tsan: de-hardcode number of unused bits in trace events

[dvyukov]

Summary

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).

I don't see a good way to ensure we cover all of u64 bits with fields.
So at least introduce named kUnusedBits consts and check that bits
sum up to 64.

Depends on D113978.

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

 
 // Nop event used as padding and does not affect state during replay.
 static constexpr Event NopEvent = {1, 0, EventType::kAccessExt, 0};
 
 // Compressed memory access can represent only some events with PCs
 // 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;
   u64 is_atomic : 1;
   u64 size_log : 2;
   u64 pc_delta : kPCBits;  // signed delta from the previous memory access PC
   u64 addr : kCompressedAddrBits;
 };
 static_assert(sizeof(EventAccess) == 8, "bad EventAccess size");
 
 // Function entry (pc != 0) or exit (pc == 0).
 struct EventFunc {
   u64 is_access : 1;  // = 0
   u64 is_func : 1;    // = 1
   u64 pc : 62;
 };
 static_assert(sizeof(EventFunc) == 8, "bad EventFunc size");
 
 // 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;
 };
 static_assert(sizeof(EventAccessExt) == 16, "bad EventAccessExt size");
 
 // 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
   EventType type : 3;  // = EventType::kAccessRange
   u64 is_read : 1;
   u64 is_free : 1;
   u64 size_lo : kSizeLoBits;
   u64 pc : kCompressedAddrBits;
   u64 addr : kCompressedAddrBits;
   u64 size_hi : 64 - kCompressedAddrBits;
 };
 static_assert(sizeof(EventAccessRange) == 16, "bad EventAccessRange size");
 
 // 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
   EventType type : 3;  // = EventType::kLock or EventType::kRLock
   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");
 
 struct Trace;
 
 struct TraceHeader {
   Trace* trace = nullptr;  // back-pointer to Trace containing this part
   INode trace_parts;       // in Trace::parts