libstdc++: Reorder compile-time checks for __formatter_str::_M_format_range.
Commit Message
If _M_format_range was called with prvalue of span S (or any contiguous_range),
the previous chain of if-contexpr will call _M_format_range<S&>, then
_M_format_range<const S&> and then format(string_view). By checking for
contiguous_range first call format(string_view) direclty, removing unnecessary
instantiations and symbols.
Similary, for all prvalues of type R, that meet __simply_formattable_range R,
we were instantiating _M_format_range<R&> and then _M_format_range<const R&>.
By moving the if for __simply_formattable_range before is_lvalue_reference_v,
we call _M_format_range<const R&> direclty.
libstdc++-v3/ChangeLog:
* include/std/format (__formatter_str::_M_format_range):
Reorder cosntexpr checks, to reduce number of instantiations.
---
Not sure, why I ordered them thay way before. But after re-reading this code
path with my span changes, I realized this order is simply better.
Testing on x86_64-linux. *format* test passed.
OK for trunk when all test passes?
libstdc++-v3/include/std/format | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
Comments
On Wed, 6 May 2026 at 14:55, Tomasz Kamiński <tkaminsk@redhat.com> wrote:
>
> If _M_format_range was called with prvalue of span S (or any contiguous_range),
> the previous chain of if-contexpr will call _M_format_range<S&>, then
> _M_format_range<const S&> and then format(string_view). By checking for
> contiguous_range first call format(string_view) direclty, removing unnecessary
> instantiations and symbols.
>
> Similary, for all prvalues of type R, that meet __simply_formattable_range R,
> we were instantiating _M_format_range<R&> and then _M_format_range<const R&>.
> By moving the if for __simply_formattable_range before is_lvalue_reference_v,
> we call _M_format_range<const R&> direclty.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/format (__formatter_str::_M_format_range):
> Reorder cosntexpr checks, to reduce number of instantiations.
> ---
> Not sure, why I ordered them thay way before. But after re-reading this code
> path with my span changes, I realized this order is simply better.
>
> Testing on x86_64-linux. *format* test passed.
> OK for trunk when all test passes?
OK, thanks
>
> libstdc++-v3/include/std/format | 12 ++++++------
> 1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
> index 60adca93b5d..0f20fcc8d88 100644
> --- a/libstdc++-v3/include/std/format
> +++ b/libstdc++-v3/include/std/format
> @@ -1438,17 +1438,17 @@ namespace __format
> {
> using _Range = remove_reference_t<_Rg>;
> using _String_view = basic_string_view<_CharT>;
> - if constexpr (!is_lvalue_reference_v<_Rg>)
> - return _M_format_range<_Range&>(__rg, __fc);
> - else if constexpr (!is_const_v<_Range>
> - && __simply_formattable_range<_Range, _CharT>)
> - return _M_format_range<const _Range&>(__rg, __fc);
> - else if constexpr (ranges::contiguous_range<_Rg>)
> + if constexpr (ranges::contiguous_range<_Rg>)
> {
> _String_view __str(ranges::data(__rg),
> size_t(ranges::distance(__rg)));
> return format(__str, __fc);
> }
> + else if constexpr (!is_const_v<_Range>
> + && __simply_formattable_range<_Range, _CharT>)
> + return _M_format_range<const _Range&>(__rg, __fc);
> + else if constexpr (!is_lvalue_reference_v<_Rg>)
> + return _M_format_range<_Range&>(__rg, __fc);
> else
> {
> auto __handle_debug = [this, &__rg]<typename _NOut>(_NOut __nout)
> --
> 2.54.0
>
@@ -1438,17 +1438,17 @@ namespace __format
{
using _Range = remove_reference_t<_Rg>;
using _String_view = basic_string_view<_CharT>;
- if constexpr (!is_lvalue_reference_v<_Rg>)
- return _M_format_range<_Range&>(__rg, __fc);
- else if constexpr (!is_const_v<_Range>
- && __simply_formattable_range<_Range, _CharT>)
- return _M_format_range<const _Range&>(__rg, __fc);
- else if constexpr (ranges::contiguous_range<_Rg>)
+ if constexpr (ranges::contiguous_range<_Rg>)
{
_String_view __str(ranges::data(__rg),
size_t(ranges::distance(__rg)));
return format(__str, __fc);
}
+ else if constexpr (!is_const_v<_Range>
+ && __simply_formattable_range<_Range, _CharT>)
+ return _M_format_range<const _Range&>(__rg, __fc);
+ else if constexpr (!is_lvalue_reference_v<_Rg>)
+ return _M_format_range<_Range&>(__rg, __fc);
else
{
auto __handle_debug = [this, &__rg]<typename _NOut>(_NOut __nout)