This allows std::format to support __int128 when __STRICT_ANSI__ is
defined, which previously failed because __int128 is not an integral
type in strict mode.
With these changes, std::to_chars still rejects 128-bit integers in
strict mode, but std::format will be able to use __detail::__to_chars_i
for unsigned __int128.
libstdc++-v3/ChangeLog:
* include/bits/charconv.h (__integer_to_chars_is_unsigned):
New variable template.
(__to_chars_len, __to_chars_10_impl): Use variable template in
assertions to allow unsigned __int128 in strict mode.
* include/std/charconv (__to_chars, __to_chars_16)
(__to_chars_10, __to_chars_8, __to_chars_2): Likewise.
Some of the helper functions use static constexpr local variables, which
is not permitted in a core constant expression. Removing the 'static'
seems to have negligible performance effect for __to_chars and
__to_chars_16. For __from_chars_alnum_to_val removing the 'static'
causes a significant performance impact for base 36 conversions. Use a
consteval lambda instead.
libstdc++-v3/ChangeLog:
* include/bits/charconv.h (__to_chars_10_impl): Add constexpr
for C++23. Remove 'static' from array.
* include/std/charconv (__cpp_lib_constexpr_charconv): Define.
(__to_chars, __to_chars_16): Remove 'static' from array, add
constexpr.
(__to_chars_10, __to_chars_8, __to_chars_2, __to_chars_i)
(to_chars, __raise_and_add, __from_chars_pow2_base)
(__from_chars_alnum, from_chars): Add constexpr.
(__from_chars_alnum_to_val): Avoid local static during constant
evaluation. Add constexpr.
* include/std/version (__cpp_lib_constexpr_charconv): Define.
* testsuite/20_util/from_chars/constexpr.cc: New test.
* testsuite/20_util/to_chars/constexpr.cc: New test.
* testsuite/20_util/to_chars/version.cc: New test.
__to_chars_10_impl is quite fast. According to the IACA the main loop
takes only 6.0 cycles, the whole function with one iteration takes
10.0 cycles. Replacing the __first[pos] and __first[pos - 1] with
__first[0] and __first[1] drops the function time to 7.53 cycles.
2019-09-09 Antony Polukhin <antoshkka@gmail.com>
* include/bits/charconv.h (__detail::__to_chars_10_impl): Replace
final offsets with constants.
From-SVN: r275514
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.
From-SVN: r272186
