[PATCH] Replace std::to_string for integers with optimized version

[Christophe Lyon]

Hi,


On Wed, 12 Jun 2019 at 16:54, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> 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/Makefile.am: Add new <bits/charconv.h> header.
>         * include/Makefile.in: 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 <charconv>.
>         (__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<T>::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/
>         to_string.cc: Fix reference in comment. Remove unused variable.
>         * testsuite/21_strings/basic_string/numeric_conversions/char/
>         to_string_int.cc: New test.
>
> Tested x86_64-linux, committed to trunk.
>

The new test to_string_int.cc fails on arm-none-eabi:
PASS: 21_strings/basic_string/numeric_conversions/char/to_string_int.cc
(test for excess errors)
spawn /aci-gcc-fsf/builds/gcc-fsf-gccsrc/obj-arm-none-eabi/gcc3/utils/bin/qemu-wrapper.sh
./to_string_int.exe
/libstdc++-v3/testsuite/21_strings/basic_string/numeric_conversions/char/to_string_int.cc:105:
void check_value(T) [with T = long long int]: Assertion 's ==
expected' failed.
FAIL: 21_strings/basic_string/numeric_conversions/char/to_string_int.cc
execution test

Christophe