[Preamble] Empty preamble is not an error.

[hokein]

Summary

Empty preamble is valid for source file which doesn't have any
preprocessor and #includes.

This patch makes clang treat an empty preamble as a normal preamble.

Check: ninja check-clang

A testcase is added in https://reviews.llvm.org/D50627.

[ilya-biryukov]

LGTM

include/clang/Frontend/PrecompiledPreamble.h

public:
   /// used instead, but having only this method allows a simpler API.
   virtual void HandleTopLevelDecl(DeclGroupRef DG);
   /// Creates wrapper class for PPCallbacks so we can also process information
   /// about includes that are inside of a preamble
   virtual std::unique_ptr<PPCallbacks> createPPCallbacks();
 };
 
 enum class BuildPreambleError {
-  PreambleIsEmpty = 1,
-  CouldntCreateTempFile,
+  CouldntCreateTempFile = 1,
   CouldntCreateTargetInfo,
   BeginSourceFileFailed,
   CouldntEmitPCH
 };
 
 class BuildPreambleErrorCategory final : public std::error_category {
 public:
   const char *name() const noexcept override;

lib/Frontend/ASTUnit.cpp

PreambleInvocationIn.getFrontendOpts().SkipFunctionBodies =
         PreviousSkipFunctionBodies;
 
     if (NewPreamble) {
       Preamble = std::move(*NewPreamble);
       PreambleRebuildCounter = 1;
     } else {
       switch (static_cast<BuildPreambleError>(NewPreamble.getError().value())) {
       case BuildPreambleError::CouldntCreateTempFile:
-      case BuildPreambleError::PreambleIsEmpty:
         // Try again next time.
         PreambleRebuildCounter = 1;
         return nullptr;
       case BuildPreambleError::CouldntCreateTargetInfo:
       case BuildPreambleError::BeginSourceFileFailed:
       case BuildPreambleError::CouldntEmitPCH:
         // These erros are more likely to repeat, retry after some period.
         PreambleRebuildCounter = DefaultPreambleRebuildInterval;

lib/Frontend/PrecompiledPreamble.cpp

 llvm::ErrorOr<PrecompiledPreamble> PrecompiledPreamble::Build(
     const CompilerInvocation &Invocation,
     const llvm::MemoryBuffer *MainFileBuffer, PreambleBounds Bounds,
     DiagnosticsEngine &Diagnostics, IntrusiveRefCntPtr<vfs::FileSystem> VFS,
     std::shared_ptr<PCHContainerOperations> PCHContainerOps, bool StoreInMemory,
     PreambleCallbacks &Callbacks) {
   assert(VFS && "VFS is null");
 
-  if (!Bounds.Size)
-    return BuildPreambleError::PreambleIsEmpty;
-
   auto PreambleInvocation = std::make_shared<CompilerInvocation>(Invocation);
   FrontendOptions &FrontendOpts = PreambleInvocation->getFrontendOpts();
   PreprocessorOptions &PreprocessorOpts =
       PreambleInvocation->getPreprocessorOpts();
 
   llvm::Optional<TempPCHFile> TempFile;
   if (!StoreInMemory) {
     // Create a temporary file for the precompiled preamble. In rare
bool PrecompiledPreamble::CanReuse(const CompilerInvocation &Invocation,
   assert(
       Bounds.Size <= MainFileBuffer->getBufferSize() &&
       "Buffer is too large. Bounds were calculated from a different buffer?");
 
   auto PreambleInvocation = std::make_shared<CompilerInvocation>(Invocation);
   PreprocessorOptions &PreprocessorOpts =
       PreambleInvocation->getPreprocessorOpts();
 
-  if (!Bounds.Size)
-    return false;
-
   // We've previously computed a preamble. Check whether we have the same
   // preamble now that we did before, and that there's enough space in
   // the main-file buffer within the precompiled preamble to fit the
   // new main file.
   if (PreambleBytes.size() != Bounds.Size ||
       PreambleEndsAtStartOfLine != Bounds.PreambleEndsAtStartOfLine ||
       memcmp(PreambleBytes.data(), MainFileBuffer->getBufferStart(),
              Bounds.Size) != 0)

 }
 
 const char *BuildPreambleErrorCategory::name() const noexcept {
   return "build-preamble.error";
 }
 
 std::string BuildPreambleErrorCategory::message(int condition) const {
   switch (static_cast<BuildPreambleError>(condition)) {
-  case BuildPreambleError::PreambleIsEmpty:
-    return "Preamble is empty";
   case BuildPreambleError::CouldntCreateTempFile:
     return "Could not create temporary file for PCH";
   case BuildPreambleError::CouldntCreateTargetInfo:
     return "CreateTargetInfo() return null";
   case BuildPreambleError::BeginSourceFileFailed:
     return "BeginSourceFile() return an error";
   case BuildPreambleError::CouldntEmitPCH:
     return "Could not emit PCH";
   }
   llvm_unreachable("unexpected BuildPreambleError");
 }