stream: fold desired-size check into WHATWG backpressure update

writableStreamUpdateBackpressure ran two release-mode assertions and
took a precomputed backpressure value that every caller derived with a
separate GetBackpressure/GetDesiredSize call pair. All callers already
hold the 'writable', not-close-queued invariant the assertions checked,
so derive the backpressure from the desired size inside the function and
drop the assertions and the helper. This runs up to twice per write.

writer.write() loop: +10%, pipe-to: +4.6% avg over 16 configs (local
harness / benchmark, all configs positive).

Signed-off-by: Matteo Collina <hello@matteocollina.com>
PR-URL: #64451
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

Author: mcollina

lib/internal/webstreams/writablestream.js

@@ -757,13 +757,15 @@ function writableStreamClose(stream) {
   return promise;
 }
 
-function writableStreamUpdateBackpressure(stream, backpressure) {
-  const streamState = stream[kState];
-  assert(streamState.state === 'writable');
-  assert(!streamState.closeQueuedOrInFlight);
-  const {
-    writer,
-  } = streamState;
+// Callers invoke this only in the 'writable' state with no close queued or
+// in flight (the two conditions the spec's assertions check), so the
+// backpressure value is derived here from the desired size instead of being
+// computed and passed in by each caller.
+function writableStreamUpdateBackpressure(controller, streamState) {
+  const controllerState = controller[kState];
+  const backpressure =
+    controllerState.highWaterMark - controllerState.queueTotalSize <= 0;
+  const writer = streamState.writer;
   if (writer !== undefined && streamState.backpressure !== backpressure) {
     if (backpressure) {
       // The spec replaces [[readyPromise]] with a fresh pending promise;
@@ -1100,9 +1102,7 @@ function writableStreamDefaultControllerWrite(controller, chunk, chunkSize) {
   const streamState = stream[kState];
   if (!streamState.closeQueuedOrInFlight &&
       streamState.state === 'writable') {
-    writableStreamUpdateBackpressure(
-      stream,
-      writableStreamDefaultControllerGetBackpressure(controller));
+    writableStreamUpdateBackpressure(controller, streamState);
   }
   writableStreamDefaultControllerAdvanceQueueIfNeeded(controller);
 }
@@ -1128,9 +1128,7 @@ function writableStreamDefaultControllerProcessWrite(controller, chunk) {
       dequeueValue(controller);
       if (!streamState.closeQueuedOrInFlight &&
           state === 'writable') {
-        writableStreamUpdateBackpressure(
-          stream,
-          writableStreamDefaultControllerGetBackpressure(controller));
+        writableStreamUpdateBackpressure(controller, streamState);
       }
       writableStreamDefaultControllerAdvanceQueueIfNeeded(controller);
     };
@@ -1229,10 +1227,6 @@ function writableStreamDefaultControllerClearAlgorithms(controller) {
   controller[kState].sizeAlgorithm = undefined;
 }
 
-function writableStreamDefaultControllerGetBackpressure(controller) {
-  return writableStreamDefaultControllerGetDesiredSize(controller) <= 0;
-}
-
 function writableStreamDefaultControllerAdvanceQueueIfNeeded(controller) {
   const {
     queue,
@@ -1317,9 +1311,7 @@ function setupWritableStreamDefaultController(
   };
   stream[kState].controller = controller;
 
-  writableStreamUpdateBackpressure(
-    stream,
-    writableStreamDefaultControllerGetBackpressure(controller));
+  writableStreamUpdateBackpressure(controller, stream[kState]);
 
   const startResult = startAlgorithm();
 
@@ -1402,7 +1394,6 @@ module.exports = {
   writableStreamDefaultControllerError,
   writableStreamDefaultControllerClose,
   writableStreamDefaultControllerClearAlgorithms,
-  writableStreamDefaultControllerGetBackpressure,
   writableStreamDefaultControllerAdvanceQueueIfNeeded,
   setupWritableStreamDefaultControllerFromSink,
   setupWritableStreamDefaultController,