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From: Jonathan Wakely <jwakely@redhat.com>
To: Patrick Palka <ppalka@redhat.com>
Cc: libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org
Subject: Re: [PATCH 3/4] libstdc++: Add floating-point std::to_chars implementation
Date: Mon, 20 Jul 2020 13:31:23 +0100	[thread overview]
Message-ID: <20200720123123.GA3215@redhat.com> (raw)
In-Reply-To: <alpine.DEB.2.23.453.2007192126500.1521071@idea>

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). 





  reply	other threads:[~2020-07-20 12:31 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 [this message]
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
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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