[JSC] Honor a GC request deferred by DeferGCForAWhile at the next VM entry

Source/JavaScriptCore/heap/Heap.cpp

@@ -3005,6 +3005,13 @@ void Heap::collectIfNecessaryOrDefer(GCDeferralContext* deferralContext)
     }
 }
 
+void Heap::performPendingDeferredGCWork()
+{
+    ASSERT(hasPendingDeferredGCWork());
+    m_didDeferGCWork = false;
+    collectIfNecessaryOrDefer();
+}
+
 void Heap::decrementDeferralDepthAndGCIfNeededSlow()
 {
     // Can't do anything if we're still deferred.

Source/JavaScriptCore/heap/Heap.h

@@ -438,6 +438,13 @@ class Heap {
 
     void collectIfNecessaryOrDefer(GCDeferralContext* = nullptr);
 
+    // A GC request raised while collection was deferred by DeferGCForAWhile (e.g. the extra-memory
+    // reports in ScriptExecutable::prepareForExecutionImpl) only sets m_didDeferGCWork; nothing acts on it
+    // until the next allocation slow path or ~DeferGC, which an embedder-driven loop may not reach for a
+    // long time. VM entry is a safe point to honor it.
+    bool hasPendingDeferredGCWork() const { return m_didDeferGCWork && !m_deferralDepth; }
+    JS_EXPORT_PRIVATE void performPendingDeferredGCWork();
+
     void completeAllJITPlans();
 
     // Note that:

Source/JavaScriptCore/runtime/OptionsList.h

@@ -238,6 +238,7 @@ bool hasCapacityToUseLargeGigacage();
     v(Bool, testTheFTL, false, Normal, nullptr) \
     v(Bool, verboseSanitizeStack, false, Normal, nullptr) \
     v(Bool, useGenerationalGC, true, Normal, nullptr) \
+    v(Bool, usePollingDeferredGCWorkAtVMEntry, true, Normal, "Honor a GC request left pending by a DeferGCForAWhile scope when the VM is next entered, instead of waiting for the next allocation slow path or ~DeferGC."_s) \
     v(Bool, useConcurrentGC, true, Normal, nullptr) \
     v(Bool, collectContinuously, false, Normal, nullptr) \
     v(Double, collectContinuouslyPeriodMS, 1, Normal, nullptr) \

Source/JavaScriptCore/runtime/VMEntryScopeInlines.h

@@ -37,6 +37,8 @@ ALWAYS_INLINE VMEntryScope::VMEntryScope(VM& vm, JSGlobalObject* globalObject)
     if (!vm.entryScope)
         setUpSlow();
     vm.clearLastException();
+    if (vm.heap.hasPendingDeferredGCWork() && Options::usePollingDeferredGCWorkAtVMEntry()) [[unlikely]]
+        vm.heap.performPendingDeferredGCWork();
 }
 
 ALWAYS_INLINE VMEntryScope::~VMEntryScope()