Stop pre-emptively creating directories

Checking for directory existence is expensive and frequently indicates
success.  Instead of pre-emptively creating the directory containing a
file to be written, attempt to create the file and only do the directory
scaffolding if the write fails.

Appears to reduce file write time by 10-20% for a file-I/O heavy partner
build.

Thanks to @rbuckton for the suggestion!

Author: amcasey

src/compiler/program.ts

@@ -151,13 +151,16 @@ namespace ts {
         function writeFile(fileName: string, data: string, writeByteOrderMark: boolean, onError?: (message: string) => void) {
             try {
                 performance.mark("beforeIOWrite");
-                ensureDirectoriesExist(getDirectoryPath(normalizePath(fileName)));
 
-                if (isWatchSet(options) && system.createHash && system.getModifiedTime) {
-                    writeFileIfUpdated(fileName, data, writeByteOrderMark);
+                // PERF: Checking for directory existence is expensive.
+                // Instead, assume the directory exists and fall back
+                // to creating it if the file write fails.
+                try {
+                    writeFileWorker(fileName, data, writeByteOrderMark);
                 }
-                else {
-                    system.writeFile(fileName, data, writeByteOrderMark);
+                catch (_) {
+                    ensureDirectoriesExist(getDirectoryPath(normalizePath(fileName)));
+                    writeFileWorker(fileName, data, writeByteOrderMark);
                 }
 
                 performance.mark("afterIOWrite");
@@ -170,6 +173,15 @@ namespace ts {
             }
         }
 
+        function writeFileWorker(fileName: string, data: string, writeByteOrderMark: boolean) {
+            if (isWatchSet(options) && system.createHash && system.getModifiedTime) {
+                writeFileIfUpdated(fileName, data, writeByteOrderMark);
+            }
+            else {
+                system.writeFile(fileName, data, writeByteOrderMark);
+            }
+        }
+
         function getDefaultLibLocation(): string {
             return getDirectoryPath(normalizePath(system.getExecutingFilePath()));
         }

src/compiler/sys.ts

@@ -541,11 +541,20 @@ namespace ts {
         // patch writefile to create folder before writing the file
         const originalWriteFile = sys.writeFile;
         sys.writeFile = (path, data, writeBom) => {
-            const directoryPath = getDirectoryPath(normalizeSlashes(path));
-            if (directoryPath && !sys.directoryExists(directoryPath)) {
-                recursiveCreateDirectory(directoryPath, sys);
+            // PERF: Checking for directory existence is expensive.
+            // Instead, assume the directory exists and fall back
+            // to creating it if the file write fails.
+            try {
+                originalWriteFile.call(sys, path, data, writeBom);
+            }
+            catch (_) {
+                const directoryPath = getDirectoryPath(normalizeSlashes(path));
+                if (directoryPath && !sys.directoryExists(directoryPath)) {
+                    recursiveCreateDirectory(directoryPath, sys);
+                }
+
+                originalWriteFile.call(sys, path, data, writeBom);
             }
-            originalWriteFile.call(sys, path, data, writeBom);
         };
     }