Add a primop ‘concatLists’
This can serve as a generic efficient list builder. For instance, the function ‘catAttrs’ in Nixpkgs can be rewritten from attr: l: fold (s: l: if hasAttr attr s then [(getAttr attr s)] ++ l else l) [] l to attr: l: builtins.concatLists (map (s: if hasAttr attr s then [(getAttr attr s)] else []) l) Statistics before: time elapsed: 1.08683 size of a value: 24 environments allocated: 1384376 (35809568 bytes) list elements: 6946783 (55574264 bytes) list concatenations: 37434 values allocated: 1760440 (42250560 bytes) attribute sets allocated: 392040 right-biased unions: 186334 values copied in right-biased unions: 591137 symbols in symbol table: 18273 number of thunks: 1297673 number of thunks avoided: 1380759 number of attr lookups: 430802 number of primop calls: 628912 number of function calls: 1333544 Statistics after (including new catAttrs): time elapsed: 0.959854 size of a value: 24 environments allocated: 1010198 (26829296 bytes) list elements: 1984878 (15879024 bytes) list concatenations: 30488 values allocated: 1589760 (38154240 bytes) attribute sets allocated: 392040 right-biased unions: 186334 values copied in right-biased unions: 591137 symbols in symbol table: 18274 number of thunks: 1040925 number of thunks avoided: 1038428 number of attr lookups: 438419 number of primop calls: 474844 number of function calls: 959366
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@ -912,16 +912,29 @@ void ExprOpUpdate::eval(EvalState & state, Env & env, Value & v)
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void ExprOpConcatLists::eval(EvalState & state, Env & env, Value & v)
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{
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state.nrListConcats++;
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Value v1; e1->eval(state, env, v1);
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state.forceList(v1);
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Value v2; e2->eval(state, env, v2);
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state.forceList(v2);
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state.mkList(v, v1.list.length + v2.list.length);
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for (unsigned int n = 0; n < v1.list.length; ++n)
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v.list.elems[n] = v1.list.elems[n];
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for (unsigned int n = 0; n < v2.list.length; ++n)
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v.list.elems[n + v1.list.length] = v2.list.elems[n];
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Value * lists[2] = { &v1, &v2 };
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state.concatLists(v, 2, lists);
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}
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void EvalState::concatLists(Value & v, unsigned int nrLists, Value * * lists)
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{
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nrListConcats++;
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unsigned int len = 0;
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for (unsigned int n = 0; n < nrLists; ++n) {
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forceList(*lists[n]);
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len += lists[n]->list.length;
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}
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mkList(v, len);
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for (unsigned int n = 0, pos = 0; n < nrLists; ++n) {
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unsigned int l = lists[n]->list.length;
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memcpy(v.list.elems + pos, lists[n]->list.elems, l * sizeof(Value *));
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pos += l;
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}
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}
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@ -232,6 +232,8 @@ public:
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void mkAttrs(Value & v, unsigned int expected);
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void mkThunk_(Value & v, Expr * expr);
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void concatLists(Value & v, unsigned int nrLists, Value * * lists);
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/* Print statistics. */
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void printStats();
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@ -931,6 +931,7 @@ static void prim_filter(EvalState & state, Value * * args, Value & v)
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}
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/* Return true if a list contains a given element. */
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static void prim_elem(EvalState & state, Value * * args, Value & v)
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{
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bool res = false;
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@ -944,6 +945,14 @@ static void prim_elem(EvalState & state, Value * * args, Value & v)
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}
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/* Concatenate a list of lists. */
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static void prim_concatLists(EvalState & state, Value * * args, Value & v)
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{
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state.forceList(*args[0]);
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state.concatLists(v, args[0]->list.length, args[0]->list.elems);
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}
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/* Return the length of a list. This is an O(1) time operation. */
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static void prim_length(EvalState & state, Value * * args, Value & v)
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{
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@ -1160,6 +1169,7 @@ void EvalState::createBaseEnv()
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addPrimOp("map", 2, prim_map);
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addPrimOp("__filter", 2, prim_filter);
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addPrimOp("__elem", 2, prim_elem);
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addPrimOp("__concatLists", 1, prim_concatLists);
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addPrimOp("__length", 1, prim_length);
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// Integer arithmetic
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