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

[Jonathan Wakely]

On 13/06/19 22:41 +0200, Christophe Lyon wrote:
>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

Does the target support "%lld" in printf?

Does the .sum file show UNSUPPORTED for the existing
21_strings/basic_string/numeric_conversions/char/to_string.cc test?

Does the attached patch make the test show what fails?

I suspect the problem is that the test relies on snprintf to check the
answers are correct, even though the actual library code doesn't need
snprintf any longer. Previously the std::to_string functions were all
guarded by _GLIBCXX_USE_C99_STDIO and so I'm guessing were not
supported for arm-none-eabi. Now the overloads for integral types
should work without any C library support, and so the new test is
enabled unconditionally. But the test itself still uses the C library.

Maybe I should have a simple test that doesn't use snprintf and just
checks some hardcoded values produce the expected results, and a more
involved test that compares the results to snprintf but uses 
{ dg-require-string-conversions "" } like the existing test.



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