Re: [committed 1/3] libstdc++: Fix std::span constraint for sizeof(size_t) < sizeof(int) [PR108221]
On Tue, 10 Jan 2023, 12:26 Jakub Jelinek via Libstdc++, < libstd...@gcc.gnu.org> wrote: > On Tue, Jan 10, 2023 at 11:46:55AM +, Jonathan Wakely via Gcc-patches > wrote: > > Tested x86_64-linux. Pushed to trunk. > > > > -- >8 -- > > > > The default constructor has a constraint that is always false if > > arithmetic on size_t values promotes to int. Rewrite the constraint > > exactly as written in the standard, which works correctly. > > > > libstdc++-v3/ChangeLog: > > > > PR libstdc++/108221 > > * include/std/span (span::span()): Un-simplify constraint to > > work for size_t of lesser rank than int. > > --- > > libstdc++-v3/include/std/span | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/libstdc++-v3/include/std/span > b/libstdc++-v3/include/std/span > > index 251fed91abf..b336332b190 100644 > > --- a/libstdc++-v3/include/std/span > > +++ b/libstdc++-v3/include/std/span > > @@ -145,7 +145,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > > > >constexpr > >span() noexcept > > - requires ((_Extent + 1u) <= 1u) > > + requires (_Extent == dynamic_extent || _Extent == 0) > >: _M_ptr(nullptr), _M_extent(0) > >{ } > > If it would be C++23 only, you could use ((_Extent + 1uz) <= 1uz). > That would still promote _Extent and 1uz to int before the addition, and so would not wrap to zero when _Extent == -1uz size_t(_Extent + 1) <= 1 would work, but I'm just going to KISS and do what the standard says. As this is evaluated at compile time only, it is unfortunate it is > 3 operations compared to former 2, but not a big deal. If this was > in code that would be emitted at runtime, GCC already optimizes > (x == -1uz || x == 0) > or > (x == 0 || x == -1uz) > to > ((x + 1uz) <= 1uz) > > Jakub > >
Re: [committed 1/3] libstdc++: Fix std::span constraint for sizeof(size_t) < sizeof(int) [PR108221]
On Tue, Jan 10, 2023 at 11:46:55AM +, Jonathan Wakely via Gcc-patches wrote: > Tested x86_64-linux. Pushed to trunk. > > -- >8 -- > > The default constructor has a constraint that is always false if > arithmetic on size_t values promotes to int. Rewrite the constraint > exactly as written in the standard, which works correctly. > > libstdc++-v3/ChangeLog: > > PR libstdc++/108221 > * include/std/span (span::span()): Un-simplify constraint to > work for size_t of lesser rank than int. > --- > libstdc++-v3/include/std/span | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span > index 251fed91abf..b336332b190 100644 > --- a/libstdc++-v3/include/std/span > +++ b/libstdc++-v3/include/std/span > @@ -145,7 +145,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > >constexpr >span() noexcept > - requires ((_Extent + 1u) <= 1u) > + requires (_Extent == dynamic_extent || _Extent == 0) >: _M_ptr(nullptr), _M_extent(0) >{ } If it would be C++23 only, you could use ((_Extent + 1uz) <= 1uz). As this is evaluated at compile time only, it is unfortunate it is 3 operations compared to former 2, but not a big deal. If this was in code that would be emitted at runtime, GCC already optimizes (x == -1uz || x == 0) or (x == 0 || x == -1uz) to ((x + 1uz) <= 1uz) Jakub
[committed 1/3] libstdc++: Fix std::span constraint for sizeof(size_t) < sizeof(int) [PR108221]
Tested x86_64-linux. Pushed to trunk. -- >8 -- The default constructor has a constraint that is always false if arithmetic on size_t values promotes to int. Rewrite the constraint exactly as written in the standard, which works correctly. libstdc++-v3/ChangeLog: PR libstdc++/108221 * include/std/span (span::span()): Un-simplify constraint to work for size_t of lesser rank than int. --- libstdc++-v3/include/std/span | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span index 251fed91abf..b336332b190 100644 --- a/libstdc++-v3/include/std/span +++ b/libstdc++-v3/include/std/span @@ -145,7 +145,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION constexpr span() noexcept - requires ((_Extent + 1u) <= 1u) + requires (_Extent == dynamic_extent || _Extent == 0) : _M_ptr(nullptr), _M_extent(0) { } -- 2.39.0