module: avoid allocating a cache key string for every require()

The relative resolve cache was keyed by a concatenated
${parent.path}\x00${request} string, allocating a new key for every
require() call including fully cached ones. Key the cache by the
parent directory first (a Map keyed by the already-retained
module.path string) and then by the request (a dictionary object,
whose property access internalizes dynamically-constructed
specifiers). Faster on every measured workload shape and slightly
smaller in memory, since the concatenated keys are no longer
retained.

Signed-off-by: Sam Attard <sattard@anthropic.com>

Author: MarshallOfSound

lib/internal/modules/cjs/loader.js

@@ -223,7 +223,7 @@ let { startTimer, endTimer } = debugWithTimer('module_timer', (start, end) => {
 const { tracingChannel } = require('diagnostics_channel');
 const onRequire = getLazy(() => tracingChannel('module.require'));
 
-const relativeResolveCache = { __proto__: null };
+const relativeResolveCache = new SafeMap();
 
 let requireDepth = 0;
 let isPreloading = false;
@@ -1273,17 +1273,23 @@ function loadBuiltinWithHooks(id, url, format) {
  * @returns {object}
  */
 Module._load = function(request, parent, isMain, internalOptions = kEmptyObject) {
-  let relResolveCacheIdentifier;
+  let relResolveCacheByDir;
   if (parent) {
     debug('Module._load REQUEST %s parent: %s', request, parent.id);
-    // Fast path for (lazy loaded) modules in the same directory. The indirect
-    // caching is required to allow cache invalidation without changing the old
-    // cache key names.
-    relResolveCacheIdentifier = `${parent.path}\x00${request}`;
-    const filename = relativeResolveCache[relResolveCacheIdentifier];
-    reportModuleToWatchMode(filename);
-    reportModuleToWatchModeFromWorker(filename);
+    // Fast path for (lazy loaded) modules in the same directory. Keyed by
+    // parent directory and then request, so no concatenated cache key
+    // string is allocated per require() call.
+    relResolveCacheByDir = relativeResolveCache.get(parent.path);
+    if (relResolveCacheByDir === undefined) {
+      // A plain object handles dynamically built specifier strings
+      // better than a Map here.
+      relResolveCacheByDir = { __proto__: null };
+      relativeResolveCache.set(parent.path, relResolveCacheByDir);
+    }
+    const filename = relResolveCacheByDir[request];
     if (filename !== undefined) {
+      reportModuleToWatchMode(filename);
+      reportModuleToWatchModeFromWorker(filename);
       const cachedModule = Module._cache[filename];
       if (cachedModule !== undefined) {
         updateChildren(parent, cachedModule, true);
@@ -1292,7 +1298,7 @@ Module._load = function(request, parent, isMain, internalOptions = kEmptyObject)
         }
         return cachedModule.exports;
       }
-      delete relativeResolveCache[relResolveCacheIdentifier];
+      delete relResolveCacheByDir[request];
     }
   }
 
@@ -1395,8 +1401,8 @@ Module._load = function(request, parent, isMain, internalOptions = kEmptyObject)
     module[kFormat] ??= format;
   }
 
-  if (parent !== undefined) {
-    relativeResolveCache[relResolveCacheIdentifier] = filename;
+  if (relResolveCacheByDir !== undefined) {
+    relResolveCacheByDir[request] = filename;
   }
 
   let threw = true;
@@ -1407,7 +1413,9 @@ Module._load = function(request, parent, isMain, internalOptions = kEmptyObject)
     if (threw) {
       delete Module._cache[filename];
       if (parent !== undefined) {
-        delete relativeResolveCache[relResolveCacheIdentifier];
+        if (relResolveCacheByDir !== undefined) {
+          delete relResolveCacheByDir[request];
+        }
         const children = parent?.children;
         if (ArrayIsArray(children)) {
           const index = ArrayPrototypeIndexOf(children, module);