On Mon, 20 Jul 2020, Patrick Palka wrote:
> On Mon, 20 Jul 2020, Jonathan Wakely wrote:
>
> > On 20/07/20 08:53 -0400, Patrick Palka via Libstdc++ wrote:
> > > 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.
> >
> > Actually that should be:
> >
> > #ifdef _GLIBCXX_LONG_DOUBLE_COMPAT
> > extern "C" from_chars_result
> > _ZSt10from_charsPKcS0_ReSt12chars_format(const char* first, const char*
last,
> > long double& value,
> > chars_format fmt) noexcept
> > __attribute__((alias ("_ZSt10from_charsPKcS0_RdSt12chars_format")));
> > #endif
> >
> > With the right parameter list I don't need to disable the warning.
>
> Sounds good. Here's patch v5 that defines such aliases, tested so far
> on x86_64-pc-linux-gnu and on powerpc64le-unknown-linux-gnu.
Here's v6 which is rebased against the floating-point from_chars patch
and which also works around a false-positive -Wmaybe-uninitialized
warning in __floating_to_chars_hex, as well as performs some minor
commentary/style cleanups:
