diff --git a/compiler-rt/lib/tsan/rtl/tsan_trace.h b/compiler-rt/lib/tsan/rtl/tsan_trace.h
--- a/compiler-rt/lib/tsan/rtl/tsan_trace.h
+++ b/compiler-rt/lib/tsan/rtl/tsan_trace.h
@@ -99,6 +99,8 @@
 // close enough to each other. Otherwise we fall back to EventAccessExt.
 struct EventAccess {
   static constexpr uptr kPCBits = 15;
+  static_assert(kPCBits + kCompressedAddrBits + 5 == 64,
+                "unused bits in EventAccess");
 
   u64 is_access : 1;  // = 1
   u64 is_read : 1;
@@ -119,13 +121,23 @@
 
 // Extended memory access with full PC.
 struct EventAccessExt {
+  // Note: precisely specifying the unused parts of the bitfield is critical for
+  // performance. If we don't specify them, compiler will generate code to load
+  // the old value and shuffle it to extract the unused bits to apply to the new
+  // value. If we specify the unused part and store 0 in there, all that
+  // unnecessary code goes away (store of the 0 const is combined with other
+  // constant parts).
+  static constexpr uptr kUnusedBits = 11;
+  static_assert(kCompressedAddrBits + kUnusedBits + 9 == 64,
+                "unused bits in EventAccessExt");
+
   u64 is_access : 1;   // = 0
   u64 is_func : 1;     // = 0
   EventType type : 3;  // = EventType::kAccessExt
   u64 is_read : 1;
   u64 is_atomic : 1;
   u64 size_log : 2;
-  u64 _ : 11;
+  u64 _ : kUnusedBits;
   u64 addr : kCompressedAddrBits;
   u64 pc;
 };
@@ -134,6 +146,8 @@
 // Access to a memory range.
 struct EventAccessRange {
   static constexpr uptr kSizeLoBits = 13;
+  static_assert(kCompressedAddrBits + kSizeLoBits + 7 == 64,
+                "unused bits in EventAccessRange");
 
   u64 is_access : 1;   // = 0
   u64 is_func : 1;     // = 0
@@ -150,6 +164,13 @@
 // Mutex lock.
 struct EventLock {
   static constexpr uptr kStackIDLoBits = 15;
+  static constexpr uptr kStackIDHiBits =
+      sizeof(StackID) * kByteBits - kStackIDLoBits;
+  static constexpr uptr kUnusedBits = 3;
+  static_assert(kCompressedAddrBits + kStackIDLoBits + 5 == 64,
+                "unused bits in EventLock");
+  static_assert(kCompressedAddrBits + kStackIDHiBits + kUnusedBits == 64,
+                "unused bits in EventLock");
 
   u64 is_access : 1;   // = 0
   u64 is_func : 1;     // = 0
@@ -157,29 +178,37 @@
   u64 pc : kCompressedAddrBits;
   u64 stack_lo : kStackIDLoBits;
   u64 stack_hi : sizeof(StackID) * kByteBits - kStackIDLoBits;
-  u64 _ : 3;
+  u64 _ : kUnusedBits;
   u64 addr : kCompressedAddrBits;
 };
 static_assert(sizeof(EventLock) == 16, "bad EventLock size");
 
 // Mutex unlock.
 struct EventUnlock {
+  static constexpr uptr kUnusedBits = 15;
+  static_assert(kCompressedAddrBits + kUnusedBits + 5 == 64,
+                "unused bits in EventUnlock");
+
   u64 is_access : 1;   // = 0
   u64 is_func : 1;     // = 0
   EventType type : 3;  // = EventType::kUnlock
-  u64 _ : 15;
+  u64 _ : kUnusedBits;
   u64 addr : kCompressedAddrBits;
 };
 static_assert(sizeof(EventUnlock) == 8, "bad EventUnlock size");
 
 // Time change event.
 struct EventTime {
+  static constexpr uptr kUnusedBits = 37;
+  static_assert(kUnusedBits + sizeof(Sid) * kByteBits + kEpochBits + 5 == 64,
+                "unused bits in EventTime");
+
   u64 is_access : 1;   // = 0
   u64 is_func : 1;     // = 0
   EventType type : 3;  // = EventType::kTime
   u64 sid : sizeof(Sid) * kByteBits;
   u64 epoch : kEpochBits;
-  u64 _ : 64 - 5 - sizeof(Sid) * kByteBits - kEpochBits;
+  u64 _ : kUnusedBits;
 };
 static_assert(sizeof(EventTime) == 8, "bad EventTime size");