tsan: minor MetaMap tweaks

1. Add some comments.
2. Use kInvalidStackID instead of literal 0.
3. Add more LIKELY/UNLIKELY.

Reviewed By: vitalybuka, melver

Differential Revision: https://reviews.llvm.org/D107371

Author: dvyukov

compiler-rt/lib/tsan/rtl/tsan_sync.cpp

@@ -26,7 +26,7 @@ void SyncVar::Init(ThreadState *thr, uptr pc, uptr addr, u64 uid,
   this->uid = uid;
   this->next = 0;
 
-  creation_stack_id = 0;
+  creation_stack_id = kInvalidStackID;
   if (save_stack && !SANITIZER_GO)  // Go does not use them
     creation_stack_id = CurrentStackId(thr, pc);
   if (common_flags()->detect_deadlocks)
@@ -35,7 +35,7 @@ void SyncVar::Init(ThreadState *thr, uptr pc, uptr addr, u64 uid,
 
 void SyncVar::Reset(Processor *proc) {
   uid = 0;
-  creation_stack_id = 0;
+  creation_stack_id = kInvalidStackID;
   owner_tid = kInvalidTid;
   last_lock = 0;
   recursion = 0;
@@ -212,12 +212,12 @@ SyncVar *MetaMap::GetSync(ThreadState *thr, uptr pc, uptr addr, bool create,
     }
     if (!create)
       return nullptr;
-    if (*meta != idx0) {
+    if (UNLIKELY(*meta != idx0)) {
       idx0 = *meta;
       continue;
     }
 
-    if (myidx == 0) {
+    if (LIKELY(myidx == 0)) {
       const u64 uid = atomic_fetch_add(&uid_gen_, 1, memory_order_relaxed);
       myidx = sync_alloc_.Alloc(&thr->proc()->sync_cache);
       mys = sync_alloc_.Map(myidx);

compiler-rt/lib/tsan/rtl/tsan_sync.h

@@ -46,6 +46,8 @@ enum MutexFlags {
                                  MutexFlagNotStatic,
 };
 
+// SyncVar is a descriptor of a user synchronization object
+// (mutex or an atomic variable).
 struct SyncVar {
   SyncVar();
 
@@ -101,10 +103,8 @@ struct SyncVar {
   }
 };
 
-/* MetaMap allows to map arbitrary user pointers onto various descriptors.
-   Currently it maps pointers to heap block descriptors and sync var descs.
-   It uses 1/2 direct shadow, see tsan_platform.h.
-*/
+// MetaMap maps app addresses to heap block (MBlock) and sync var (SyncVar)
+// descriptors. It uses 1/2 direct shadow, see tsan_platform.h for the mapping.
 class MetaMap {
  public:
   MetaMap();