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From: Christophe Lyon <christophe.lyon@linaro.org>
To: Patrick Palka <ppalka@redhat.com>
Cc: Jonathan Wakely <jwakely@redhat.com>,
	 "Jonathan Wakely via Libstdc++" <libstdc++@gcc.gnu.org>,
	GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH 3/4] libstdc++: Add floating-point std::to_chars implementation
Date: Fri, 18 Dec 2020 14:24:25 +0100	[thread overview]
Message-ID: <CAKdteOYvXsb0aeWyHrnEbaStdi=dtLXxUmgvAgDhxSRGCDy9bA@mail.gmail.com> (raw)
In-Reply-To: <CAMOnLZbHa9B1_jjbwHJA4H621emmn=aFzjtkF7TEE1g=cPFHFA@mail.gmail.com>

Hi,


On Fri, 18 Dec 2020 at 05:13, Patrick Palka via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Thu, Dec 17, 2020 at 9:32 AM Jonathan Wakely <jwakely@redhat.com> wrote:
> >
> > On 19/08/20 17:57 -0400, Patrick Palka via Libstdc++ wrote:
> > >On Wed, 22 Jul 2020, Patrick Palka wrote:
> > >
> > >> 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:
> > >
> > >Here's v7, with the following changes:
> > >
> > >* Remove extraneous indentation (two spaces when inside the std
> > >  namespace, two spaces after a template header) for better consistency
> > >  with floating_from_chars.cc
> > >* Guard the calls to fe[gs]etround with a preprocessor test for
> > >  _GLIBCXX_USE_C99_FENV_TR1;
> > >* Properly XFAIL the new long_double.cc test on targets with a
> > >  large long double type but without __int128, most notably i386
> > >* Reword the commit message slightly.
> > >
> > >-- >8 --
> > >
> > >Subject: [PATCH] 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 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 only 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 number through
> > >Ryu a second time when the general formatting mode resolves to fixed
> > >(which we determine by doing a scientific formatting first and
> > >inspecting the scientific exponent).  We avoid going through Ryu twice
> > >by instead transforming the scientific form to the corresponding fixed
> > >form via in-place string manipulation.
> > >
> > >This implementation is non-conforming in a couple of ways:
> > >
> > >1. For the shortest hexadecimal formatting, we currently follow the
> > >   Microsoft implementation's decision to be 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 for the number 1.08p+0 is 2.1p-1, but we output the former
> > >   instead of the latter, as does printf.
> > >
> > >2. The Ryu routine generic_binary_to_decimal that we use for performing
> > >   shortest formatting for large floating point types is implemented
> > >   using the __int128 type, but some targets with a large long double
> > >   type lack __int128 (e.g. i686), so we can't perform shortest
> > >   formatting of long double on such targets through Ryu.  As a
> > >   temporary stopgap this patch makes the long double to_chars overloads
> > >   just dispatch to the double overloads on these targets, which means
> > >   we lose precision in the output.  (We could potentially fix this by
> > >   writing a specialized version of Ryu's generic_binary_to_decimal
> > >   routine that uses uint64_t instead of __int128.)  [Though I wonder if
> > >   there's a better way to work around the lack of __int128 on i686
> > >   specifically?]
> > >
> > >3. Our shortest formatting for __ibm128 doesn't guarantee the round-trip
> > >   property if the difference between the high- and low-order exponent
> > >   is large.  This is because we treat __ibm128 as if it has a
> > >   contiguous 105-bit mantissa by merging the mantissas of the high-
> > >   and low-order parts (using code extracted from glibc), so we
> > >   potentially lose precision from the low-order part.  This seems to be
> > >   consistent with how glibc printf formats __ibm128.
> > >
> > >libstdc++-v3/ChangeLog:
> > >
> > >       * config/abi/pre/gnu.ver: Add new exports.
> > >       * 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.
> > >       (to_chars): Define for float, double and long double.
> > >       * testsuite/20_util/to_chars/long_double.cc: New test.
> >
> > Sorry it took so long to review, this is OK for trunk.
> >
> > The patch needs some minor changes to rebase it on the current trunk:
> > The linker script has additions since you send this patch, so the
> > context in the patch is wrong and it doesn't apply, and in <charconv>
> > the first line of context in the patch needs to have 'noexcept' added.
> > That rebase should be easy though.
> >
> > I'll look at adding __float128 support for powerpc64le.
>
> Thanks a lot.  I committed the patch series just now, after rebasing
> and retesting on x86_64, aarch64 and ppc64le.
>

My newlib-based toolchains (arm-eabi, aarch64-elf) fail to build after
this commit, because:
libstdc++-v3/src/c++17/floating_to_chars.cc:951:40: error:
'FE_TONEAREST' was not declared in this scope
  951 |             if (saved_rounding_mode != FE_TONEAREST)

I'm (still) using newlib-3.3.0, and I think newlib's fenv.h was updated
this year for arm/aarch64, so I suspect bumping to 3.4.0 would
avoid the problem. However, is this something you want to support?
(I mean the possibility that FE_TONEAREST is not supported etc...)

Thanks,

Christophe

  reply	other threads:[~2020-12-18 13:24 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-14 19:41 [PATCH 1/4] libstdc++: Import parts of the Ryu library Patrick Palka
2020-07-14 19:41 ` [PATCH 2/4] libstdc++: Apply modifications to our local copy of Ryu Patrick Palka
2020-07-15 18:12   ` Patrick Palka
2020-12-17 14:35     ` Jonathan Wakely
2020-07-14 19:41 ` [PATCH 3/4] libstdc++: Add floating-point std::to_chars implementation Patrick Palka
2020-07-15 18:21   ` Patrick Palka
2020-07-17  4:23     ` Patrick Palka
2020-07-17 16:24       ` Patrick Palka
2020-07-20  3:37         ` Patrick Palka
2020-07-20 12:31           ` Jonathan Wakely
2020-07-20 12:53             ` Patrick Palka
2020-07-20 14:13               ` Jonathan Wakely
2020-07-20 14:46                 ` Patrick Palka
2020-07-22 15:56                   ` Patrick Palka
2020-08-19 21:57                     ` Patrick Palka
2020-12-17 14:32                       ` Jonathan Wakely
2020-12-18  4:13                         ` Patrick Palka
2020-12-18 13:24                           ` Christophe Lyon [this message]
2020-12-18 14:58                             ` Jonathan Wakely
2020-12-18 15:00                             ` Patrick Palka
2020-12-18 16:52                               ` Christophe Lyon
2020-12-18 17:03                                 ` Patrick Palka
2020-12-18 18:28                                   ` Christophe Lyon
2020-12-20 21:44                           ` Maciej W. Rozycki
2020-12-21 17:06                             ` Patrick Palka
2020-12-21 23:09                               ` Maciej W. Rozycki
2020-07-14 19:41 ` [PATCH 4/4] libstdc++: Import MSVC floating-point std::to_chars testcases Patrick Palka
2020-07-14 19:49   ` Patrick Palka
2020-12-17 14:37   ` Jonathan Wakely
2020-07-14 19:46 ` [PATCH 1/4] libstdc++: Import parts of the Ryu library Patrick Palka
2020-07-25 11:44 ` Václav Haisman
2020-07-26 13:09   ` Patrick Palka
2020-12-17 14:34 ` Jonathan Wakely

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