This prevents some duplicate evaluation in nix-env and
nix-instantiate.
Also, when traversing ~/.nix-defexpr, only read regular files with the
extension .nix. Previously it was reading files like
.../channels/binary-caches/<name>. The only reason this didn't cause
problems is pure luck (namely, <name> shadows an actual Nix
expression, the binary-caches files happen to be syntactically valid
Nix expressions, and we iterate over the directory contents in just
the right order).
tree). This saves a lot of memory. The vector should be sorted so
that names can be looked up using binary search, but this is not the
case yet. (Surprisingly, looking up attributes using linear search
doesn't have a big impact on performance.)
Memory consumption for
$ nix-instantiate /etc/nixos/nixos/tests -A bittorrent.test --readonly-mode
on x86_64-linux with GC enabled is now 185 MiB (compared to 946
MiB on the trunk).
a pointer to a Value, rather than the Value directly. This improves
the effectiveness of garbage collection a lot: if the Value is
stored inside the set directly, then any live pointer to the Value
causes all other attributes in the set to be live as well.
values. This improves sharing and gives another speed up.
Evaluation of the NixOS system attribute is now almost 7 times
faster than the old evaluator.
efficiently. The symbol table ensures that there is only one copy
of each symbol, thus allowing symbols to be compared efficiently
using a pointer equality test.
