Avoid copying the data of in-memory preambles

[ilya-biryukov]

Summary

Preambles are large and we should avoid copying them.

[bkramer]

include/clang/Frontend/PrecompiledPreamble.h

101 ↗

(On Diff #123778)

continues to own

[ilya-biryukov]

• Fixed a typo.

[bkramer]

lg

cfe/trunk/include/clang/Frontend/PrecompiledPreamble.h

public:
   /// MainFileBuffer) of the main file.
   bool CanReuse(const CompilerInvocation &Invocation,
                 const llvm::MemoryBuffer *MainFileBuffer, PreambleBounds Bounds,
                 vfs::FileSystem *VFS) const;
 
   /// Changes options inside \p CI to use PCH from this preamble. Also remaps
   /// main file to \p MainFileBuffer and updates \p VFS to ensure the preamble
   /// is accessible.
+  /// For in-memory preambles, PrecompiledPreamble instance continues to own
+  /// the MemoryBuffer with the Preamble after this method returns. The caller
+  /// is reponsible for making sure the PrecompiledPreamble instance outlives
+  /// the compiler run and the AST that will be using the PCH.
   void AddImplicitPreamble(CompilerInvocation &CI,
                            IntrusiveRefCntPtr<vfs::FileSystem> &VFS,
                            llvm::MemoryBuffer *MainFileBuffer) const;
 
 private:
   PrecompiledPreamble(PCHStorage Storage, std::vector<char> PreambleBytes,
                       bool PreambleEndsAtStartOfLine,
                       llvm::StringMap<PreambleFileHash> FilesInPreamble);

cfe/trunk/lib/Frontend/PrecompiledPreamble.cpp

if (Storage.getKind() == PCHStorage::Kind::TempFile) {
     VFS = createVFSOverlayForPreamblePCH(PCHPath, std::move(*Buf), VFS);
   } else {
     assert(Storage.getKind() == PCHStorage::Kind::InMemory);
     // For in-memory preamble, we have to provide a VFS overlay that makes it
     // accessible.
     StringRef PCHPath = getInMemoryPreamblePath();
     PreprocessorOpts.ImplicitPCHInclude = PCHPath;
 
-    // FIMXE(ibiryukov): Preambles can be large. We should allow shared access
-    // to the preamble data instead of copying it here.
-    auto Buf = llvm::MemoryBuffer::getMemBufferCopy(Storage.asMemory().Data);
+    auto Buf = llvm::MemoryBuffer::getMemBuffer(Storage.asMemory().Data);
     VFS = createVFSOverlayForPreamblePCH(PCHPath, std::move(Buf), VFS);
   }
 }
 
 void PreambleCallbacks::AfterExecute(CompilerInstance &CI) {}
 void PreambleCallbacks::AfterPCHEmitted(ASTWriter &Writer) {}
 void PreambleCallbacks::HandleTopLevelDecl(DeclGroupRef DG) {}
 void PreambleCallbacks::HandleMacroDefined(const Token &MacroNameTok,