Fix autoupgrade logic for Objective-C class properties module flag

[mehdi_amini]

Summary

Previous we were issuing an error when linking a module containing
the new Objective-C metadata structure for class properties with an
"old" one.
Now instead we downgrade the module flag so that the Objective-C
runtime does not expect the new metadata structure.

This is consistent with what ld64 is doing on binary files.

[manmanren]

Thanks for working on this!

This makes sense. I wonder what happens when we downgrade. Will the class properties in the new code no longer functional?

Manman

llvm/lib/IR/AutoUpgrade.cpp

1566 ↗

(On Diff #71538)

Please update the comment.

[mehdi_amini]

Greg told me that only the metadata that contains the description of the properties won't be available.
Note that ld64 issue a warning when we do the same with MachO object file, but there is no easy way to do it for LTO, we may add it afterward.

[manmanren]

LGTM. Please update the comment.

Cheers,
Manman

llvm/trunk/lib/IR/AutoUpgrade.cpp

if (!ID)
       continue;
     if (ID->getString() == "Objective-C Image Info Version")
       HasObjCFlag = true;
     if (ID->getString() == "Objective-C Class Properties")
       HasClassProperties = true;
   }
   // "Objective-C Class Properties" is recently added for Objective-C. We
   // upgrade ObjC bitcodes to contain a "Objective-C Class Properties" module
-  // flag of value 0, so we can correclty report error when trying to link
-  // an ObjC bitcode without this module flag with an ObjC bitcode with this
-  // module flag.
+  // flag of value 0, so we can correclty downgrade this flag when trying to
+  // link an ObjC bitcode without this module flag with an ObjC bitcode with
+  // this module flag.
   if (HasObjCFlag && !HasClassProperties) {
-    M.addModuleFlag(llvm::Module::Error, "Objective-C Class Properties",
+    M.addModuleFlag(llvm::Module::Override, "Objective-C Class Properties",
                     (uint32_t)0);
     return true;
   }
   return false;
 }
 
 static bool isOldLoopArgument(Metadata *MD) {
   auto *T = dyn_cast_or_null<MDTuple>(MD);

llvm/trunk/test/Bitcode/upgrade-module-flag.ll

 ; RUN: llvm-as < %s | llvm-dis | FileCheck %s
 ; RUN: verify-uselistorder < %s
 
 !llvm.module.flags = !{!0}
 
 !0 = !{i32 1, !"Objective-C Image Info Version", i32 0}
 
 ; CHECK: !0 = !{i32 1, !"Objective-C Image Info Version", i32 0}
-; CHECK: !1 = !{i32 1, !"Objective-C Class Properties", i32 0}
+; CHECK: !1 = !{i32 4, !"Objective-C Class Properties", i32 0}

llvm/trunk/test/Linker/Inputs/objectivec-class-property-flag-mismatch.ll

+
+!llvm.module.flags = !{!0, !1}
+
+!0 = !{i32 1, !"Objective-C Image Info Version", i32 0}
+!1 = !{i32 1, !"Objective-C Class Properties", i32 64}

llvm/trunk/test/Linker/objectivec-class-property-flag-mismatch.ll

+; RUN: llvm-as < %s -o %t1.bc
+; RUN: llvm-as < %p/Inputs/objectivec-class-property-flag-mismatch.ll -o %t2.bc
+
+
+; RUN: llvm-link %t1.bc %t2.bc -S | FileCheck %s
+; RUN: llvm-link %t2.bc %t1.bc -S | FileCheck %s
+
+
+; CHECK: !0 = !{i32 1, !"Objective-C Image Info Version", i32 0}
+; CHECK: !1 = !{i32 4, !"Objective-C Class Properties", i32 0}
+
+
+
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 1, !"Objective-C Image Info Version", i32 0}