[Remarks] Restructure bitstream remarks to be fully standalone (#156715)

Currently there are two serialization modes for bitstream Remarks:
standalone and separate. The separate mode splits remark metadata (e.g.
the string table) from actual remark data. The metadata is written into
the object file by the AsmPrinter, while the remark data is stored in a
separate remarks file. This means we can't use bitstream remarks with
tools like opt that don't generate an object file. Also, it is confusing
to post-process bitstream remarks files, because only the standalone
files can be read by llvm-remarkutil. We always need to use dsymutil
to convert the separate files to standalone files, which only works for
MachO. It is not possible for clang/opt to directly emit bitstream
remark files in standalone mode, because the string table can only be
serialized after all remarks were emitted.

Therefore, this change completely removes the separate serialization
mode. Instead, the remark string table is now always written to the end
of the remarks file. This requires us to tell the serializer when to
finalize remark serialization. This automatically happens when the
serializer goes out of scope. However, often the remark file goes out of
scope before the serializer is destroyed. To diagnose this, I have added
an assert to alert users that they need to explicitly call
finalizeLLVMOptimizationRemarks.

This change paves the way for further improvements to the remark
infrastructure, including more tooling (e.g. #159784), size optimizations
for bitstream remarks, and more.

Pull Request: https://github.com/llvm/llvm-project/pull/156715

1 files changed, 5 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index 77bf0c8..106363fa 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -29,6 +29,7 @@
 #include "llvm/Frontend/Driver/CodeGenOptions.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DebugInfo.h"
+#include "llvm/IR/LLVMRemarkStreamer.h"
 #include "llvm/IR/LegacyPassManager.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/ModuleSummaryIndex.h"
@@ -1384,6 +1385,10 @@ runThinLTOBackend(CompilerInstance &CI, ModuleSummaryIndex *CombinedIndex,
     Conf.CGFileType = getCodeGenFileType(Action);
     break;
   }
+
+  // FIXME: Both ExecuteAction and thinBackend set up optimization remarks for
+  // the same context.
+  finalizeLLVMOptimizationRemarks(M->getContext());
   if (Error E =
           thinBackend(Conf, -1, AddStream, *M, *CombinedIndex, ImportList,
                       ModuleToDefinedGVSummaries[M->getModuleIdentifier()],