[clang][CodeGen] Shift relink option implementation away from module cloning (#81693)

We recently implemented a new option allowing relinking of bitcode
modules via the "-mllvm -relink-builtin-bitcode-postop"
option.

This implementation relied on llvm::CloneModule() in order to pass
copies to modules and preserve the original modules for later relinking.
However, cloning modules has been found to be prohibitively expensive,
significantly increasing compilation time for large bitcode libraries.

In this patch, we shift the relink option implementation to instead link
the original modules initially, and reload modules from the file system
if relinking is requested. This approach results in significantly
reduced overhead.

We accomplish this by creating a new ReloadModules() routine that can be
called from a BackendConsumer class, to mimic the behavior of
ASTConsumer's loadLinkModules(), but without access to the
CompilerInstance.

Because loading the bitcodes from the filesystem requires access to the
FileManager class, we also forward a reference to the CompilerInstance
class to the BackendConsumer. This mirrors what is already done for
several CompilerInstance members, such as TargetOptions and
CodeGenOptions.

Finally, we needed to add a const specifier to the
FileManager::getBufferForFile() routine to allow it to be called using
the const reference returned from CompilerInstance::getFileManager()

1 files changed, 1 insertions, 1 deletions

diff --git a/clang/lib/Basic/FileManager.cpp b/clang/lib/Basic/FileManager.cpp
index 6097a27..cd520a6 100644
--- a/clang/lib/Basic/FileManager.cpp
+++ b/clang/lib/Basic/FileManager.cpp
@@ -547,7 +547,7 @@ FileManager::getBufferForFile(FileEntryRef FE, bool isVolatile,
 llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
 FileManager::getBufferForFileImpl(StringRef Filename, int64_t FileSize,
                                   bool isVolatile,
-                                  bool RequiresNullTerminator) {
+                                  bool RequiresNullTerminator) const {
   if (FileSystemOpts.WorkingDir.empty())
     return FS->getBufferForFile(Filename, FileSize, RequiresNullTerminator,
                                 isVolatile);