Bug 1558440 - Use SaturateUint32 for the visible content heuristic. r=bzbarsky

Turns out we do have saturated arithmetic in mfbt, I just missed it.

Also, use just an uint32 for the heuristic. Text length is a uint32 anyway, and
it's unlikely we want to decide anything when the value is over the max uint32
value.

Differential Revision: https://phabricator.services.mozilla.com/D34496

--HG--
extra : moz-landing-system : lando

dom/base/Document.h

@@ -9,6 +9,7 @@
 #include "mozilla/EventStates.h"  // for EventStates
 #include "mozilla/FlushType.h"    // for enum
 #include "mozilla/Pair.h"         // for Pair
+#include "mozilla/Saturate.h"     // for SaturateUint32
 #include "nsAutoPtr.h"            // for member
 #include "nsCOMArray.h"           // for member
 #include "nsCompatibility.h"      // for member
@@ -3699,15 +3700,13 @@ class Document : public nsINode,
 
   void PropagateUseCounters(Document* aParentDocument);
 
-  void AddToVisibleContentHeuristic(size_t aNumber) {
-    if (MOZ_UNLIKELY(SIZE_MAX - mVisibleContentHeuristic < aNumber)) {
-      mVisibleContentHeuristic = SIZE_MAX;
-    } else {
-      mVisibleContentHeuristic += aNumber;
-    }
+  void AddToVisibleContentHeuristic(uint32_t aNumber) {
+    mVisibleContentHeuristic += aNumber;
   }
 
-  size_t GetVisibleContentHeuristic() const { return mVisibleContentHeuristic; }
+  uint32_t GetVisibleContentHeuristic() const {
+    return mVisibleContentHeuristic.value();
+  }
 
   // Called to track whether this document has had any interaction.
   // This is used to track whether we should permit "beforeunload".
@@ -4913,7 +4912,7 @@ class Document : public nsINode,
   // <style>.
   //
   // Note that this is only measured during load.
-  size_t mVisibleContentHeuristic = 0;
+  SaturateUint32 mVisibleContentHeuristic{0};
 
   // Whether the user has interacted with the document or not:
   bool mUserHasInteracted;

mfbt/Saturate.h

@@ -114,7 +114,7 @@ class SaturateOp {
 
 /**
  * |Saturate<T>| is a value type for saturation arithmetics. It's
- * build on top of |SaturateOp<T>|.
+ * built on top of |SaturateOp<T>|.
  */
 template <typename T>
 class Saturate {