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, 2 insertions, 1 deletions

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 0d4efc47e6d6..b833413ea5ca 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -713,7 +713,7 @@ static void prim_attrNames(EvalState & state, Value * * args, Value & v)
 
 
 /* Dynamic version of the `.' operator. */
-static void prim_getAttr(EvalState & state, Value * * args, Value & v)
+void prim_getAttr(EvalState & state, Value * * args, Value & v)
 {
     string attr = state.forceStringNoCtx(*args[0]);
     state.forceAttrs(*args[1]);
@@ -722,6 +722,7 @@ static void prim_getAttr(EvalState & state, Value * * args, Value & v)
     if (i == args[1]->attrs->end())
         throw EvalError(format("attribute `%1%' missing") % attr);
     // !!! add to stack trace?
+    if (state.countCalls && i->pos) state.attrSelects[*i->pos]++;
     state.forceValue(*i->value);
     v = *i->value;
 }