Apparently this thing is getting common:

  jemalloc - Use dev-libs/jemalloc for memory management

28 uses:

  app-admin/rsyslog: Use dev-libs/jemalloc for allocations.
  app-editors/neovim: Use dev-libs/jemalloc for allocations
  app-office/libreoffice: Use dev-libs/jemalloc for allocations
  app-text/groonga: Enable dev-libs/jemalloc for memory management
  dev-db/mariadb: Use dev-libs/jemalloc for allocations.
  dev-db/mariadb-galera: Use dev-libs/jemalloc for allocations.
  dev-db/mysql: Use dev-libs/jemalloc for allocations.
  dev-db/mysql-cluster: Use dev-libs/jemalloc for allocations.
  dev-db/percona-server: Use dev-libs/jemalloc for allocations.
  dev-db/redis: Use dev-libs/jemalloc for allocations.
  dev-db/rethinkdb: Use dev-libs/jemalloc for allocations (default).
  dev-lang/ruby: Use dev-libs/jemalloc for memory allocation.
  dev-lang/rust: Use dev-libs/jemalloc as the standard memory allocator
  dev-libs/bitset: Use dev-libs/jemalloc for allocations
  mail-client/thunderbird: Enable or disable jemalloc
  mail-filter/rspamd: Use dev-libs/jemalloc for allocations
  media-gfx/blender: Use dev-libs/jemalloc for memory allocation.
  net-analyzer/bro: Support for the Jemalloc allocator
  net-libs/nghttp2: Use dev-libs/jemalloc for memory allocation
  net-misc/aria2: Use dev-libs/jemalloc to replace the malloc()
implementation with a possibly faster one
  sci-libs/tensorflow: Enable support for dev-libs/jemalloc
  sci-physics/root: Enable support for dev-libs/jemalloc
  sys-cluster/ceph: Use dev-libs/jemalloc for allocations
  sys-cluster/hpx: Use dev-libs/jemalloc for memory allocation
  www-client/firefox: Enable or disable jemalloc
  www-client/seamonkey: Enable or disable jemalloc
  www-servers/uwsgi: Use dev-libs/jemalloc instead of libc's malloc
  www-servers/varnish: Use dev-libs/jemalloc for allocations

-- 
Best regards,
Michał Górny