[ThinLTO] Add -objc-arc-contract pass

[EShuman]

Summary

Add the -objc-arc-contract pass to enable distributed ThinLTO compilation of sources containing ARC.
The details are here – https://discourse.llvm.org/t/objective-c-and-distributed-thinlto/72482

I need a hand in creating a test (test case file is in attachment)

reverse.o.ll30 KBDownload

[tejohnson]

Regarding the test, take a look at clang/test/CodeGen/thinlto-distributed-newpm.ll. You can probably just do something similar and check that this pass is invoked under the expected options.

llvm/lib/LTO/LTOBackend.cpp

 #include "llvm/Support/Program.h"
 #include "llvm/Support/ThreadPool.h"
 #include "llvm/Support/ToolOutputFile.h"
 #include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/TargetParser/SubtargetFeature.h"
 #include "llvm/Transforms/IPO/WholeProgramDevirt.h"
+#include "llvm/Transforms/ObjCARC.h"
 #include "llvm/Transforms/Scalar/LoopPassManager.h"
 #include "llvm/Transforms/Utils/FunctionImportUtils.h"
 #include "llvm/Transforms/Utils/SplitModule.h"
 #include <optional>
 
 using namespace llvm;
 using namespace lto;
 
static void codegen(const Config &Conf, TargetMachine *TM,
 
   legacy::PassManager CodeGenPasses;
   TargetLibraryInfoImpl TLII(Triple(Mod.getTargetTriple()));
   CodeGenPasses.add(new TargetLibraryInfoWrapperPass(TLII));
   CodeGenPasses.add(
       createImmutableModuleSummaryIndexWrapperPass(&CombinedIndex));
   if (Conf.PreCodeGenPassesHook)
     Conf.PreCodeGenPassesHook(CodeGenPasses);
+
+  // Add ObjC ARC final-cleanup optimizations. This is done as part of the
+  // "codegen" passes so that it isn't run multiple times when there is
+  // inlining happening.
+  if (Conf.OptLevel > 0)
+    CodeGenPasses.add(createObjCARCContractPass());
+
   if (TM->addPassesToEmitFile(CodeGenPasses, *Stream->OS,
                               DwoOut ? &DwoOut->os() : nullptr,
                               Conf.CGFileType))
     report_fatal_error("Failed to setup codegen");
   CodeGenPasses.run(Mod);
 
   if (DwoOut)
     DwoOut->keep();