ARRISEOS-45892 limit conservative scan range (#410)

* ARRISEOS-45892 limit conservative scan range

Turns out that conservative GC root scanning is
scanning too deep into the stack & this results
in GC being prevented (for more details see the
ticket & upstream issue:
WebPlatformForEmbedded#1363

This workaround introduces 'stack guards' that prevent
this in particular cases, that is - if there is gloop
main loop routine in the stack (see RunLoopGLib), the
GC root search will not go deeper that this frame. This
is enough to make GC work when invoked synchronously,
via VM::shrinkFootprintWhenIdle; therefore for the
workaround to work, wpe plugin needs to call that
(another part of the workaround is implemented in
WebKitBrowser plugin)

* ARRISEOS-45892 limit conservative scan range

Small correction: immediate garbageCollectNow is
still useful, since under heavy load shrinkFootprintWhenIdle
might be executed quite late.

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
@@ -423,6 +424,7 @@ 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();
 }