On 28/09/20 09:11 -0400, Patrick Palka via Libstdc++ wrote:
On Mon, 28 Sep 2020, Jonathan Wakely wrote:On 28/09/20 00:48 -0400, Patrick Palka via Libstdc++ wrote: > Since the standard range adaptors are specified to derive from the empty > class view_base, making their first data member store the underlying > view is suboptimal, for if the underlying view also derives from > view_base then the two view_base subobjects will be adjacent, thus > preventing the compiler from applying the empty base optimization to > elide away the storage for these two empty bases. > > This patch improves the situation by declaring the _M_base data member > last instead of first in each range adaptor that has more than one data > member, so that the empty base optimization can apply more often. > > Tested on x86_64-pc-linux-gnu with and wihout -m32. > > libstdc++-v3/ChangeLog: > > * include/std/ranges (filter_view::_M_base): Declare this data > member last. > (transform_view::_M_base): Likewise. > (take_view::_M_base): Likewise. > (take_while_view::_M_base): Likewise. > (drop_view::_M_base): Likewise. > (drop_while_view::_M_base): Likewise. > (join_view::_M_base): Likewise. > (split_view::_M_base): Likewise. > * testsuite/std/ranges/adaptors/sizeof.cc: Adjust expected > sizes. > --- > libstdc++-v3/include/std/ranges | 17 ++++++++--------- > .../testsuite/std/ranges/adaptors/sizeof.cc | 18 +++++++++--------- > 2 files changed, 17 insertions(+), 18 deletions(-) > > diff --git a/libstdc++-v3/include/std/ranges > b/libstdc++-v3/include/std/ranges > index 964a2b616a6..6fd8a85c2bf 100644 > --- a/libstdc++-v3/include/std/ranges > +++ b/libstdc++-v3/include/std/ranges > @@ -1250,9 +1250,9 @@ namespace views > { return __y.__equal(__x); } > }; > > - _Vp _M_base = _Vp(); > [[no_unique_address]] __detail::__box<_Pred> _M_pred; > [[no_unique_address]] __detail::_CachedPosition<_Vp> _M_cached_begin; > + _Vp _M_base = _Vp(); The constructor's mem-initializer-list needs to be reordered, to avoid warnings with -Wsystem-headers.Oops, fixed thusly. I noticed meanwhile that I forgot to move reverse_view's _M_base member, so the below patch adds that as well.
Nice to see testsuite/std/ranges/adaptors/sizeof.cc already proving useful by demonstrating the reduced sizes.
Tested on x86_64-pc-linux-gnu, and also verified that there are no new -Wsystem-headers warnings.
OK, thanks.
