[CallGraphUpdater][Attributor][FIX] Allow standalone function replacement

[jdoerfert]

Summary

NOTE: This is not a working patch. We need to revisit the ways we update the new PM callgraph, potentially the things we want to do are just not possible.

We did mix replacing a function and deleting the old one but that should
be two different operations so we can reuse them individually.

This is supposed to fix the creation of deep-wrappers which currently
delete the original function but it actually will cause a crash in the
way we update the new PM call graph.

Mail discussion: http://lists.llvm.org/pipermail/llvm-dev/2020-October/146117.html

llvm/lib/Transforms/IPO/Attributor.cpp

for (auto &CallSitePair : CallSitePairs) {
       ModifiedFns.insert(OldCB.getFunction());
       CGUpdater.replaceCallSite(OldCB, NewCB);
       OldCB.replaceAllUsesWith(&NewCB);
       OldCB.eraseFromParent();
     }
 
     // Replace the function in the call graph (if any).
     CGUpdater.replaceFunctionWith(*OldFn, *NewFn);
+    CGUpdater.removeFunction(*OldFn);
 
     // If the old function was modified and needed to be reanalyzed, the new one
     // does now.
     if (ModifiedFns.erase(OldFn))
       ModifiedFns.insert(NewFn);
 
     Changed = ChangeStatus::CHANGED;
   }

llvm/lib/Transforms/Utils/CallGraphUpdater.cpp

if (CG) {
 
     // And update the SCC we're iterating as well.
     CGSCC->ReplaceNode(OldCGN, NewCGN);
   } else if (LCG) {
     // Directly substitute the functions in the call graph.
     LazyCallGraph::Node &OldLCGN = LCG->get(OldFn);
     SCC->getOuterRefSCC().replaceNodeFunction(OldLCGN, NewFn);
   }
-  removeFunction(OldFn);
 }
 
 bool CallGraphUpdater::replaceCallSite(CallBase &OldCS, CallBase &NewCS) {
   // This is only necessary in the (old) CG.
   if (!CG)
     return true;
 
   Function *Caller = OldCS.getCaller();

llvm/unittests/Analysis/CGSCCPassManagerTest.cpp

CGPM.addPass(LambdaSCCPassNoPreserve(
         // Create an unsused constant that is referencing the old (=replaced)
         // function.
         ConstantExpr::getBitCast(FnF, Type::getInt8PtrTy(FnF->getContext()));
 
         // Use the CallGraphUpdater to update the call graph.
         CallGraphUpdater CGU;
         CGU.initialize(CG, C, AM, UR);
         ASSERT_NO_FATAL_FAILURE(CGU.replaceFunctionWith(*FnF, *FnewF));
-        ASSERT_TRUE(FnF->isDeclaration());
+        ASSERT_FALSE(FnF->isDeclaration());
         ASSERT_EQ(FnF->getNumUses(), 0U);
       }));
 
   ModulePassManager MPM(/*DebugLogging*/ true);
   MPM.addPass(createModuleToPostOrderCGSCCPassAdaptor(std::move(CGPM)));
   MPM.run(*M, MAM);
 }