[2.38] limit conservative scan range

Turns out that conservative GC root scanning is going too deep into the stack
and this may prevent the collection of objects - see:

WebPlatformForEmbedded#1363

The change introduces 'stack guards' that prevent this in particular cases,
that is - if there is gloop main loop routine in the stack the GC root stack search
will not go deeper that this frame. This avoids some 'false positives' conservative roots
that can make the app hold a lot of memory.

Source/JavaScriptCore/heap/MachineStackMarker.cpp

@@ -27,6 +27,7 @@
 #include <wtf/BitVector.h>
 #include <wtf/PageBlock.h>
 #include <wtf/StdLibExtras.h>
+#include <wtf/ConservativeScanStackGuards.h>
 
 namespace JSC {
 
@@ -44,7 +45,7 @@ void MachineThreads::gatherFromCurrentThread(ConservativeRoots& conservativeRoot
         conservativeRoots.add(registersBegin, registersEnd, jitStubRoutines, codeBlocks);
     }
 
-    conservativeRoots.add(currentThreadState.stackTop, currentThreadState.stackOrigin, jitStubRoutines, codeBlocks);
+    conservativeRoots.add(currentThreadState.stackTop, WTF::ConservativeScanStackGuards::updatedStackOrigin(currentThreadState.stackTop, currentThreadState.stackOrigin), jitStubRoutines, codeBlocks);
 }
 
 static inline int osRedZoneAdjustment()

Source/WTF/wtf/CMakeLists.txt

@@ -46,6 +46,7 @@ set(WTF_PUBLIC_HEADERS
     ConcurrentPtrHashSet.h
     ConcurrentVector.h
     Condition.h
+    ConservativeScanStackGuards.h
     CountingLock.h
     CrossThreadCopier.h
     CrossThreadQueue.h
@@ -422,6 +423,7 @@ set(WTF_SOURCES
     CompilationThread.cpp
     ConcurrentBuffer.cpp
     ConcurrentPtrHashSet.cpp
+    ConservativeScanStackGuards.cpp
     CountingLock.cpp
     CrossThreadCopier.cpp
     CrossThreadTaskHandler.cpp

Source/WTF/wtf/ConservativeScanStackGuards.cpp

@@ -0,0 +1,6 @@
+#include "ConservativeScanStackGuards.h"
+
+namespace WTF {
+    std::atomic_bool ConservativeScanStackGuards::active {true};
+    thread_local std::deque<void*>  ConservativeScanStackGuards::guard_ptr_stack;
+}

Source/WTF/wtf/ConservativeScanStackGuards.h

@@ -0,0 +1,65 @@
+#pragma once
+
+#include "Logging.h"
+
+#include <atomic>
+#include <deque>
+
+namespace WTF {
+    class ConservativeScanStackGuards {
+        public:
+            struct ConservativeScanStackGuard {
+                ConservativeScanStackGuard() {
+                    ConservativeScanStackGuards::setStackGuard(this);
+                }
+                ~ConservativeScanStackGuard() {
+                    ConservativeScanStackGuards::resetStackGuard(this);
+                }
+
+                private:
+                    // Prevent heap allocation
+                    void *operator new   (size_t);
+                    void *operator new[] (size_t);
+                    void  operator delete   (void *);
+                    void  operator delete[] (void*);
+            };
+
+            friend struct ConservativeScanStackGuard;
+
+            static void* updatedStackOrigin(void *stackTop, void *stackOrigin) {
+                if (!active.load() || guard_ptr_stack.empty()) {
+                    return stackOrigin;
+                }
+
+                void *ret = stackOrigin;
+
+                void *guard_ptr = guard_ptr_stack.back();
+
+                if (guard_ptr > stackTop && guard_ptr < stackOrigin) {
+                    WTFLogAlways("ConservativeScanStackGuards: guard IN RANGE: stackTop: %p guard: %p stackOrigin: %p; correcting stackOrigin\n", stackTop, guard_ptr, stackOrigin);
+                    ret = guard_ptr;
+                }
+                return ret;
+            }
+
+            static void setActive(bool act) {
+                active.store(act);
+            }
+
+        private:
+
+            static thread_local std::deque<void*> guard_ptr_stack;
+            static std::atomic_bool active;
+
+            static void setStackGuard(void *ptr) {
+                guard_ptr_stack.push_back(ptr);
+            }
+
+            static void resetStackGuard(void *ptr) {
+                if (ptr != guard_ptr_stack.back()) {
+                    WTFLogAlways("ConservativeScanStackGuards::resetStackGuard expected %p, had: %p", guard_ptr_stack.back(), ptr);
+                }
+                guard_ptr_stack.pop_back();
+            }
+    };
+}

Source/WTF/wtf/glib/RunLoopGLib.cpp

@@ -31,6 +31,8 @@
 #include <wtf/MainThread.h>
 #include <wtf/glib/RunLoopSourcePriority.h>
 
+#include "wtf/ConservativeScanStackGuards.h"
+
 namespace WTF {
 
 typedef struct {
@@ -103,6 +105,7 @@ void RunLoop::run()
     ASSERT(!runLoop.m_mainLoops.isEmpty());
 
     GMainLoop* innermostLoop = runLoop.m_mainLoops[0].get();
+    ConservativeScanStackGuards::ConservativeScanStackGuard guard;
     if (!g_main_loop_is_running(innermostLoop)) {
         g_main_context_push_thread_default(mainContext);
         g_main_loop_run(innermostLoop);

Source/WebKit/WebProcess/WebProcess.cpp

@@ -1042,6 +1042,10 @@ void WebProcess::isJITEnabled(CompletionHandler<void(bool)>&& completionHandler)
 
 void WebProcess::garbageCollectJavaScriptObjects()
 {
+    {
+        JSLockHolder lock(commonVM());
+        commonVM().shrinkFootprintWhenIdle();
+    }
     GCController::singleton().garbageCollectNow();
 }