Avoid copying the data of in-memory preambles

Summary: Preambles are large and we should avoid copying them.

Reviewers: bkramer, klimek

Reviewed By: bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D40302

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@318945 91177308-0d34-0410-b5e6-96231b3b80d8

Author: ilya-biryukov

include/clang/Frontend/PrecompiledPreamble.h

@@ -98,6 +98,10 @@ class PrecompiledPreamble {
   /// 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;

lib/Frontend/PrecompiledPreamble.cpp

@@ -699,9 +699,7 @@ void PrecompiledPreamble::setupPreambleStorage(
     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);
   }
 }