https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87258
Bug ID: 87258
Summary: vector<bool> useless offset
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Keywords: ABI, missed-optimization
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: glisse at gcc dot gnu.org
Target Milestone: ---
Hello,
I guess nobody will care much since this is vector<bool>, but I don't
understand why _M_start is a _Bit_iterator. This means that it includes a
useless _M_offset that is always 0 (that's easy to check since operator[]
ignores it). It wastes some space. And since begin() returns _M_start, it also
makes it hard for the compiler to know that the beginning offset is always 0,
and we often generate a lot of dead code to handle the non-zero case.
Removing the offset would break the ABI, so I guess that's not going to fly.
(_M_finish is also not very space efficient with pointer+length where just the
size would work, but maybe it is faster?) As a comparison, sizeof(vector<bool>)
is 40 with libstdc++ and 24 with libc++ on x86_64 linux.
Maybe begin could return _Bit_iterator(_M_start._M_p, 0) instead of _M_start to
give more information to the compiler?
