Replace std::to_string for integers with optimized version

Programming / Compilers / GCC - redi [138bc75d-0d04-0410-961f-82ee72b054a4] - 12 June 2019 14:52 EDT

The std::to_chars functions from C++17 can be used to implement std::to_string with much better performance than calling snprintf. Only the __detail::__to_chars_len and __detail::__to_chars_10 functions are needed for to_string, because it always outputs base 10 representations.

The return type of __detail::__to_chars_10 should not be declared before C++17, so the function body is extracted into a new function that can be reused by to_string and __detail::__to_chars_10.

The existing tests for to_chars rely on to_string to check for correct answers. Now that they use the same code that doesn't actually ensure correctness, so add new tests for std::to_string that compare against printf output.

- include/ Add new header.
- include/ Regenerate.
- include/bits/basic_string.h (to_string(int), to_string(unsigned)) (to_string(long), to_string(unsigned long), to_string(long long)) (to_string(unsigned long long)): Rewrite to use __to_chars_10_impl.
- include/bits/charconv.h: New header. (__detail::__to_chars_len): Move here from . (__detail::__to_chars_10_impl): New function extracted from __detail::__to_chars_10.
- include/std/charconv (__cpp_lib_to_chars): Add, but comment out. (__to_chars_unsigned_type): New class template that reuses __make_unsigned_selector_base::__select to pick a type. (__unsigned_least_t): Redefine as __to_chars_unsigned_type::type. (__detail::__to_chars_len): Move to new header. (__detail::__to_chars_10): Add inline specifier. Move code doing the output to __detail::__to_chars_10_impl and call that.
- include/std/version (__cpp_lib_to_chars): Add, but comment out.
- testsuite/21_strings/basic_string/numeric_conversions/char/ Fix reference in comment. Remove unused variable.
- testsuite/21_strings/basic_string/numeric_conversions/char/ New test.

00f08bc3d9b Replace std::to_string for integers with optimized version
libstdc++-v3/ChangeLog | 24 +++
libstdc++-v3/include/ | 1 +
libstdc++-v3/include/ | 1 +
libstdc++-v3/include/bits/basic_string.h | 64 +++++---
libstdc++-v3/include/bits/charconv.h | 106 +++++++++++++
libstdc++-v3/include/std/charconv | 75 +++-------
libstdc++-v3/include/std/version | 1 +
.../numeric_conversions/char/ | 4 +-
.../numeric_conversions/char/ | 164 +++++++++++++++++++++
9 files changed, 365 insertions(+), 75 deletions(-)


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