Simplify StackFrameDepth's handling of stack limits.

third_party/WebKit/Source/platform/heap/StackFrameDepth.h

Index: third_party/WebKit/Source/platform/heap/StackFrameDepth.h
diff --git a/third_party/WebKit/Source/platform/heap/StackFrameDepth.h b/third_party/WebKit/Source/platform/heap/StackFrameDepth.h
index 05cd6c6e58ce53527882bdc7d3606acd4e4f2db0..94379aa71692505ddc0cfdd4a82680c1f28216cf 100644
--- a/third_party/WebKit/Source/platform/heap/StackFrameDepth.h
+++ b/third_party/WebKit/Source/platform/heap/StackFrameDepth.h
@@ -14,15 +14,16 @@
 namespace blink {
 
 // StackFrameDepth keeps track of current call stack frame depth.
-// Use isSafeToRecurse() to query if there is a room in current
-// call stack for more recursive call.
+// It is specifically used to control stack usage while tracing
+// the object graph during a GC.
+//
+// Use isSafeToRecurse() to determine if it is safe to consume
+// more stack by invoking another recursive call.
 class PLATFORM_EXPORT StackFrameDepth final {
     STATIC_ONLY(StackFrameDepth);
 public:
     inline static bool isSafeToRecurse()
     {
-        ASSERT(s_stackFrameLimit || !s_isEnabled);
-
         // Asssume that the stack grows towards lower addresses, which
         // all the ABIs currently supported do.
         //
@@ -34,27 +35,18 @@ public:
     static void enableStackLimit();
     static void disableStackLimit()
     {
-        s_stackFrameLimit = 0;
-#if ENABLE(ASSERT)
-        s_isEnabled = false;
-#endif
+        s_stackFrameLimit = kMinimumStackLimit;
     }
 
 #if ENABLE(ASSERT)
-    inline static bool isEnabled() { return s_isEnabled; }
+    inline static bool isEnabled() { return s_stackFrameLimit != kMinimumStackLimit; }
     inline static bool isAcceptableStackUse()
     {
-        // ASan adds extra stack usage leading to too noisy asserts.
 #if defined(ADDRESS_SANITIZER)
+        // ASan adds extra stack usage leading to too noisy asserts.
         return true;
 #else
-        // If a conservative fallback stack size is in effect, use
-        // a larger stack limit so as to avoid false positives.
-        if (!s_isEnabled || isSafeToRecurse())
-            return true;
-        if (s_isUsingFallbackStackSize)
-            return (s_stackFrameLimit - currentStackFrame()) < 3 * kSafeStackFrameSize;
-        return false;
+        return !isEnabled() || isSafeToRecurse();
 #endif
     }
 #endif
@@ -85,16 +77,19 @@ public:
 
 private:
     // The maximum depth of eager, unrolled trace() calls that is
-    // considered safe and allowed.
+    // considered safe and allowed for targets with an unknown
+    // thread stack size.
     static const int kSafeStackFrameSize = 32 * 1024;
 
+    // The stack pointer is assumed to grow towards lower addresses;
+    // |kMinimumStackLimit| then being the limit that a stack
+    // pointer will always exceed.
+    static const uintptr_t kMinimumStackLimit = ~0ul;
+
     static uintptr_t getFallbackStackLimit();
 
+    // The (pointer-valued) stack limit.
     static uintptr_t s_stackFrameLimit;
-#if ENABLE(ASSERT)
-    static bool s_isEnabled;
-    static bool s_isUsingFallbackStackSize;
-#endif
 };
 
 class StackFrameDepthScope {