Bug 1645416 - Fix the TracePayload layout on 32bits and re-enable the static assert that check the struct has a size that is a power of two. r=achronop

Differential Revision: https://phabricator.services.mozilla.com/D80696

dom/media/AsyncLogger.h

@@ -51,21 +51,30 @@ class AsyncLogger {
     char mPayload[PAYLOAD_TOTAL_SIZE - MPSC_MSG_RESERVERD];
   };
 
+// On 32bits the struct is packed differently and there is a hole we need to
+// account for.
+#if !defined(HAVE_64BIT_BUILD) && !(defined(XP_LINUX) && !defined(MOZ_WIDGET_ANDROID))
+#  define PADDING 8
+#else
+#  define PADDING 0
+#endif
+
   // The order of the fields is important here to minimize padding.
   struct TracePayload {
-    // The thread on which this tracepoint was gathered.
-    int mTID;
-    // If this marker is of phase X (COMPLETE), this holds the duration of the
-    // event in microseconds. Else, the value is not used.
-    uint32_t mDurationUs;
     // If this marker is of phase B or E (begin or end), this is the time at
     // which it was captured.
     TimeStamp mTimestamp;
+    // If this marker is of phase X (COMPLETE), this holds the duration of the
+    // event in microseconds. Else, the value is not used.
+    uint32_t mDurationUs;
+    // The thread on which this tracepoint was gathered.
+    int mTID;
     // An arbitrary string, usually containing a function signature or a
     // recognizable tag of some sort, to be displayed when analyzing the
     // profile.
     char mName[PAYLOAD_TOTAL_SIZE - sizeof(TracingPhase) - sizeof(int) -
-               sizeof(uint32_t) - sizeof(TimeStamp) - MPSC_MSG_RESERVERD];
+               sizeof(uint32_t) - sizeof(TimeStamp) - MPSC_MSG_RESERVERD -
+               PADDING];
     // A trace payload can be either:
     // - Begin - this marks the beginning of a temporal region
     // - End - this marks the end of a temporal region
@@ -73,6 +82,7 @@ class AsyncLogger {
     // temporal region
     TracingPhase mPhase;
   };
+#undef PADDING
   // aLogModuleName is the name of the MOZ_LOG module.
   explicit AsyncLogger(const char* aLogModuleName,
                        AsyncLogger::AsyncLoggerOutputMode aMode =

dom/media/MPSCQueue.h

@@ -22,7 +22,7 @@ namespace mozilla {
 // allows having a simpler design, we count on the fact that jemalloc will get
 // the memory from a thread-local source most of the time.  We'll replace
 // this with a fixed-size ring buffer if this becomes an issue.
-const size_t MPSC_MSG_RESERVERD = sizeof(void*);
+const size_t MPSC_MSG_RESERVERD = sizeof(std::atomic<void*>);
 
 template <typename T>
 class MPSCQueue {
@@ -32,8 +32,8 @@ class MPSCQueue {
     Message(const Message& aMessage) = delete;
     void operator=(const Message& aMessage) = delete;
 
-    T data;
     std::atomic<Message*> mNext;
+    T data;
   };
 
   // The goal here is to make it easy on the allocator. We pack a pointer in the
@@ -43,9 +43,9 @@ class MPSCQueue {
   // making it cheap and, more importantly, lock-free enough. This has been
   // measured to be cheap and reliable enough, but will be replaced in the
   // longer run.
-#if !defined(XP_WIN) && !defined(MOZ_WIDGET_ANDROID)
+#if !(defined(ANDROID) && defined(__i386__))
   static_assert(IsPowerOfTwo(sizeof(MPSCQueue<T>::Message)),
-                "MPSCQueue internal allocations must have a size that is a"
+                "MPSCQueue internal allocations must have a size that is a "
                 "power of two ");
 #endif