5 +-
> > > libstdc++-v3/src/c++17/floating_to_chars.cc | 1514 +++++++++++++++++
> > > .../testsuite/20_util/to_chars/long_double.cc | 197 +++
> > > libstdc++-v3/testsuite/util/testsuite_abi.cc | 3 +-
> > > 9 files changed, 1774 insertions(+), 5 deletions(-)
> > > create mode 100644 libstdc++-v3/src/c++17/floating_to_chars.cc
> > > create mode 100644 libstdc++-v3/testsuite/20_util/to_chars/long_double.cc
> > >
> > > diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
> > > index ee5e0336f2c..e3926e1c9c2 100644
> > > --- a/libstdc++-v3/acinclude.m4
> > > +++ b/libstdc++-v3/acinclude.m4
> > > @@ -3846,7 +3846,7 @@ changequote([,])dnl
> > > fi
> > >
> > > # For libtool versioning info, format is CURRENT:REVISION:AGE
> > > -libtool_VERSION=6:28:0
> > > +libtool_VERSION=6:29:0
> > >
> > > # Everything parsed; figure out what files and settings to use.
> > > case $enable_symvers in
> > > diff --git a/libstdc++-v3/config/abi/pre/gnu.ver
> > > b/libstdc++-v3/config/abi/pre/gnu.ver
> > > index edf4485e607..9a1bcfd25d1 100644
> > > --- a/libstdc++-v3/config/abi/pre/gnu.ver
> > > +++ b/libstdc++-v3/config/abi/pre/gnu.ver
> > > @@ -2299,6 +2299,18 @@ GLIBCXX_3.4.28 {
> > >
> > > } GLIBCXX_3.4.27;
> > >
> > > +GLIBCXX_3.4.29 {
> > > + # to_chars(char*, char*, [float|double|long double])
> > > + _ZSt8to_charsPcS_[fdeg];
> > > +
> > > + # to_chars(char*, char*, [float|double|long double], chars_format)
> > > + _ZSt8to_charsPcS_[fdeg]St12chars_format;
> > > +
> > > + # to_chars(char*, char*, [float|double|long double], chars_format,
> > > int)
> > > + _ZSt8to_charsPcS_[fdeg]St12chars_formati;
> > > +
> > > +} GLIBCXX_3.4.28;
> > > +
> > > # Symbols in the support library (libsupc++) have their own tag.
> > > CXXABI_1.3 {
> > >
> > > diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
> > > index dd54bd406a9..73f771e7335 100755
> > > --- a/libstdc++-v3/configure
> > > +++ b/libstdc++-v3/configure
> > > @@ -75231,7 +75231,7 @@ $as_echo "$as_me: WARNING: === Symbol versioning
> > > will be disabled." >&2;}
> > > fi
> > >
> > > # For libtool versioning info, format is CURRENT:REVISION:AGE
> > > -libtool_VERSION=6:28:0
> > > +libtool_VERSION=6:29:0
> > >
> > > # Everything parsed; figure out what files and settings to use.
> > > case $enable_symvers in
> > > diff --git a/libstdc++-v3/include/std/charconv
> > > b/libstdc++-v3/include/std/charconv
> > > index cc7dd0e3758..bd59924f7e7 100644
> > > --- a/libstdc++-v3/include/std/charconv
> > > +++ b/libstdc++-v3/include/std/charconv
> > > @@ -688,6 +688,49 @@ namespace __detail
> > > operator^=(chars_format& __lhs, chars_format __rhs) noexcept
> > > { return __lhs = __lhs ^ __rhs; }
> > >
> > > + // Floating-point std::to_chars
> > > +
> > > + // Overloads for float.
> > > + to_chars_result to_chars(char* __first, char* __last, float __value)
> > > noexcept;
> > > + to_chars_result to_chars(char* __first, char* __last, float __value,
> > > + chars_format __fmt) noexcept;
> > > + to_chars_result to_chars(char* __first, char* __last, float __value,
> > > + chars_format __fmt, int __precision) noexcept;
> > > +
> > > + // Overloads for double.
> > > + to_chars_result to_chars(char* __first, char* __last, double __value)
> > > noexcept;
> > > + to_chars_result to_chars(char* __first, char* __last, double __value,
> > > + chars_format __fmt) noexcept;
> > > + to_chars_result to_chars(char* __first, char* __last, double __value,
> > > + chars_format __fmt, int __precision) noexcept;
> > > +
> > > + // Overloads for long double.
> > > + to_chars_result to_chars(char* __first, char* __last, long double
> > > __value)
> > > + noexcept;
> > > + to_chars_result to_chars(char* __first, char* __last, long double
> > > __value,
> > > + chars_format __fmt) noexcept;
> > > + to_chars_result to_chars(char* __first, char* __last, long double
> > > __value,
> > > + chars_format __fmt, int __precision) noexcept;
> > > +
> > > + // If long double has the same binary format as double, then we just
> > > define
> > > + // the long double overloads as wrappers around the corresponding
> > > double
> > > + // overloads.
> > > +#if __LDBL_MANT_DIG__ == __DBL_MANT_DIG__
> > > + inline to_chars_result
> > > + to_chars(char* __first, char* __last, long double __value) noexcept
> > > + { return to_chars(__first, __last, double(__value)); }
> > > +
> > > + inline to_chars_result
> > > + to_chars(char* __first, char* __last, long double __value,
> > > + chars_format __fmt) noexcept
> > > + { return to_chars(__first, __last, double(__value), __fmt); }
> > > +
> > > + inline to_chars_result
> > > + to_chars(char* __first, char* __last, long double __value,
> > > + chars_format __fmt, int __precision) noexcept
> > > + { return to_chars(__first, __last, double(__value), __fmt,
> > > __precision); }
> > > +#endif
> >
> > Hmm, I think this approach for supporting -mlong-double-64 might
> > introduce an ODR violation because each long double to_chars overload
> > could potentially have two different definitions available in a program,
> > one out-of-line in floating_to_chars.cc (compiled without
> > -mlong-double-64) and another inline in <charconv> (compiled with
> > -mlong-double-64)..
>
> But they have different mangled names, so there's no ODR violation.
> The 64-bit long double is mangled as 'e' and the 128-bit long double
> is mangled as __float128. You *will* get an ODR violation on targets
> where there's no -mlong-double-64 switch, where double and long double
> are always the same representation.
>
> What I'm doing for std::from_chars is adding this in the new
> src/c++17/floating_from_chars.cc file:
>
> #ifdef _GLIBCXX_LONG_DOUBLE_COMPAT
> #pragma GCC diagnostic ignored "-Wattribute-alias"
> extern "C" from_chars_result _ZSt10from_charsPKcS0_ReSt12chars_format(double)
> __attribute__((alias ("_ZSt10from_charsPKcS0_RdSt12chars_format")));
> #endif
>
> This just defines the _ZSt10from_charsPKcS0_ReSt12chars_format symbol
> (i.e. from_chars for 64-bit long double) as an alias of
> _ZSt10from_charsPKcS0_RdSt12chars_format (i.e. from_chars for 64-bit
> double).
Aha, that makes sense. I'll follow suit for std::to_chars.
