libstdc++: Optimize std::to_string using std::string::resize_and_overwrite
This uses std::string::__resize_and_overwrite to avoid initializing the string buffer with characters that are immediately overwritten. This results in about 6% better performance for the std_to_string case in int-benchmark.cc from https://github.com/fmtlib/format-benchmark This requires a change to a testcase. The previous implementation guaranteed that the string returned from std::to_string(integral-type) would have no excess capacity, because it was constructed with the correct length. The new implementation constructs an empty string and then resizes it with resize_and_overwrite, which over-allocates. This means that the "no-excess capacity" guarantee no longer holds. We can also greatly improve the performance of std::to_wstring by using std::to_string and then widening it with a new helper function, instead of using std::swprintf to do the formatting. libstdc++-v3/ChangeLog: * include/bits/basic_string.h (to_string(integral-type)): Use resize_and_overwrite when available. (__to_wstring_numeric): New helper functions. (to_wstring): Use std::to_string then __to_wstring_numeric. * testsuite/21_strings/basic_string/numeric_conversions/char/to_string_int.cc: Remove check for no excess capacity.
This commit is contained in:
Jonathan Wakely
parent
95c2b0cc9e
commit
51ec07b116
2 changed files with 123 additions and 52 deletions
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@ -4197,8 +4197,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
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const bool __neg = __val < 0;
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const unsigned __uval = __neg ? (unsigned)~__val + 1u : __val;
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const auto __len = __detail::__to_chars_len(__uval);
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string __str(__neg + __len, '-');
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__detail::__to_chars_10_impl(&__str[__neg], __len, __uval);
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string __str;
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__str.__resize_and_overwrite(__neg + __len, [=](char* __p, size_t __n) {
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__p[0] = '-';
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__detail::__to_chars_10_impl(__p + (int)__neg, __len, __uval);
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return __n;
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});
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return __str;
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}
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@ -4209,8 +4213,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
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noexcept // any 32-bit value fits in the SSO buffer
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#endif
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{
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string __str(__detail::__to_chars_len(__val), '\0');
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__detail::__to_chars_10_impl(&__str[0], __str.size(), __val);
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const auto __len = __detail::__to_chars_len(__val);
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string __str;
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__str.__resize_and_overwrite(__len, [__val](char* __p, size_t __n) {
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__detail::__to_chars_10_impl(__p, __n, __val);
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return __n;
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});
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return __str;
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}
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@ -4224,8 +4232,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
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const bool __neg = __val < 0;
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const unsigned long __uval = __neg ? (unsigned long)~__val + 1ul : __val;
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const auto __len = __detail::__to_chars_len(__uval);
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string __str(__neg + __len, '-');
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__detail::__to_chars_10_impl(&__str[__neg], __len, __uval);
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string __str;
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__str.__resize_and_overwrite(__neg + __len, [=](char* __p, size_t __n) {
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__p[0] = '-';
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__detail::__to_chars_10_impl(__p + (int)__neg, __len, __uval);
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return __n;
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});
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return __str;
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}
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@ -4236,8 +4248,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
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noexcept // any 32-bit value fits in the SSO buffer
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#endif
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{
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string __str(__detail::__to_chars_len(__val), '\0');
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__detail::__to_chars_10_impl(&__str[0], __str.size(), __val);
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const auto __len = __detail::__to_chars_len(__val);
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string __str;
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__str.__resize_and_overwrite(__len, [__val](char* __p, size_t __n) {
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__detail::__to_chars_10_impl(__p, __n, __val);
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return __n;
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});
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return __str;
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}
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@ -4249,8 +4265,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
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const unsigned long long __uval
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= __neg ? (unsigned long long)~__val + 1ull : __val;
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const auto __len = __detail::__to_chars_len(__uval);
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string __str(__neg + __len, '-');
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__detail::__to_chars_10_impl(&__str[__neg], __len, __uval);
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string __str;
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__str.__resize_and_overwrite(__neg + __len, [=](char* __p, size_t __n) {
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__p[0] = '-';
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__detail::__to_chars_10_impl(__p + (int)__neg, __len, __uval);
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return __n;
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});
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return __str;
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}
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@ -4258,8 +4278,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
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inline string
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to_string(unsigned long long __val)
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{
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string __str(__detail::__to_chars_len(__val), '\0');
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__detail::__to_chars_10_impl(&__str[0], __str.size(), __val);
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const auto __len = __detail::__to_chars_len(__val);
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string __str;
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__str.__resize_and_overwrite(__len, [__val](char* __p, size_t __n) {
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__detail::__to_chars_10_impl(__p, __n, __val);
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return __n;
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});
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return __str;
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}
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@ -4335,80 +4359,129 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
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inline long double
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stold(const wstring& __str, size_t* __idx = 0)
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{ return __gnu_cxx::__stoa(&std::wcstold, "stold", __str.c_str(), __idx); }
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#endif
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#ifdef _GLIBCXX_USE_WCHAR_T
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#pragma GCC diagnostic push
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#pragma GCC diagnostic ignored "-Wc++17-extensions"
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_GLIBCXX20_CONSTEXPR
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inline void
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__to_wstring_numeric(const char* __s, int __len, wchar_t* __wout)
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{
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// This condition is true if exec-charset and wide-exec-charset share the
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// same values for the ASCII subset or the EBCDIC invariant character set.
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if constexpr (wchar_t('0') == L'0' && wchar_t('-') == L'-'
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&& wchar_t('.') == L'.' && wchar_t('e') == L'e')
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{
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for (int __i = 0; __i < __len; ++__i)
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__wout[__i] = (wchar_t) __s[__i];
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}
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else
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{
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wchar_t __wc[256];
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for (int __i = '0'; __i <= '9'; ++__i)
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__wc[__i] = L'0' + __i;
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__wc['.'] = L'.';
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__wc['+'] = L'+';
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__wc['-'] = L'-';
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__wc['a'] = L'a';
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__wc['b'] = L'b';
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__wc['c'] = L'c';
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__wc['d'] = L'd';
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__wc['e'] = L'e';
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__wc['f'] = L'f';
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__wc['n'] = L'n'; // for "nan" and "inf"
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__wc['p'] = L'p'; // for hexfloats "0x1p1"
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__wc['x'] = L'x';
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__wc['A'] = L'A';
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__wc['B'] = L'B';
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__wc['C'] = L'C';
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__wc['D'] = L'D';
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__wc['E'] = L'E';
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__wc['F'] = L'F';
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__wc['N'] = L'N';
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__wc['P'] = L'P';
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__wc['X'] = L'X';
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for (int __i = 0; __i < __len; ++__i)
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__wout[__i] = __wc[(int)__s[__i]];
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}
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}
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#if __cpp_lib_constexpr_string >= 201907L
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constexpr
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#endif
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inline wstring
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#if __cplusplus >= 201703L
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__to_wstring_numeric(string_view __s)
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#else
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__to_wstring_numeric(const string& __s)
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#endif
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{
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if constexpr (wchar_t('0') == L'0' && wchar_t('-') == L'-'
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&& wchar_t('.') == L'.' && wchar_t('e') == L'e')
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return wstring(__s.data(), __s.data() + __s.size());
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else
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{
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wstring __ws;
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auto __f = __s.data();
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__ws.__resize_and_overwrite(__s.size(),
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[__f] (wchar_t* __to, int __n) {
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std::__to_wstring_numeric(__f, __n, __to);
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return __n;
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});
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return __ws;
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}
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}
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#pragma GCC diagnostic pop
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#ifndef _GLIBCXX_HAVE_BROKEN_VSWPRINTF
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// DR 1261.
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_GLIBCXX_NODISCARD
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inline wstring
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to_wstring(int __val)
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{ return __gnu_cxx::__to_xstring<wstring>(&std::vswprintf, 4 * sizeof(int),
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L"%d", __val); }
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{ return std::__to_wstring_numeric(std::to_string(__val)); }
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_GLIBCXX_NODISCARD
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inline wstring
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to_wstring(unsigned __val)
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{ return __gnu_cxx::__to_xstring<wstring>(&std::vswprintf,
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4 * sizeof(unsigned),
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L"%u", __val); }
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{ return std::__to_wstring_numeric(std::to_string(__val)); }
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_GLIBCXX_NODISCARD
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inline wstring
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to_wstring(long __val)
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{ return __gnu_cxx::__to_xstring<wstring>(&std::vswprintf, 4 * sizeof(long),
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L"%ld", __val); }
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{ return std::__to_wstring_numeric(std::to_string(__val)); }
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_GLIBCXX_NODISCARD
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inline wstring
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to_wstring(unsigned long __val)
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{ return __gnu_cxx::__to_xstring<wstring>(&std::vswprintf,
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4 * sizeof(unsigned long),
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L"%lu", __val); }
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{ return std::__to_wstring_numeric(std::to_string(__val)); }
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_GLIBCXX_NODISCARD
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inline wstring
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to_wstring(long long __val)
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{ return __gnu_cxx::__to_xstring<wstring>(&std::vswprintf,
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4 * sizeof(long long),
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L"%lld", __val); }
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{ return std::__to_wstring_numeric(std::to_string(__val)); }
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_GLIBCXX_NODISCARD
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inline wstring
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to_wstring(unsigned long long __val)
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{ return __gnu_cxx::__to_xstring<wstring>(&std::vswprintf,
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4 * sizeof(unsigned long long),
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L"%llu", __val); }
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{ return std::__to_wstring_numeric(std::to_string(__val)); }
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#if _GLIBCXX_USE_C99_STDIO
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_GLIBCXX_NODISCARD
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inline wstring
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to_wstring(float __val)
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{
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const int __n =
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__gnu_cxx::__numeric_traits<float>::__max_exponent10 + 20;
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return __gnu_cxx::__to_xstring<wstring>(&std::vswprintf, __n,
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L"%f", __val);
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}
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{ return std::__to_wstring_numeric(std::to_string(__val)); }
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_GLIBCXX_NODISCARD
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inline wstring
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to_wstring(double __val)
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{
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const int __n =
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__gnu_cxx::__numeric_traits<double>::__max_exponent10 + 20;
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return __gnu_cxx::__to_xstring<wstring>(&std::vswprintf, __n,
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L"%f", __val);
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}
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{ return std::__to_wstring_numeric(std::to_string(__val)); }
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_GLIBCXX_NODISCARD
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inline wstring
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to_wstring(long double __val)
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{
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const int __n =
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__gnu_cxx::__numeric_traits<long double>::__max_exponent10 + 20;
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return __gnu_cxx::__to_xstring<wstring>(&std::vswprintf, __n,
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L"%Lf", __val);
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}
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#endif // _GLIBCXX_HAVE_BROKEN_VSWPRINTF
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#endif // _GLIBCXX_USE_WCHAR_T && _GLIBCXX_USE_C99_WCHAR
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{ return std::__to_wstring_numeric(std::to_string(__val)); }
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#endif
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#endif // _GLIBCXX_USE_WCHAR_T
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_GLIBCXX_END_NAMESPACE_CXX11
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_GLIBCXX_END_NAMESPACE_VERSION
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@ -104,8 +104,6 @@ template<typename T>
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const std::string expected = test::to_string(val);
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VERIFY( s == expected );
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VERIFY( s[s.size()] == '\0' ); // null-terminator not overwritten!
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if (s.size() > empty_string_capacity)
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VERIFY( s.capacity() == s.size() ); // GNU-specific guarantee
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}
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#ifdef SIMULATOR_TEST
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