[clang][modules] Serialize VFS overlay paths into PCMs

With implicitly built modules, the importing `CompilerInstance` assumes PCMs were built in a "compatible way" (i.e. with similarly set up instance). Either because their context hash matches, or because this instance has just built them.

There are some use-cases, however, where this assumption doesn't hold, libclang/c-index-test being one of them. There, the importing instance (or `ASTUnit`) is being set up while the PCM file is being deserialized. Until now, we've assumed the serialized paths to input files are the actual on-disk files, meaning the default physical VFS was always able to resolve them. This won't be the case after D135636. Therefore, this patch makes sure `ASTUnit` is initialized with the same VFS as the PCM it's deserializing - by storing paths to the VFS overlay files into the PCM itself.

For the VFS overlay files to be adopted at the very start of PCM deserialization, they are stored in a new section in the unhashed control block, together with header search paths and system header prefixes. The move to the unhashed control block should be safe: if two modules were built with different header search paths and they produced different results, the hashed part of the PCM file will reflect that.

Reviewed By: akyrtzi, benlangmuir

Differential Revision: https://reviews.llvm.org/D135634

1 files changed, 9 insertions, 2 deletions

diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 9536319..a637b92 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -4737,12 +4737,19 @@ IntrusiveRefCntPtr<llvm::vfs::FileSystem>
 clang::createVFSFromCompilerInvocation(
     const CompilerInvocation &CI, DiagnosticsEngine &Diags,
     IntrusiveRefCntPtr<llvm::vfs::FileSystem> BaseFS) {
-  if (CI.getHeaderSearchOpts().VFSOverlayFiles.empty())
+  return createVFSFromOverlayFiles(CI.getHeaderSearchOpts().VFSOverlayFiles,
+                                   Diags, std::move(BaseFS));
+}
+
+IntrusiveRefCntPtr<llvm::vfs::FileSystem> clang::createVFSFromOverlayFiles(
+    ArrayRef<std::string> VFSOverlayFiles, DiagnosticsEngine &Diags,
+    IntrusiveRefCntPtr<llvm::vfs::FileSystem> BaseFS) {
+  if (VFSOverlayFiles.empty())
     return BaseFS;
 
   IntrusiveRefCntPtr<llvm::vfs::FileSystem> Result = BaseFS;
   // earlier vfs files are on the bottom
-  for (const auto &File : CI.getHeaderSearchOpts().VFSOverlayFiles) {
+  for (const auto &File : VFSOverlayFiles) {
     llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Buffer =
         Result->getBufferForFile(File);
     if (!Buffer) {