Add some basic profiling support to the evaluator

Setting the environment variable NIX_COUNT_CALLS to 1 enables some
basic profiling in the evaluator.  It will count calls to functions
and primops as well as evaluations of attributes.

For example, to see where evaluation of a NixOS configuration spends
its time:

$ NIX_SHOW_STATS=1 NIX_COUNT_CALLS=1 ./src/nix-instantiate/nix-instantiate '<nixos>' -A system --readonly-mode
...
calls to 39 primops:
    239532 head
    233962 tail
    191252 hasAttr
...
calls to 1595 functions:
    224157 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:19'
    221767 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:14'
    221767 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:10'
...
evaluations of 7088 attributes:
    167377 undefined position
    132459 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/attrsets.nix:119:41'
     47322 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/attrsets.nix:13:21'
...

1 files changed, 3 insertions, 0 deletions

diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index edb7c0fa6c..dc38a53ca2 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -17,6 +17,9 @@ namespace nix {
 #define foreach(it_type, it, collection)                                \
     for (it_type it = (collection).begin(); it != (collection).end(); ++it)
 
+#define foreach_reverse(it_type, it, collection)                                \
+    for (it_type it = (collection).rbegin(); it != (collection).rend(); ++it)
+
 
 /* Return an environment variable. */
 string getEnv(const string & key, const string & def = "");