Implement Dragonbox

[jk-jeon]

From #1426 (comment).

(Sorry for the repetition, I realized a bug so have closed the previous one and now I'm opening this.)

All tests in the repo (except for os-test and format-test, which I believe are not related to this PR) passed, but there might be bugs. I'm not sure which branch I should choose for this kind of large PR, so I just have chose the master branch.

Performance is as measured as before.
(clang-cl)

(msvc)

Currently, I didn't touch write function, but I think it would be wiser to do that for the following reasons. First, as now format_float follows completely different paths for the shortest formatting and the fixed-precision formatting, I think there is little reason to unify them into one function. Second, it seems that currently the template argument T for format_float is always chosen to be double, but this introduces a needless branch inside format_float. Hence, I think it would be wiser to thus make separate functions for three different cases: the shortest formatting for float, the shortest formatting for double (and possibly long double when it is no different from double), and the fixed formatting.

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

This is great, thanks!

[vitaut]

On my system (macOS + clang) the results look even better:

[vitaut]

I think it would be wiser to thus make separate functions for three different cases: the shortest formatting for float, the shortest formatting for double (and possibly long double when it is no different from double), and the fixed formatting.

I totally agree, the current implementation is more of a historical artefact and should be split.

[vitaut]

Fuzzer caught one issue on formatting 1.35631564E-19f with dragonbox:

fmt::print("{}", 1.35631564E-19f);

triggers an assertion failure in

fmt/include/fmt/format-inl.h

Line 2003 in bb0db5e

FMT_ASSERT(n <= small_division_by_pow10_info<N>::divisor, "n is too large");

Stack trace:

  * frame #0: 0x00007fff6767033a libsystem_kernel.dylib`__pthread_kill + 10
    frame #1: 0x00007fff6772ce60 libsystem_pthread.dylib`pthread_kill + 430
    frame #2: 0x00007fff675f7808 libsystem_c.dylib`abort + 120
    frame #3: 0x00007fff64856458 libc++abi.dylib`abort_message + 231
    frame #4: 0x00007fff648477e9 libc++abi.dylib`demangling_terminate_handler() + 48
    frame #5: 0x00007fff64855887 libc++abi.dylib`std::__terminate(void (*)()) + 8
    frame #6: 0x00007fff64855838 libc++abi.dylib`std::terminate() + 56
    frame #7: 0x0000000100004242 a.out`fmt::v7::detail::assert_fail(file="include/fmt/format-inl.h", line=2003, message="n is too large") at format-inl.h:57:3
    frame #8: 0x0000000100029502 a.out`unsigned int fmt::v7::detail::dragonbox::small_division_by_pow10<1>(n=69) at format-inl.h:2003:3
    frame #9: 0x00000001000095fc a.out`fmt::v7::detail::dragonbox::decimal_fp<float> fmt::v7::detail::dragonbox::to_decimal<float>(x=1.35631564E-19) at format-inl.h:2528:9
    frame #10: 0x0000000100008283 a.out`int fmt::v7::detail::format_float<double>(value=1.3563156426940112E-19, precision=-1, specs=float_specs @ 0x00007ffeefbfef80, buf=0x00007ffeefbff120) at format-inl.h:2683:11
    frame #11: 0x00000001000164ad a.out`fmt::v7::detail::buffer_appender<char> fmt::v7::detail::write<char, fmt::v7::detail::buffer_appender<char>, float, 0>(out=buffer_appender<char> @ 0x00007ffeefbff110, value=1.35631564E-19) at format.h:1851:13
    frame #12: 0x0000000100014d7a a.out`fmt::v7::detail::buffer_appender<char> fmt::v7::detail::default_arg_formatter<fmt::v7::detail::buffer_appender<char>, char>::operator(this=0x00007ffeefbff400, value=1.35631564E-19)<float>(float) at format.h:1989:12
    frame #13: 0x0000000100006cee a.out`fmt::v7::basic_format_context<fmt::v7::detail::buffer_appender<char>, char>::iterator fmt::v7::vformat_to<fmt::v7::detail::arg_formatter<fmt::v7::detail::buffer_appender<char>, char>, char, fmt::v7::basic_format_context<fmt::v7::detail::buffer_appender<char>, char> >(fmt::v7::detail::arg_formatter<fmt::v7::detail::buffer_appender<char>, char>::iterator, fmt::v7::basic_string_view<char>, fmt::v7::basic_format_args<fmt::v7::basic_format_context<fmt::v7::detail::buffer_appender<char>, char> >, fmt::v7::detail::locale_ref) [inlined] decltype(vis=0x00007ffeefbff400, arg=0x00007ffeefbff550)) fmt::v7::visit_format_arg<fmt::v7::detail::default_arg_formatter<fmt::v7::detail::buffer_appender<char>, char>, fmt::v7::basic_format_context<fmt::v7::detail::buffer_appender<char>, char> >(fmt::v7::detail::default_arg_formatter<fmt::v7::detail::buffer_appender<char>, char>&&, fmt::v7::basic_format_arg<fmt::v7::basic_format_context<fmt::v7::detail::buffer_appender<char>, char> > const&) at core.h:1329:12
    frame #14: 0x0000000100006b48 a.out`fmt::v7::basic_format_context<fmt::v7::detail::buffer_appender<char>, char>::iterator fmt::v7::vformat_to<fmt::v7::detail::arg_formatter<fmt::v7::detail::buffer_appender<char>, char>, char, fmt::v7::basic_format_context<fmt::v7::detail::buffer_appender<char>, char> >(out=fmt::v7::detail::arg_formatter<fmt::v7::detail::buffer_appender<char>, char>::iterator @ 0x00007ffeefbff450, format_str=(data_ = "{}", size_ = 2), args=basic_format_args<fmt::v7::basic_format_context<fmt::v7::detail::buffer_appender<char>, char> > @ 0x00007ffeefbff430, loc=(locale_ = 0x0000000000000000)) at format.h:3461
    frame #15: 0x000000010000625f a.out`fmt::v7::detail::buffer_appender<char> fmt::v7::detail::vformat_to<char>(buf=0x00007ffeefbff690, format_str=(data_ = "{}", size_ = 2), args=basic_format_args<fmt::v7::basic_format_context<fmt::v7::detail::buffer_appender<char>, char> > @ 0x00007ffeefbff5c8) at format.h:3641:10
    frame #16: 0x000000010000633f a.out`fmt::v7::vprint(f=0x00007fff8dc74e28, format_str=(data_ = "{}", size_ = 2), args=format_args @ 0x00007ffeefbff670) at format-inl.h:3045:3
    frame #17: 0x0000000100006537 a.out`fmt::v7::vprint(format_str=(data_ = "{}", size_ = 2), args=format_args @ 0x00007ffeefbff8e0) at format-inl.h:3075:3
    frame #18: 0x0000000100003f60 a.out`void fmt::v7::print<char [3], float, char>(format_str=<no value available>, args=0x00007ffeefbff9ac) [3], float&&) at core.h:2121:16
    frame #19: 0x0000000100003eb5 a.out`main at test.cc:21:3
    frame #20: 0x00007fff67528cc9 libdyld.dylib`start + 1