* [committed 1/3] libstdc++: Fix std::span constraint for sizeof(size_t) < sizeof(int) [PR108221] @ 2023-01-10 11:46 Jonathan Wakely 2023-01-10 12:25 ` Jakub Jelinek 0 siblings, 1 reply; 3+ messages in thread From: Jonathan Wakely @ 2023-01-10 11:46 UTC (permalink / raw) To: libstdc++, gcc-patches 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 ^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [committed 1/3] libstdc++: Fix std::span constraint for sizeof(size_t) < sizeof(int) [PR108221] 2023-01-10 11:46 [committed 1/3] libstdc++: Fix std::span constraint for sizeof(size_t) < sizeof(int) [PR108221] Jonathan Wakely @ 2023-01-10 12:25 ` Jakub Jelinek 2023-01-10 13:03 ` Jonathan Wakely 0 siblings, 1 reply; 3+ messages in thread From: Jakub Jelinek @ 2023-01-10 12:25 UTC (permalink / raw) To: Jonathan Wakely; +Cc: libstdc++, gcc-patches On Tue, Jan 10, 2023 at 11:46:55AM +0000, 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 ^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [committed 1/3] libstdc++: Fix std::span constraint for sizeof(size_t) < sizeof(int) [PR108221] 2023-01-10 12:25 ` Jakub Jelinek @ 2023-01-10 13:03 ` Jonathan Wakely 0 siblings, 0 replies; 3+ messages in thread From: Jonathan Wakely @ 2023-01-10 13:03 UTC (permalink / raw) To: Jakub Jelinek; +Cc: Jonathan Wakely, libstdc++, gcc-patches [-- Attachment #1: Type: text/plain, Size: 1807 bytes --] On Tue, 10 Jan 2023, 12:26 Jakub Jelinek via Libstdc++, < libstdc++@gcc.gnu.org> wrote: > On Tue, Jan 10, 2023 at 11:46:55AM +0000, 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 > > ^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2023-01-10 13:03 UTC | newest] Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2023-01-10 11:46 [committed 1/3] libstdc++: Fix std::span constraint for sizeof(size_t) < sizeof(int) [PR108221] Jonathan Wakely 2023-01-10 12:25 ` Jakub Jelinek 2023-01-10 13:03 ` Jonathan Wakely
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