https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91788
Bug ID: 91788
Summary: std::variant index +1-1
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: glisse at gcc dot gnu.org
Target Milestone: ---
(this is a detail, it probably has a negligible impact)
constexpr size_t index() const noexcept
{ return size_t(typename _Base::__index_type(this->_M_index + 1)) - 1; }
IIUC, the whole +1-1 is here so that for a valueless variant, index_type(-1)
becomes size_t(-1). I think there are cases where we could do better. For
instance, for a never valueless type, we could just return _M_index. If there
are fewer than 128 alternatives, we could use a sign extension: "return (signed
char)_M_index;". Maybe some well-placed __builtin_unreachable to specify the
range of _M_index would work as well.
