[Bug libstdc++/124463] [15/16 Regression] Big (10x) performance regression for std::deque<unsigned char>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124463

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Bisected to my r15-4473-g3abe751ea86e34 change