Move EvalState from the stack to the heap

EvalState contains a few counters (e.g. nrValues) that increase
quickly enough that they end up being interpreted as pointers by the
garbage collector. Moving it to the heap makes them invisible to the
garbage collector.

This reduces the max RSS doing 100 evaluations of
nixos.tests.firefox.x86_64-linux.drvPath from 455 MiB to 292 MiB.

Note: ideally, allocations would be much further up in the 64-bit
address space to reduce the odds of an integer being misinterpreted as
a pointer. Maybe we can use some linker magic to move the .bss segment
to a higher address.

1 files changed, 14 insertions, 14 deletions

diff --git a/src/nix-prefetch-url/nix-prefetch-url.cc b/src/nix-prefetch-url/nix-prefetch-url.cc
index fa7ee254500c..50b2c2803ec9 100644
--- a/src/nix-prefetch-url/nix-prefetch-url.cc
+++ b/src/nix-prefetch-url/nix-prefetch-url.cc
@@ -95,9 +95,9 @@ int main(int argc, char * * argv)
             throw UsageError("too many arguments");
 
         auto store = openStore();
-        EvalState state(myArgs.searchPath, store);
+        auto state = std::make_unique<EvalState>(myArgs.searchPath, store);
 
-        Bindings & autoArgs = *myArgs.getAutoArgs(state);
+        Bindings & autoArgs = *myArgs.getAutoArgs(*state);
 
         /* If -A is given, get the URI from the specified Nix
            expression. */
@@ -107,33 +107,33 @@ int main(int argc, char * * argv)
                 throw UsageError("you must specify a URI");
             uri = args[0];
         } else {
-            Path path = resolveExprPath(lookupFileArg(state, args.empty() ? "." : args[0]));
+            Path path = resolveExprPath(lookupFileArg(*state, args.empty() ? "." : args[0]));
             Value vRoot;
-            state.evalFile(path, vRoot);
-            Value & v(*findAlongAttrPath(state, attrPath, autoArgs, vRoot));
-            state.forceAttrs(v);
+            state->evalFile(path, vRoot);
+            Value & v(*findAlongAttrPath(*state, attrPath, autoArgs, vRoot));
+            state->forceAttrs(v);
 
             /* Extract the URI. */
-            auto attr = v.attrs->find(state.symbols.create("urls"));
+            auto attr = v.attrs->find(state->symbols.create("urls"));
             if (attr == v.attrs->end())
                 throw Error("attribute set does not contain a 'urls' attribute");
-            state.forceList(*attr->value);
+            state->forceList(*attr->value);
             if (attr->value->listSize() < 1)
                 throw Error("'urls' list is empty");
-            uri = state.forceString(*attr->value->listElems()[0]);
+            uri = state->forceString(*attr->value->listElems()[0]);
 
             /* Extract the hash mode. */
-            attr = v.attrs->find(state.symbols.create("outputHashMode"));
+            attr = v.attrs->find(state->symbols.create("outputHashMode"));
             if (attr == v.attrs->end())
                 printInfo("warning: this does not look like a fetchurl call");
             else
-                unpack = state.forceString(*attr->value) == "recursive";
+                unpack = state->forceString(*attr->value) == "recursive";
 
             /* Extract the name. */
             if (name.empty()) {
-                attr = v.attrs->find(state.symbols.create("name"));
+                attr = v.attrs->find(state->symbols.create("name"));
                 if (attr != v.attrs->end())
-                    name = state.forceString(*attr->value);
+                    name = state->forceString(*attr->value);
             }
         }
 
@@ -158,7 +158,7 @@ int main(int argc, char * * argv)
 
         if (storePath.empty()) {
 
-            auto actualUri = resolveMirrorUri(state, uri);
+            auto actualUri = resolveMirrorUri(*state, uri);
 
             /* Download the file. */
             DownloadRequest req(actualUri);