[PATCH] D60302: [CodeGen][ObjC] Emit the retainRV marker as a module flag instead of named metadata.

[Akira Hatanaka via Phabricator via cfe-commits]

ahatanak created this revision.
ahatanak added reviewers: steven_wu, dexonsmith, rjmccall.
ahatanak added a project: clang.
Herald added subscribers: jkorous, kristof.beyls, javed.absar, mehdi_amini.

This fixes a bug which causes the ARC contract pass to not insert the assembly marker that is needed for the runtime to do the `objc_autoreleaseReturnValue/ objc_retainAutoreleasedReturnValue` optimization when compiling on ARM64/ARM with LTO enabled. This happens because `IRLinker::linkNamedMDNodes()` adds all the operands from the source modules to the metadata's operand list in the merged module (see the example below) and ARC contract fails to extract the marker string if the metadata has more than one operand.

  clang.arc.retainAutoreleasedReturnValueMarker = !{!0, !0}
  !0 = !{!"mov\09fp, fp\09\09# marker for objc_retainAutoreleaseReturnValue"}

To fix the bug, this patch emits the marker as a module flag.

rdar://problem/49464214


Repository:
  rC Clang

https://reviews.llvm.org/D60302

Files:
  lib/CodeGen/CGObjC.cpp
  test/CodeGenObjC/arc-unsafeclaim.m


Index: test/CodeGenObjC/arc-unsafeclaim.m
===================================================================
--- test/CodeGenObjC/arc-unsafeclaim.m
+++ test/CodeGenObjC/arc-unsafeclaim.m
@@ -231,4 +231,5 @@
 
 // This is always at the end of the module.
 
-// CHECK-OPTIMIZED: !clang.arc.retainAutoreleasedReturnValueMarker = !{!0}
+// CHECK-OPTIMIZED: !llvm.module.flags = !{!0,
+// CHECK-OPTIMIZED: !0 = !{i32 1, !"clang.arc.retainAutoreleasedReturnValueMarker", !"mov{{.*}}marker for objc_retainAutoreleaseReturnValue"}
Index: lib/CodeGen/CGObjC.cpp
===================================================================
--- lib/CodeGen/CGObjC.cpp
+++ lib/CodeGen/CGObjC.cpp
@@ -2161,14 +2161,10 @@
     // with this marker yet, so leave a breadcrumb for the ARC
     // optimizer to pick up.
     } else {
-      llvm::NamedMDNode *metadata =
-        CGF.CGM.getModule().getOrInsertNamedMetadata(
-                            "clang.arc.retainAutoreleasedReturnValueMarker");
-      assert(metadata->getNumOperands() <= 1);
-      if (metadata->getNumOperands() == 0) {
-        auto &ctx = CGF.getLLVMContext();
-        metadata->addOperand(llvm::MDNode::get(ctx,
-                                     llvm::MDString::get(ctx, assembly)));
+      const char *markerKey = "clang.arc.retainAutoreleasedReturnValueMarker";
+      if (!CGF.CGM.getModule().getModuleFlag(markerKey)) {
+        auto *str = llvm::MDString::get(CGF.getLLVMContext(), assembly);
+        CGF.CGM.getModule().addModuleFlag(llvm::Module::Error, markerKey, str);
       }
     }
   }


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