stream: group bool fields at tail of clientStream to reduce allocator size class 288B→256B (grpc#9281)

Author: gidotencate

stream.go

@@ -689,30 +689,17 @@ type clientStream struct {
 
 	cancel context.CancelFunc // cancels all attempts
 
-	sentLast bool // sent an end stream
-
-	receivedFirstMsg bool // set after the first message is received
-
 	methodConfig *MethodConfig
 
 	ctx context.Context // the application's context, wrapped by stats/tracing
 
 	retryThrottler *retryThrottler // The throttler active when the RPC began.
 
 	binlogs []binarylog.MethodLogger
-	// serverHeaderBinlogged is a boolean for whether server header has been
-	// logged. Server header will be logged when the first time one of those
-	// happens: stream.Header(), stream.Recv().
-	//
-	// It's only read and used by Recv() and Header(), so it doesn't need to be
-	// synchronized.
-	serverHeaderBinlogged bool
 
 	mu                      sync.Mutex
-	firstAttempt            bool // if true, transparent retry is valid
-	numRetries              int  // exclusive of transparent retry attempt(s)
-	numRetriesSincePushback int  // retries since pushback; to reset backoff
-	finished                bool // TODO: replace with atomic cmpxchg or sync.Once?
+	numRetries              int // exclusive of transparent retry attempt(s)
+	numRetriesSincePushback int // retries since pushback; to reset backoff
 	// attempt is the active client stream attempt.
 	// The only place where it is written is the newAttemptLocked method and this method never writes nil.
 	// So, attempt can be nil only inside newClientStream function when clientStream is first created.
@@ -722,13 +709,33 @@ type clientStream struct {
 	// place where we need to check if the attempt is nil.
 	attempt *csAttempt
 	// TODO(hedging): hedging will have multiple attempts simultaneously.
-	committed        bool // active attempt committed for retry?
 	onCommit         func()
 	replayBuffer     []replayOp // operations to replay on retry
 	replayBufferSize int        // current size of replayBuffer
+
+	// Bool fields are grouped at the tail to eliminate the alignment padding
+	// that would otherwise follow each bool when the next field is pointer- or
+	// int-sized. See https://github.com/grpc/grpc-go/issues/9280 for benchmarks.
+	// Add new bool fields here, not inline above.
+
+	// Not guarded by mu.
+	sentLast         bool // sent an end stream
+	receivedFirstMsg bool // set after the first message is received
+	// serverHeaderBinlogged is a boolean for whether server header has been
+	// logged. Server header will be logged when the first time one of those
+	// happens: stream.Header(), stream.Recv().
+	//
+	// It's only read and used by Recv() and Header(), so it doesn't need to be
+	// synchronized.
+	serverHeaderBinlogged bool
 	// nameResolutionDelay indicates if there was a delay in the name resolution.
 	// This field is only valid on client side, it's always false on server side.
 	nameResolutionDelay bool
+
+	// Guarded by mu.
+	firstAttempt bool // if true, transparent retry is valid
+	finished     bool // TODO: replace with atomic cmpxchg or sync.Once?
+	committed    bool // active attempt committed for retry?
 }
 
 type replayOp struct {