Add ability to fill YG with invalid values after collection

Summary:
Add a config in the GC to overwrite the YG with `kInvalidHeapValue`
after all objects have been evacuated. This is sufficiently lightweight
to be turned on in production if needed to identify memory safety
issues.

Reviewed By: jpporto

Differential Revision: D38009831

fbshipit-source-id: 483daa6b72697d02d9116b19966fddb79730d916

include/hermes/VM/HadesGC.h

@@ -743,6 +743,10 @@ class HadesGC final : public GCBase {
   /// If true, turn off promoteYGToOG_ as soon as the first OG GC occurs.
   bool revertToYGAtTTI_;
 
+  /// If true, overwrite the allocation region in the YG with kInvalidHeapValue
+  /// at the end of each YG collection.
+  bool overwriteDeadYGObjects_;
+
   /// Target OG occupancy ratio at the end of an OG collection.
   const double occupancyTarget_;
 

lib/VM/gcs/HadesGC.cpp

@@ -1296,6 +1296,7 @@ HadesGC::HadesGC(
           kConcurrentGC ? std::make_unique<Executor>() : nullptr},
       promoteYGToOG_{!gcConfig.getAllocInYoung()},
       revertToYGAtTTI_{gcConfig.getRevertToYGAtTTI()},
+      overwriteDeadYGObjects_{gcConfig.getOverwriteDeadYGObjects()},
       occupancyTarget_(gcConfig.getOccupancyTarget()),
       ygAverageSurvivalBytes_{
           /*weight*/ 0.5,
@@ -2503,6 +2504,10 @@ void HadesGC::youngGenCollection(
     // This was modified by debitExternalMemoryFromFinalizer, called by
     // finalizers. The difference in the value before to now was the swept bytes
     externalBytes.after = getYoungGenExternalBytes();
+
+    if (overwriteDeadYGObjects_)
+      memset(yg.start(), kInvalidHeapValue, yg.used());
+
     // Now the copy list is drained, and all references point to the old
     // gen. Clear the level of the young gen.
     yg.resetLevel();

public/hermes/Public/GCConfig.h

@@ -184,6 +184,9 @@ enum class GCEventKind {
   /* old gen (false). */                                                  \
   F(constexpr, bool, AllocInYoung, true)                                  \
                                                                           \
+  /* Whether to fill the YG with invalid data after each collection. */   \
+  F(constexpr, bool, OverwriteDeadYGObjects, false)                       \
+                                                                          \
   /* Whether to revert, if necessary, to young-gen allocation at TTI. */  \
   F(constexpr, bool, RevertToYGAtTTI, false)                              \
                                                                           \