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diff --git a/‎src/compiler/builder.ts
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+218-63Lines changed: 218 additions & 63 deletions b/‎src/compiler/builder.ts
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+218-63Lines changed: 218 additions & 63 deletions
diff --git a/‎src/compiler/builderState.ts
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+34-7Lines changed: 34 additions & 7 deletions b/‎src/compiler/builderState.ts
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+34-7Lines changed: 34 additions & 7 deletions
diff --git a/‎src/compiler/core.ts
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+12Lines changed: 12 additions & 0 deletions b/‎src/compiler/core.ts
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+12Lines changed: 12 additions & 0 deletions
diff --git a/‎src/compiler/diagnosticMessages.json
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+4Lines changed: 4 additions & 0 deletions b/‎src/compiler/diagnosticMessages.json
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+4Lines changed: 4 additions & 0 deletions
diff --git a/‎src/compiler/program.ts
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+56-39Lines changed: 56 additions & 39 deletions b/‎src/compiler/program.ts
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+56-39Lines changed: 56 additions & 39 deletions
diff --git a/‎src/compiler/sys.ts
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+10-9Lines changed: 10 additions & 9 deletions b/‎src/compiler/sys.ts
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+10-9Lines changed: 10 additions & 9 deletions
@@ -50,11 +50,15 @@ namespace ts {
         /**
          * Cache of all files excluding default library file for the current program
          */
-        allFilesExcludingDefaultLibraryFile: ReadonlyArray<SourceFile> | undefined;
+        allFilesExcludingDefaultLibraryFile?: ReadonlyArray<SourceFile>;
         /**
          * Cache of all the file names
          */
-        allFileNames: ReadonlyArray<string> | undefined;
+        allFileNames?: ReadonlyArray<string>;
+    }
+
+    export function cloneMapOrUndefined<T>(map: ReadonlyMap<T> | undefined) {
+        return map ? cloneMap(map) : undefined;
     }
 }
 
@@ -230,9 +234,32 @@ namespace ts.BuilderState {
             fileInfos,
             referencedMap,
             exportedModulesMap,
-            hasCalledUpdateShapeSignature,
-            allFilesExcludingDefaultLibraryFile: undefined,
-            allFileNames: undefined
+            hasCalledUpdateShapeSignature
+        };
+    }
+
+    /**
+     * Releases needed properties
+     */
+    export function releaseCache(state: BuilderState) {
+        state.allFilesExcludingDefaultLibraryFile = undefined;
+        state.allFileNames = undefined;
+    }
+
+    /**
+     * Creates a clone of the state
+     */
+    export function clone(state: Readonly<BuilderState>): BuilderState {
+        const fileInfos = createMap<FileInfo>();
+        state.fileInfos.forEach((value, key) => {
+            fileInfos.set(key, { ...value });
+        });
+        // Dont need to backup allFiles info since its cache anyway
+        return {
+            fileInfos,
+            referencedMap: cloneMapOrUndefined(state.referencedMap),
+            exportedModulesMap: cloneMapOrUndefined(state.exportedModulesMap),
+            hasCalledUpdateShapeSignature: cloneMap(state.hasCalledUpdateShapeSignature),
         };
     }
 
@@ -505,14 +532,14 @@ namespace ts.BuilderState {
 
         // Start with the paths this file was referenced by
         seenFileNamesMap.set(sourceFileWithUpdatedShape.path, sourceFileWithUpdatedShape);
-        const queue = getReferencedByPaths(state, sourceFileWithUpdatedShape.path);
+        const queue = getReferencedByPaths(state, sourceFileWithUpdatedShape.resolvedPath);
         while (queue.length > 0) {
             const currentPath = queue.pop()!;
             if (!seenFileNamesMap.has(currentPath)) {
                 const currentSourceFile = programOfThisState.getSourceFileByPath(currentPath)!;
                 seenFileNamesMap.set(currentPath, currentSourceFile);
                 if (currentSourceFile && updateShapeSignature(state, programOfThisState, currentSourceFile, cacheToUpdateSignature, cancellationToken, computeHash!, exportedModulesMapCache)) { // TODO: GH#18217
-                    queue.push(...getReferencedByPaths(state, currentPath));
+                    queue.push(...getReferencedByPaths(state, currentSourceFile.resolvedPath));
                 }
             }
         }
 
@@ -1387,6 +1387,18 @@ namespace ts {
         return result;
     }
 
+    export function copyProperties<T1 extends T2, T2>(first: T1, second: T2) {
+        for (const id in second) {
+            if (hasOwnProperty.call(second, id)) {
+                (first as any)[id] = second[id];
+            }
+        }
+    }
+
+    export function maybeBind<T, A extends any[], R>(obj: T, fn: ((this: T, ...args: A) => R) | undefined): ((...args: A) => R) | undefined {
+        return fn ? fn.bind(obj) : undefined;
+    }
+
     export interface MultiMap<T> extends Map<T[]> {
         /**
          * Adds the value to an array of values associated with the key, and returns the array.
 
@@ -3957,6 +3957,10 @@
         "category": "Error",
         "code": 6370
     },
+    "Updating unchanged output timestamps of project '{0}'...": {
+        "category": "Message",
+        "code": 6371
+    },
 
     "The expected type comes from property '{0}' which is declared here on type '{1}'": {
         "category": "Message",
 
@@ -69,6 +69,7 @@ namespace ts {
     export function createCompilerHost(options: CompilerOptions, setParentNodes?: boolean): CompilerHost {
         return createCompilerHostWorker(options, setParentNodes);
     }
+
     /*@internal*/
     // TODO(shkamat): update this after reworking ts build API
     export function createCompilerHostWorker(options: CompilerOptions, setParentNodes?: boolean, system = sys): CompilerHost {
@@ -93,7 +94,6 @@ namespace ts {
                 }
                 text = "";
             }
-
             return text !== undefined ? createSourceFile(fileName, text, languageVersion, setParentNodes) : undefined;
         }
 
@@ -203,18 +203,25 @@ namespace ts {
         return compilerHost;
     }
 
+    interface CompilerHostLikeForCache {
+        fileExists(fileName: string): boolean;
+        readFile(fileName: string, encoding?: string): string | undefined;
+        directoryExists?(directory: string): boolean;
+        createDirectory?(directory: string): void;
+        writeFile?: WriteFileCallback;
+    }
+
     /*@internal*/
-    export function changeCompilerHostToUseCache(
-        host: CompilerHost,
+    export function changeCompilerHostLikeToUseCache(
+        host: CompilerHostLikeForCache,
         toPath: (fileName: string) => Path,
-        useCacheForSourceFile: boolean
+        getSourceFile?: CompilerHost["getSourceFile"]
     ) {
         const originalReadFile = host.readFile;
         const originalFileExists = host.fileExists;
         const originalDirectoryExists = host.directoryExists;
         const originalCreateDirectory = host.createDirectory;
         const originalWriteFile = host.writeFile;
-        const originalGetSourceFile = host.getSourceFile;
         const readFileCache = createMap<string | false>();
         const fileExistsCache = createMap<boolean>();
         const directoryExistsCache = createMap<boolean>();
@@ -242,19 +249,17 @@ namespace ts {
             return setReadFileCache(key, fileName);
         };
 
-        if (useCacheForSourceFile) {
-            host.getSourceFile = (fileName, languageVersion, onError, shouldCreateNewSourceFile) => {
-                const key = toPath(fileName);
-                const value = sourceFileCache.get(key);
-                if (value) return value;
+        const getSourceFileWithCache: CompilerHost["getSourceFile"] | undefined = getSourceFile ? (fileName, languageVersion, onError, shouldCreateNewSourceFile) => {
+            const key = toPath(fileName);
+            const value = sourceFileCache.get(key);
+            if (value) return value;
 
-                const sourceFile = originalGetSourceFile.call(host, fileName, languageVersion, onError, shouldCreateNewSourceFile);
-                if (sourceFile && (isDeclarationFileName(fileName) || fileExtensionIs(fileName, Extension.Json))) {
-                    sourceFileCache.set(key, sourceFile);
-                }
-                return sourceFile;
-            };
-        }
+            const sourceFile = getSourceFile(fileName, languageVersion, onError, shouldCreateNewSourceFile);
+            if (sourceFile && (isDeclarationFileName(fileName) || fileExtensionIs(fileName, Extension.Json))) {
+                sourceFileCache.set(key, sourceFile);
+            }
+            return sourceFile;
+        } : undefined;
 
         // fileExists for any kind of extension
         host.fileExists = fileName => {
@@ -265,23 +270,25 @@ namespace ts {
             fileExistsCache.set(key, !!newValue);
             return newValue;
         };
-        host.writeFile = (fileName, data, writeByteOrderMark, onError, sourceFiles) => {
-            const key = toPath(fileName);
-            fileExistsCache.delete(key);
+        if (originalWriteFile) {
+            host.writeFile = (fileName, data, writeByteOrderMark, onError, sourceFiles) => {
+                const key = toPath(fileName);
+                fileExistsCache.delete(key);
 
-            const value = readFileCache.get(key);
-            if (value && value !== data) {
-                readFileCache.delete(key);
-                sourceFileCache.delete(key);
-            }
-            else if (useCacheForSourceFile) {
-                const sourceFile = sourceFileCache.get(key);
-                if (sourceFile && sourceFile.text !== data) {
+                const value = readFileCache.get(key);
+                if (value && value !== data) {
+                    readFileCache.delete(key);
                     sourceFileCache.delete(key);
                 }
-            }
-            originalWriteFile.call(host, fileName, data, writeByteOrderMark, onError, sourceFiles);
-        };
+                else if (getSourceFileWithCache) {
+                    const sourceFile = sourceFileCache.get(key);
+                    if (sourceFile && sourceFile.text !== data) {
+                        sourceFileCache.delete(key);
+                    }
+                }
+                originalWriteFile.call(host, fileName, data, writeByteOrderMark, onError, sourceFiles);
+            };
+        }
 
         // directoryExists
         if (originalDirectoryExists && originalCreateDirectory) {
@@ -306,7 +313,7 @@ namespace ts {
             originalDirectoryExists,
             originalCreateDirectory,
             originalWriteFile,
-            originalGetSourceFile,
+            getSourceFileWithCache,
             readFileWithCache
         };
     }
@@ -735,7 +742,7 @@ namespace ts {
         performance.mark("beforeProgram");
 
         const host = createProgramOptions.host || createCompilerHost(options);
-        const configParsingHost = parseConfigHostFromCompilerHost(host);
+        const configParsingHost = parseConfigHostFromCompilerHostLike(host);
 
         let skipDefaultLib = options.noLib;
         const getDefaultLibraryFileName = memoize(() => host.getDefaultLibFileName(options));
@@ -3104,18 +3111,28 @@ namespace ts {
         }
     }
 
+    interface CompilerHostLike {
+        useCaseSensitiveFileNames(): boolean;
+        getCurrentDirectory(): string;
+        fileExists(fileName: string): boolean;
+        readFile(fileName: string): string | undefined;
+        readDirectory?(rootDir: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string> | undefined, includes: ReadonlyArray<string>, depth?: number): string[];
+        trace?(s: string): void;
+        onUnRecoverableConfigFileDiagnostic?: DiagnosticReporter;
+    }
+
     /* @internal */
-    export function parseConfigHostFromCompilerHost(host: CompilerHost): ParseConfigFileHost {
+    export function parseConfigHostFromCompilerHostLike(host: CompilerHostLike, directoryStructureHost: DirectoryStructureHost = host): ParseConfigFileHost {
         return {
-            fileExists: f => host.fileExists(f),
+            fileExists: f => directoryStructureHost.fileExists(f),
             readDirectory(root, extensions, excludes, includes, depth) {
-                Debug.assertDefined(host.readDirectory, "'CompilerHost.readDirectory' must be implemented to correctly process 'projectReferences'");
-                return host.readDirectory!(root, extensions, excludes, includes, depth);
+                Debug.assertDefined(directoryStructureHost.readDirectory, "'CompilerHost.readDirectory' must be implemented to correctly process 'projectReferences'");
+                return directoryStructureHost.readDirectory!(root, extensions, excludes, includes, depth);
             },
-            readFile: f => host.readFile(f),
+            readFile: f => directoryStructureHost.readFile(f),
             useCaseSensitiveFileNames: host.useCaseSensitiveFileNames(),
             getCurrentDirectory: () => host.getCurrentDirectory(),
-            onUnRecoverableConfigFileDiagnostic: () => undefined,
+            onUnRecoverableConfigFileDiagnostic: host.onUnRecoverableConfigFileDiagnostic || (() => undefined),
             trace: host.trace ? (s) => host.trace!(s) : undefined
         };
     }
 
@@ -2,6 +2,16 @@ declare function setTimeout(handler: (...args: any[]) => void, timeout: number):
 declare function clearTimeout(handle: any): void;
 
 namespace ts {
+    /**
+     * djb2 hashing algorithm
+     * https://www.cse.yorku.ca/~oz/hash.html
+     */
+    /* @internal */
+    export function generateDjb2Hash(data: string): string {
+        const chars = data.split("").map(str => str.charCodeAt(0));
+        return `${chars.reduce((prev, curr) => ((prev << 5) + prev) + curr, 5381)}`;
+    }
+
     /**
      * Set a high stack trace limit to provide more information in case of an error.
      * Called for command-line and server use cases.
@@ -1115,15 +1125,6 @@ namespace ts {
                 }
             }
 
-            /**
-             * djb2 hashing algorithm
-             * https://www.cse.yorku.ca/~oz/hash.html
-             */
-            function generateDjb2Hash(data: string): string {
-                const chars = data.split("").map(str => str.charCodeAt(0));
-                return `${chars.reduce((prev, curr) => ((prev << 5) + prev) + curr, 5381)}`;
-            }
-
             function createMD5HashUsingNativeCrypto(data: string): string {
                 const hash = _crypto!.createHash("md5");
                 hash.update(data);