Apply a patch to boehmgc to increase the initial mark stack size

If the mark stack size is too small, it greatly inhibits parallel
marking, which is very bad for performance on multi-core systems.

Author: edolstra

packaging/dependencies.nix

@@ -50,9 +50,13 @@ scope: {
         requiredSystemFeatures = [ ];
       };
 
-  boehmgc = pkgs.boehmgc.override {
-    enableLargeConfig = true;
-  };
+  boehmgc =
+    (pkgs.boehmgc.override {
+      enableLargeConfig = true;
+    }).overrideAttrs
+      (attrs: {
+        patches = attrs.patches or [ ] ++ [ ./patches/bdwgc-bigger-mark-stack.patch ];
+      });
 
   # TODO Hack until https://github.com/NixOS/nixpkgs/issues/45462 is fixed.
   boost =

packaging/patches/bdwgc-bigger-mark-stack.patch

@@ -0,0 +1,16 @@
+Increase the initial mark stack size to avoid stack overflows, since
+these inhibit parallel marking (see GC_mark_some()).
+
+diff --git a/mark.c b/mark.c
+index 50e6cfd7..907f7763 100644
+--- a/mark.c
++++ b/mark.c
+@@ -66,7 +66,7 @@ GC_INNER struct obj_kind GC_obj_kinds[MAXOBJKINDS] = {
+ };
+ 
+ # ifndef INITIAL_MARK_STACK_SIZE
+-#   define INITIAL_MARK_STACK_SIZE (1*HBLKSIZE)
++#   define INITIAL_MARK_STACK_SIZE (256*HBLKSIZE)
+                 /* INITIAL_MARK_STACK_SIZE * sizeof(mse) should be a    */
+                 /* multiple of HBLKSIZE.                                */
+                 /* The incremental collector actually likes a larger    */