[ThinLTO][NFC] Refactor FileCache (llvm#110463)

This is a prep for llvm#90933.
 
  - Change `FileCache` from a function to a type.
  - Store the cache directory in the type, which will be used when creating additional caches for two-codegen round runs that inherit this value.

llvm/include/llvm/LTO/LTO.h

@@ -298,7 +298,7 @@ class LTO {
   ///
   /// The client will receive at most one callback (via either AddStream or
   /// Cache) for each task identifier.
-  Error run(AddStreamFn AddStream, FileCache Cache = nullptr);
+  Error run(AddStreamFn AddStream, FileCache Cache = {});
 
   /// Static method that returns a list of libcall symbols that can be generated
   /// by LTO but might not be visible from bitcode symbol table.

llvm/include/llvm/Support/Caching.h

@@ -41,21 +41,48 @@ class CachedFileStream {
 using AddStreamFn = std::function<Expected<std::unique_ptr<CachedFileStream>>(
     unsigned Task, const Twine &ModuleName)>;
 
-/// This is the type of a file cache. To request an item from the cache, pass a
-/// unique string as the Key. For hits, the cached file will be added to the
-/// link and this function will return AddStreamFn(). For misses, the cache will
-/// return a stream callback which must be called at most once to produce
-/// content for the stream. The file stream produced by the stream callback will
-/// add the file to the link after the stream is written to. ModuleName is the
-/// unique module identifier for the bitcode module the cache is being checked
-/// for.
+/// This is a callable that manages file caching operations. It accepts a task
+/// ID \p Task, a unique key \p Key, and a module name \p ModuleName, and
+/// returns AddStreamFn(). This function determines whether a cache hit or miss
+/// occurs and handles the appropriate actions.
+using FileCacheFunction = std::function<Expected<AddStreamFn>(
+    unsigned Task, StringRef Key, const Twine &ModuleName)>;
+
+/// This type represents a file cache system that manages caching of files.
+/// It encapsulates a caching function and the directory path where the cache is
+/// stored. To request an item from the cache, pass a unique string as the Key.
+/// For hits, the cached file will be added to the link and this function will
+/// return AddStreamFn(). For misses, the cache will return a stream callback
+/// which must be called at most once to produce content for the stream. The
+/// file stream produced by the stream callback will add the file to the link
+/// after the stream is written to. ModuleName is the unique module identifier
+/// for the bitcode module the cache is being checked for.
 ///
 /// Clients generally look like this:
 ///
 /// if (AddStreamFn AddStream = Cache(Task, Key, ModuleName))
 ///   ProduceContent(AddStream);
-using FileCache = std::function<Expected<AddStreamFn>(
-    unsigned Task, StringRef Key, const Twine &ModuleName)>;
+///
+/// CacheDirectoryPath stores the directory path where cached files are kept.
+struct FileCache {
+  FileCache(FileCacheFunction CacheFn, const std::string &DirectoryPath)
+      : CacheFunction(std::move(CacheFn)), CacheDirectoryPath(DirectoryPath) {}
+  FileCache() = default;
+
+  Expected<AddStreamFn> operator()(unsigned Task, StringRef Key,
+                                   const Twine &ModuleName) {
+    assert(isValid() && "Invalid cache function");
+    return CacheFunction(Task, Key, ModuleName);
+  }
+  const std::string &getCacheDirectoryPath() const {
+    return CacheDirectoryPath;
+  }
+  bool isValid() const { return static_cast<bool>(CacheFunction); }
+
+private:
+  FileCacheFunction CacheFunction = nullptr;
+  std::string CacheDirectoryPath;
+};
 
 /// This type defines the callback to add a pre-existing file (e.g. in a cache).
 ///

llvm/lib/LTO/LTO.cpp

@@ -1484,7 +1484,7 @@ class InProcessThinBackend : public ThinBackendProc {
         return E;
     }
 
-    if (!Cache || !CombinedIndex.modulePaths().count(ModuleID) ||
+    if (!Cache.isValid() || !CombinedIndex.modulePaths().count(ModuleID) ||
         all_of(CombinedIndex.getModuleHash(ModuleID),
                [](uint32_t V) { return V == 0; }))
       // Cache disabled or no entry for this module in the combined index or

llvm/lib/Support/Caching.cpp

@@ -37,8 +37,8 @@ Expected<FileCache> llvm::localCache(const Twine &CacheNameRef,
   TempFilePrefixRef.toVector(TempFilePrefix);
   CacheDirectoryPathRef.toVector(CacheDirectoryPath);
 
-  return [=](unsigned Task, StringRef Key,
-             const Twine &ModuleName) -> Expected<AddStreamFn> {
+  auto Func = [=](unsigned Task, StringRef Key,
+                  const Twine &ModuleName) -> Expected<AddStreamFn> {
     // This choice of file name allows the cache to be pruned (see pruneCache()
     // in include/llvm/Support/CachePruning.h).
     SmallString<64> EntryPath;
@@ -167,4 +167,5 @@ Expected<FileCache> llvm::localCache(const Twine &CacheNameRef,
           Task);
     };
   };
+  return FileCache(Func, CacheDirectoryPathRef.str());
 }