Add ObjC ARC module flag and use it to control arc-contract pass

[steven_wu]

Summary

Currently, there is no good way to tell if the bitcode is compiled
with ARC or not. However, it is required to run objc-arc-contract
pass for ObjC ARC. Introduces a new module flag for ObjC-ARC and
use that to decide if objc-arc-contract pass is required or not.

[steven_wu]

Ping.

[steven_wu]

Fix the code according to Duncan's feedback. Attempt to add a testcase.
Not sure if there is better to check if the pass is run other than using -debug-pass=Arguments.

lib/CodeGen/BackendUtil.cpp

bool EmitAssemblyHelper::AddEmitPasses(BackendAction Action,
   else if (Action == Backend_EmitMCNull)
     CGFT = TargetMachine::CGFT_Null;
   else
     assert(Action == Backend_EmitAssembly && "Invalid action!");
 
   // 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 (LangOpts.ObjCAutoRefCount &&
+  if (TheModule->getModuleFlag("Objective-C ARC") &&
       CodeGenOpts.OptimizationLevel > 0)
     PM->add(createObjCARCContractPass());
 
   if (TM->addPassesToEmitFile(*PM, OS, CGFT,
                               /*DisableVerify=*/!CodeGenOpts.VerifyModule)) {
     Diags.Report(diag::err_fe_unable_to_interface_with_target);
     return false;
   }

lib/CodeGen/CGObjCMac.cpp

if (CGM.getLangOpts().getGC() == LangOptions::GCOnly) {
           llvm::MDString::get(VMContext, "Objective-C Garbage Collection"),
           llvm::ConstantAsMetadata::get(llvm::ConstantInt::get(
               llvm::Type::getInt32Ty(VMContext), eImageInfo_GarbageCollected))};
       Mod.addModuleFlag(llvm::Module::Require, "Objective-C GC Only",
                         llvm::MDNode::get(VMContext, Ops));
     }
   }
 
+  // Add ObjCAutoRefCount as a module flag
+  if (CGM.getLangOpts().ObjCAutoRefCount) {
+    Mod.addModuleFlag(llvm::Module::Error,
+                      "Objective-C ARC", 1u);
+  }
+
   // Indicate whether we're compiling this to run on a simulator.
   const llvm::Triple &Triple = CGM.getTarget().getTriple();
   if (Triple.isiOS() &&
       (Triple.getArch() == llvm::Triple::x86 ||
        Triple.getArch() == llvm::Triple::x86_64))
     Mod.addModuleFlag(llvm::Module::Error, "Objective-C Is Simulated",
                       eImageInfo_ImageIsSimulated);
 }

test/CodeGen/arc-passes.m

+// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -fobjc-arc -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -emit-llvm %s -o - | FileCheck %s -check-prefix=NO-ARC
+// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -fobjc-arc -emit-obj -O3 %s -o /dev/null -mllvm -debug-pass=Arguments 2>&1 | FileCheck %s -check-prefix=ARC-PASS
+// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -emit-obj -O3 %s -o /dev/null -mllvm -debug-pass=Arguments 2>&1 | FileCheck %s -check-prefix=NO-ARC-PASS
+
+// CHECK: !{i32 1, !"Objective-C ARC", i32 1}
+// NO-ARC-NOT: !{i32 1, !"Objective-C ARC", i32 1}
+// ARC-PASS: -objc-arc-contract
+// NO-ARC-PASS-NOT: -objc-arc-contract
+
+void test() {
+}