Merge pull request #7348 from thufschmitt/dont-use-vlas

Remove the usage of VLAs in the code

(cherry picked from commit ac4431e9d016e62fb5dc9ae36833bd0c6cdadeec)
Change-Id: Ifbf5fbfc2e27122362a2aaea4b62c7cf3ca46b1a

src/libexpr/eval.cc

@@ -1,6 +1,7 @@
 #include "eval.hh"
 #include "eval-settings.hh"
 #include "hash.hh"
+#include "primops.hh"
 #include "types.hh"
 #include "util.hh"
 #include "store-api.hh"
@@ -38,6 +39,7 @@
 #include <boost/coroutine2/coroutine.hpp>
 #include <boost/coroutine2/protected_fixedsize_stack.hpp>
 #include <boost/context/stack_context.hpp>
+#include <boost/container/small_vector.hpp>
 
 #endif
 
@@ -703,6 +705,23 @@ void EvalState::addConstant(const std::string & name, Value * v, Constant info)
 }
 
 
+void PrimOp::check()
+{
+    if (arity > maxPrimOpArity) {
+        throw Error("primop arity must not exceed %1%", maxPrimOpArity);
+    }
+}
+
+
+void Value::mkPrimOp(PrimOp * p)
+{
+    p->check();
+    clearValue();
+    internalType = tPrimOp;
+    primOp = p;
+}
+
+
 Value * EvalState::addPrimOp(PrimOp && primOp)
 {
     /* Hack to make constants lazy: turn them into a application of
@@ -1683,7 +1702,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 /* We have all the arguments, so call the primop with
                    the previous and new arguments. */
 
-                Value * vArgs[arity];
+                Value * vArgs[maxPrimOpArity];
                 auto n = argsDone;
                 for (Value * arg = &vCur; arg->isPrimOpApp(); arg = arg->primOpApp.left)
                     vArgs[--n] = arg->primOpApp.right;
@@ -1740,11 +1759,17 @@ void ExprCall::eval(EvalState & state, Env & env, Value & v)
     Value vFun;
     fun->eval(state, env, vFun);
 
-    Value * vArgs[args.size()];
+    // Empirical arity of Nixpkgs lambdas by regex e.g. ([a-zA-Z]+:(\s|(/\*.*\/)|(#.*\n))*){5}
+    // 2: over 4000
+    // 3: about 300
+    // 4: about 60
+    // 5: under 10
+    // This excluded attrset lambdas (`{...}:`). Contributions of mixed lambdas appears insignificant at ~150 total.
+    boost::container::small_vector<Value *, 4> vArgs(args.size());
     for (size_t i = 0; i < args.size(); ++i)
         vArgs[i] = args[i]->maybeThunk(state, env);
 
-    state.callFunction(vFun, args.size(), vArgs, v, pos);
+    state.callFunction(vFun, args.size(), vArgs.data(), v, pos);
 }
 
 
@@ -1983,8 +2008,8 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
         return result;
     };
 
-    Value values[es->size()];
-    Value * vTmpP = values;
+    boost::container::small_vector<Value, conservativeStackReservation> values(es->size());
+    Value * vTmpP = values.data();
 
     for (auto & [i_pos, i] : *es) {
         Value & vTmp = *vTmpP++;