refactor(3p/nix/libexpr): Back Nix lists with std::vector r/1377

This change does away with the previous special-casing of lists of
certain element sizes, and the use of raw C-style arrays.

Lists are now backed by a std::vector of nix::Value*, which uses the
traceable GC allocator.

This change is unfortunately quite noisy because the accessor methods
were updated/removed accordingly, so all callsites of Nix-related
lists have changed.

For some operations in primops.cc where keeping the previous code
structure would have been more difficult with a "proper" vector, the
implementation has been replaced with std::vector methods. For
example, list concatenation now uses appropriate range inserts.

Anecdotally the performance of this is about equal, to even slightly
better, than the previous implementation.

All language tests pass and the depot paths I've used for testing
still evaluate.

Change-Id: Ib5eca6c0207429cb323a330c838c3a2200b2c693
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1266
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>
Reviewed-by: Kane York <rikingcoding@gmail.com>
Reviewed-by: glittershark <grfn@gws.fyi>

1 files changed, 29 insertions, 60 deletions

diff --git a/third_party/nix/src/libexpr/eval.cc b/third_party/nix/src/libexpr/eval.cc
index 16e6907214c9..f35f56c83c7d 100644
--- a/third_party/nix/src/libexpr/eval.cc
+++ b/third_party/nix/src/libexpr/eval.cc
@@ -97,12 +97,10 @@ static void printValue(std::ostream& str, std::set<const Value*>& active,
       str << "}";
       break;
     }
-    case tList1:
-    case tList2:
-    case tListN:
+    case tList:
       str << "[ ";
       for (unsigned int n = 0; n < v.listSize(); ++n) {
-        printValue(str, active, *v.listElems()[n]);
+        printValue(str, active, *(*v.list)[n]);
         str << " ";
       }
       str << "]";
@@ -162,9 +160,7 @@ std::string showType(const Value& v) {
       return "null";
     case tAttrs:
       return "a set";
-    case tList1:
-    case tList2:
-    case tListN:
+    case tList:
       return "a list";
     case tThunk:
       return "a thunk";
@@ -641,19 +637,15 @@ Env& EvalState::allocEnv(size_t size) {
   return *env;
 }
 
-void EvalState::mkList(Value& v, size_t size) {
+void EvalState::mkList(Value& v, NixList* list) {
   clearValue(v);
-  if (size == 1) {
-    v.type = tList1;
-  } else if (size == 2) {
-    v.type = tList2;
-  } else {
-    v.type = tListN;
-    v.bigList.size = size;
-    v.bigList.elems =
-        size != 0u ? (Value**)allocBytes(size * sizeof(Value*)) : nullptr;
-  }
-  nrListElems += size;
+  v.type = tList;
+  v.list = list;
+  nrListElems += list->size();
+}
+
+void EvalState::mkList(Value& v, size_t size) {
+  EvalState::mkList(v, new (GC) NixList(size));
 }
 
 unsigned long nrThunks = 0;
@@ -877,7 +869,7 @@ void ExprLet::eval(EvalState& state, Env& env, Value& v) {
 void ExprList::eval(EvalState& state, Env& env, Value& v) {
   state.mkList(v, elems.size());
   for (size_t n = 0; n < elems.size(); ++n) {
-    v.listElems()[n] = elems[n]->maybeThunk(state, env);
+    (*v.list)[n] = elems[n]->maybeThunk(state, env);
   }
 }
 
@@ -1252,39 +1244,21 @@ void ExprOpConcatLists::eval(EvalState& state, Env& env, Value& v) {
   e1->eval(state, env, v1);
   Value v2;
   e2->eval(state, env, v2);
-  Value* lists[2] = {&v1, &v2};
-  state.concatLists(v, 2, lists, pos);
+  state.concatLists(v, {&v1, &v2}, pos);
 }
 
-void EvalState::concatLists(Value& v, size_t nrLists, Value** lists,
-                            const Pos& pos) {
+void EvalState::concatLists(Value& v, const NixList& lists, const Pos& pos) {
   nrListConcats++;
 
-  Value* nonEmpty = nullptr;
-  size_t len = 0;
-  for (size_t n = 0; n < nrLists; ++n) {
-    forceList(*lists[n], pos);
-    auto l = lists[n]->listSize();
-    len += l;
-    if (l != 0u) {
-      nonEmpty = lists[n];
-    }
-  }
+  auto outlist = new (GC) NixList();
 
-  if ((nonEmpty != nullptr) && len == nonEmpty->listSize()) {
-    v = *nonEmpty;
-    return;
+  size_t len = 0;
+  for (Value* list : lists) {
+    forceList(*list, pos);
+    outlist->insert(outlist->end(), list->list->begin(), list->list->end());
   }
 
-  mkList(v, len);
-  auto out = v.listElems();
-  for (size_t n = 0, pos = 0; n < nrLists; ++n) {
-    auto l = lists[n]->listSize();
-    if (l != 0u) {
-      memcpy(out + pos, lists[n]->listElems(), l * sizeof(Value*));
-    }
-    pos += l;
-  }
+  mkList(v, outlist);
 }
 
 void ExprConcatStrings::eval(EvalState& state, Env& env, Value& v) {
@@ -1383,7 +1357,7 @@ void EvalState::forceValueDeep(Value& v) {
       }
     } else if (v.isList()) {
       for (size_t n = 0; n < v.listSize(); ++n) {
-        recurse(*v.listElems()[n]);
+        recurse(*(*v.list)[n]);
       }
     }
   };
@@ -1563,12 +1537,11 @@ std::string EvalState::coerceToString(const Pos& pos, Value& v,
     if (v.isList()) {
       std::string result;
       for (size_t n = 0; n < v.listSize(); ++n) {
-        result += coerceToString(pos, *v.listElems()[n], context, coerceMore,
+        result += coerceToString(pos, *(*v.list)[n], context, coerceMore,
                                  copyToStore);
         if (n < v.listSize() - 1
             /* !!! not quite correct */
-            && (!v.listElems()[n]->isList() ||
-                v.listElems()[n]->listSize() != 0)) {
+            && (!(*v.list)[n]->isList() || (*v.list)[n]->listSize() != 0)) {
           result += " ";
         }
       }
@@ -1653,14 +1626,12 @@ bool EvalState::eqValues(Value& v1, Value& v2) {
     case tNull:
       return true;
 
-    case tList1:
-    case tList2:
-    case tListN:
+    case tList:
       if (v1.listSize() != v2.listSize()) {
         return false;
       }
       for (size_t n = 0; n < v1.listSize(); ++n) {
-        if (!eqValues(*v1.listElems()[n], *v2.listElems()[n])) {
+        if (!eqValues(*(*v1.list)[n], *(*v2.list)[n])) {
           return false;
         }
       }
@@ -1883,14 +1854,12 @@ size_t valueSize(Value& v) {
           }
         }
         break;
-      case tList1:
-      case tList2:
-      case tListN:
-        if (seen.find(v.listElems()) == seen.end()) {
-          seen.insert(v.listElems());
+      case tList:
+        if (seen.find(v.list) == seen.end()) {
+          seen.insert(v.list);
           sz += v.listSize() * sizeof(Value*);
           for (size_t n = 0; n < v.listSize(); ++n) {
-            sz += doValue(*v.listElems()[n]);
+            sz += doValue(*(*v.list)[n]);
           }
         }
         break;