3c57e69235
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 for float and double. For explicit-precision formatting for 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. |
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| .. | ||
| config | ||
| doc | ||
| include | ||
| libsupc++ | ||
| po | ||
| python | ||
| scripts | ||
| src | ||
| testsuite | ||
| acinclude.m4 | ||
| aclocal.m4 | ||
| ChangeLog | ||
| ChangeLog-1998 | ||
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| ChangeLog-2003 | ||
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| ChangeLog-2012 | ||
| ChangeLog-2013 | ||
| ChangeLog-2014 | ||
| ChangeLog-2015 | ||
| ChangeLog-2016 | ||
| ChangeLog-2017 | ||
| ChangeLog-2018 | ||
| ChangeLog-2019 | ||
| config.h.in | ||
| configure | ||
| configure.ac | ||
| configure.host | ||
| crossconfig.m4 | ||
| fragment.am | ||
| linkage.m4 | ||
| Makefile.am | ||
| Makefile.in | ||
| README | ||
file: libstdc++-v3/README New users may wish to point their web browsers to the file index.html in the 'doc/html' subdirectory. It contains brief building instructions and notes on how to configure the library in interesting ways.