From 4ccd48ce2478cbe1263605838969f89d5b745f0a Mon Sep 17 00:00:00 2001 From: Eelco Dolstra Date: Mon, 13 Aug 2012 00:28:08 -0400 Subject: [PATCH] Add a "filter" primop Evaluation of a NixOS configuration spends quite a lot of time in the "filter" function in Nixpkgs. As implemented in Nixpkgs, this is a O(n^2) operation, so it's a good candidate for providing a more efficient (i.e. primop) implementation. Using it gives a ~10% speed increase and a significant reduction in the number of evaluations. Statistics before (on a NixOS system config): time elapsed: 1.3258 size of a value: 24 environments allocated: 1980939 (50127080 bytes) list elements: 14679308 (117434464 bytes) list concatenations: 50828 values allocated: 2098938 (50374512 bytes) attribute sets allocated: 392040 right-biased unions: 186334 values copied in right-biased unions: 591137 symbols in symbol table: 18271 number of thunks: 1645752 number of thunks avoided: 1921196 number of attr lookups: 430798 number of primop calls: 838807 number of function calls: 1930107 Statistics after: time elapsed: 1.17982 size of a value: 24 environments allocated: 1543334 (39624560 bytes) list elements: 9612638 (76901104 bytes) list concatenations: 37434 values allocated: 1854933 (44518392 bytes) attribute sets allocated: 392040 right-biased unions: 186334 values copied in right-biased unions: 591137 symbols in symbol table: 18272 number of thunks: 1392467 number of thunks avoided: 1507311 number of attr lookups: 430801 number of primop calls: 691600 number of function calls: 1492502 --- src/libexpr/eval.cc | 2 +- src/libexpr/primops.cc | 25 +++++++++++++++++++++++++ 2 files changed, 26 insertions(+), 1 deletion(-) diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc index a1a227c8a..871dfa64c 100644 --- a/src/libexpr/eval.cc +++ b/src/libexpr/eval.cc @@ -724,7 +724,7 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v) } if (fun.type != tLambda) - throwTypeError("attempt to call something which is neither a function nor a primop (built-in operation) but %1%", + throwTypeError("attempt to call something which is not a function but %1%", showType(fun)); unsigned int size = diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc index b833413ea..db7aa9e89 100644 --- a/src/libexpr/primops.cc +++ b/src/libexpr/primops.cc @@ -906,6 +906,30 @@ static void prim_map(EvalState & state, Value * * args, Value & v) } +/* Filter a list using a predicate; that is, return a list containing + every element from the list for which the predicate function + returns true. */ +static void prim_filter(EvalState & state, Value * * args, Value & v) +{ + state.forceFunction(*args[0]); + state.forceList(*args[1]); + + // FIXME: putting this on the stack is risky. + Value * vs[args[1]->list.length]; + unsigned int k = 0; + + for (unsigned int n = 0; n < args[1]->list.length; ++n) { + Value res; + state.callFunction(*args[0], *args[1]->list.elems[n], res); + if (state.forceBool(res)) + vs[k++] = args[1]->list.elems[n]; + } + + state.mkList(v, k); + for (unsigned int n = 0; n < k; ++n) v.list.elems[n] = vs[n]; +} + + /* Return the length of a list. This is an O(1) time operation. */ static void prim_length(EvalState & state, Value * * args, Value & v) { @@ -1120,6 +1144,7 @@ void EvalState::createBaseEnv() addPrimOp("__head", 1, prim_head); addPrimOp("__tail", 1, prim_tail); addPrimOp("map", 2, prim_map); + addPrimOp("__filter", 2, prim_filter); addPrimOp("__length", 1, prim_length); // Integer arithmetic