[PATCH] libstdc+: Eliminate usage of alloca for non-localized formatting
Jonathan Wakely
jwakely@redhat.com
Mon Mar 9 21:19:55 GMT 2026
On Thu, 05 Mar 2026 at 15:45 +0100, Tomasz Kamiński wrote:
>This patch eliminate the use of __builtin_alloca for non-localized formatting
>of integers and pointers.
>
>For integers, the transcoding to _CharT moved from _M_format_int function
>to format. This makes the maximum size of the buffer known (__buf_size)
>that depends on sizeof(_Int) and allows use local array of _CharT (__wbuf)
>for the storage. The _M_format_int is modified to accept the string of _CharT.
>
>For pointers, format specifiers are subset of one allowed from the integers,
>so we simply delegate to __formatter_int::format, instead of repeated formatting.
>(_Pres_p and _Pres_P was modified by r16-7844-gbfc2b87f8244a1 to have same value
>as _Pres_x and _Pres_X). The set of allowed specifier is still limited per C++
>standard by __formatter_ptr::parse.
>
>We also fix issue in __formatter_ptr::parse, where for 'p' and 'P' the value
>of _M_alt was negated for _M_spec (result of previous parse) instead of __spec
>(result of current parse), and adjust the __formatter_ptr default constructor
>to set _M_spec._M_type and _M_spec._M_alt appropriately.
>
>libstdc++-v3/ChangeLog:
>
> * include/std/format (__formatter_int::format): Handle transcoding
> to _CharT before calling _M_format_int.
> (__formatter_int::_M_format_int): Accept basic_string_view<_CharT>
> and remove transcoding.
> (__formatter_ptr::__formatter_ptr): Configure _M_spec member.
> (__formatter_ptr::parse): Negate _M_alt for __spec and not _M_spec.
> (__formatter_ptr::format): Delegate to __formatter_int.
>
>Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
>---
>This could be done for GCC-17, but I think we should at least merge
>chagnes for __formatter_ptr cosntructor and parse for GCC16, so the __spec
>value is configured in way compatible with __formatter_int.
>
>Testing on x86_64-linux. OK for trunk when test passes?
Nice, OK for trunk, thanks.
> libstdc++-v3/include/std/format | 102 +++++++++-----------------------
> 1 file changed, 29 insertions(+), 73 deletions(-)
>
>diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
>index ec3d0018efd..9ee9bb4c9e2 100644
>--- a/libstdc++-v3/include/std/format
>+++ b/libstdc++-v3/include/std/format
>@@ -1647,7 +1647,8 @@ namespace __format
> if (_M_spec._M_type == _Pres_c)
> return _M_format_character(_S_to_character(__i), __fc);
>
>- char __buf[sizeof(_Int) * __CHAR_BIT__ + 3];
>+ constexpr size_t __buf_size = sizeof(_Int) * __CHAR_BIT__ + 3;
>+ char __buf[__buf_size];
> to_chars_result __res{};
>
> string_view __base_prefix;
>@@ -1707,8 +1708,21 @@ namespace __format
> }
> __start = __format::__put_sign(__i, _M_spec._M_sign, __start - 1);
>
>- return _M_format_int(string_view(__start, __res.ptr - __start),
>- __start_digits - __start, __fc);
>+
>+ string_view __narrow_str(__start, __res.ptr - __start);
>+ size_t __prefix_len = __start_digits - __start;
>+ if constexpr (is_same_v<char, _CharT>)
>+ return _M_format_int(__narrow_str, __prefix_len, __fc);
>+#ifdef _GLIBCXX_USE_WCHAR_T
>+ else
>+ {
>+ _CharT __wbuf[__buf_size];
>+ size_t __n = __narrow_str.size();
>+ std::__to_wstring_numeric(__narrow_str.data(), __n, __wbuf);
>+ return _M_format_int(basic_string_view<_CharT>(__wbuf, __n),
>+ __prefix_len, __fc);
>+ }
>+#endif
> }
>
> template<typename _Out>
>@@ -1795,24 +1809,10 @@ namespace __format
>
> template<typename _Out>
> typename basic_format_context<_Out, _CharT>::iterator
>- _M_format_int(string_view __narrow_str, size_t __prefix_len,
>+ _M_format_int(basic_string_view<_CharT> __str, size_t __prefix_len,
> basic_format_context<_Out, _CharT>& __fc) const
> {
> size_t __width = _M_spec._M_get_width(__fc);
>-
>- basic_string_view<_CharT> __str;
>- if constexpr (is_same_v<char, _CharT>)
>- __str = __narrow_str;
>-#ifdef _GLIBCXX_USE_WCHAR_T
>- else
>- {
>- size_t __n = __narrow_str.size();
>- auto __p = (_CharT*)__builtin_alloca(__n * sizeof(_CharT));
>- std::__to_wstring_numeric(__narrow_str.data(), __n, __p);
>- __str = {__p, __n};
>- }
>-#endif
>-
> if (_M_spec._M_localized)
> {
> const auto& __l = __fc.locale();
>@@ -2433,7 +2433,13 @@ namespace __format
> template<__format::__char _CharT>
> struct __formatter_ptr
> {
>- __formatter_ptr() = default;
>+ constexpr
>+ __formatter_ptr() noexcept
>+ : _M_spec()
>+ {
>+ _M_spec._M_type = _Pres_p;
>+ _M_spec._M_alt = true;
>+ }
>
> constexpr
> __formatter_ptr(_Spec<_CharT> __spec) noexcept
>@@ -2483,14 +2489,14 @@ namespace __format
> if (*__first == 'p')
> {
> __spec._M_type = _Pres_p;
>- _M_spec._M_alt = !_M_spec._M_alt;
>+ __spec._M_alt = !__spec._M_alt;
> ++__first;
> }
> #if __glibcxx_format >= 202304L
> else if (*__first == 'P')
> {
> __spec._M_type = _Pres_P;
>- _M_spec._M_alt = !_M_spec._M_alt;
>+ __spec._M_alt = !__spec._M_alt;
> ++__first;
> }
> #endif
>@@ -2506,57 +2512,7 @@ namespace __format
> format(const void* __v, basic_format_context<_Out, _CharT>& __fc) const
> {
> auto __u = reinterpret_cast<__UINTPTR_TYPE__>(__v);
>- char __buf[2 + sizeof(__v) * 2];
>- auto [__ptr, __ec] = std::to_chars(__buf + 2, std::end(__buf),
>- __u, 16);
>- int __n = __ptr - __buf;
>- __buf[0] = '0';
>- __buf[1] = 'x';
>-#if __glibcxx_format >= 202304L
>- if (_M_spec._M_type == __format::_Pres_P)
>- {
>- __buf[1] = 'X';
>- for (auto __p = __buf + 2; __p != __ptr; ++__p)
>-#if __has_builtin(__builtin_toupper)
>- *__p = __builtin_toupper(*__p);
>-#else
>- *__p = std::toupper(*__p);
>-#endif
>- }
>-#endif
>-
>- basic_string_view<_CharT> __str;
>- if constexpr (is_same_v<_CharT, char>)
>- __str = string_view(__buf, __n);
>-#ifdef _GLIBCXX_USE_WCHAR_T
>- else
>- {
>- auto __p = (_CharT*)__builtin_alloca(__n * sizeof(_CharT));
>- std::__to_wstring_numeric(__buf, __n, __p);
>- __str = wstring_view(__p, __n);
>- }
>-#endif
>-
>-#if __glibcxx_format >= 202304L
>- if (_M_spec._M_zero_fill)
>- {
>- size_t __width = _M_spec._M_get_width(__fc);
>- if (__width <= __str.size())
>- return __format::__write(__fc.out(), __str);
>-
>- auto __out = __fc.out();
>- // Write "0x" or "0X" prefix before zero-filling.
>- __out = __format::__write(std::move(__out), __str.substr(0, 2));
>- __str.remove_prefix(2);
>- size_t __nfill = __width - __n;
>- return __format::__write_padded(std::move(__out), __str,
>- __format::_Align_right,
>- __nfill, _CharT('0'));
>- }
>-#endif
>-
>- return __format::__write_padded_as_spec(__str, __n, __fc, _M_spec,
>- __format::_Align_right);
>+ return __formatter_int<_CharT>(_M_spec).format(__u, __fc);
> }
>
> private:
>@@ -2571,7 +2527,7 @@ namespace __format
> }
> }
>
>- __format::_Spec<_CharT> _M_spec{};
>+ __format::_Spec<_CharT> _M_spec;
> };
>
> } // namespace __format
>--
>2.53.0
>
>
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