Make the recursive behavior of getBaseObject uniform for GlobalIndirectSymbols

[ibookstein]

Summary

As discussed on https://lists.llvm.org/pipermail/llvm-dev/2020-September/145031.html

Previously, the getBaseObject() on a GlobalIFunc returned its resolver function,
but calling it on a GlobalAlias which points to a GlobalIFunc returned nullptr.
This patch fixes the issue and makes the recursive traversal more uniform by
starting with this rather than getOperand(0).

[DmitryPolukhin]

I think it should fix immediate problem that we have and makes things more consistent in general.

[ibookstein]

Looks like the following tests start failing as a result of this change:

Failed Tests (4):
  LLVM :: LTO/Resolution/X86/ifunc.ll
  LLVM :: LTO/Resolution/X86/ifunc2.ll
  LLVM :: Object/X86/nm-ir.ll
  LLVM :: ThinLTO/X86/empty-module.ll

I'll try to investigate.

[DmitryPolukhin]

Oh, it seems that LTO is already relay on getBaseObject from ifunc :(

[DmitryPolukhin]

I investigated this issue a bit under debugger. It seems that there are multiple places where resolver is used as "base" object for ifunc and resolver attributes like comdat are applied to ifunc itself. It is reasonable for aliases but not for ifunc. Unfortunately it is not something that can be changed easily. The proposal with deriving GlobalIFunc from GlobalObject makes more sense to me now.

[ibookstein]

Superseded by https://reviews.llvm.org/D109792.

llvm/lib/IR/Globals.cpp

default:
       break;
     }
   }
   return nullptr;
 }
 
 const GlobalObject *GlobalIndirectSymbol::getBaseObject() const {
   DenseSet<const GlobalAlias *> Aliases;
-  return findBaseObject(getOperand(0), Aliases);
+  return findBaseObject(this, Aliases);
 }
 
 //===----------------------------------------------------------------------===//
 // GlobalAlias Implementation
 //===----------------------------------------------------------------------===//
 
 GlobalAlias::GlobalAlias(Type *Ty, unsigned AddressSpace, LinkageTypes Link,
                          const Twine &Name, Constant *Aliasee,