https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96750
--- Comment #5 from Matt Bentley <mattreecebentley at gmail dot com> --- If anyone out there is interested in working on this, I found the smallest change possible to create the same performance as GCC8- it is literally eliminating one branch possibility in one function (move-insert). The branch in question questions whether there are existing memory blocks to re-use or if we need to create a new memory block. For example, if the reserve() function has been called there will be existing memory blocks to re-use. However the performance drop occurs whether or not there are memory blocks to reuse. The actual if statement is irrelevant. I have tested and can remove all instances of memory block storage (reserve(), erase()) and problem still exists if this one branch is still in insert. I've attached the source files to demonstrate this above, including one plf_colony.h with the branch removed (renamed to plf_colony_fast.h), so you can see what difference there is. This code is all zlib license, free to share, but is early beta so don't redistribute please. Thanks, Matt ps. For consistency I've also removed the non-move-insert and emplace instances of this branch statement, even though they won't be called by the benchmark code in a C++11-compliant compiler.
