ValueMapper: Delete unused initializers of replaced appending globals.

A full LTO link time performance and memory regression was introduced
by #137081 in cases where the modules contain large quantities of llvm.used
globals. This was unnoticed because it was not expected that this would
be a typical case, but this is exactly what coverage collection does,
and when this feature is enabled together with full LTO we end up with
quadratic memory consumption (from the unused constants) and quadratic
complexity in the function Verifier::visitGlobalValue (which visits all
the unused constants in the use list of each global value). This is a
targeted fix that avoids reintroducing the quadratic complexity from
before #137081, by having ValueMapper delete the old initializer of an
appending global if it is unused, instead of visiting every global in
the context after every link.

The repro-cfi-64 reproducer from #167037 before and after this change:

```
        Elapsed time   Max RSS (KB)
Before   12:05.11        52537184
After     3:27.68         7520696
```

Fixes #167037.

Reviewers: nikic, teresajohnson

Reviewed By: teresajohnson

Pull Request: #167629

Author: pcc

llvm/include/llvm/Transforms/Utils/ValueMapper.h

@@ -204,7 +204,7 @@ class ValueMapper {
   LLVM_ABI void scheduleMapGlobalInitializer(GlobalVariable &GV, Constant &Init,
                                              unsigned MappingContextID = 0);
   LLVM_ABI void scheduleMapAppendingVariable(GlobalVariable &GV,
-                                             Constant *InitPrefix,
+                                             GlobalVariable *OldGV,
                                              bool IsOldCtorDtor,
                                              ArrayRef<Constant *> NewMembers,
                                              unsigned MappingContextID = 0);

llvm/lib/Linker/IRMover.cpp

@@ -882,10 +882,7 @@ IRLinker::linkAppendingVarProto(GlobalVariable *DstGV,
   NG->copyAttributesFrom(SrcGV);
   forceRenaming(NG, SrcGV->getName());
 
-  Mapper.scheduleMapAppendingVariable(
-      *NG,
-      (DstGV && !DstGV->isDeclaration()) ? DstGV->getInitializer() : nullptr,
-      IsOldStructor, SrcElements);
+  Mapper.scheduleMapAppendingVariable(*NG, DstGV, IsOldStructor, SrcElements);
 
   // Replace any uses of the two global variables with uses of the new
   // global.

llvm/lib/Transforms/Utils/ValueMapper.cpp

@@ -77,7 +77,7 @@ struct WorklistEntry {
   };
   struct AppendingGVTy {
     GlobalVariable *GV;
-    Constant *InitPrefix;
+    GlobalVariable *OldGV;
   };
   struct AliasOrIFuncTy {
     GlobalValue *GV;
@@ -162,7 +162,7 @@ class Mapper {
 
   void scheduleMapGlobalInitializer(GlobalVariable &GV, Constant &Init,
                                     unsigned MCID);
-  void scheduleMapAppendingVariable(GlobalVariable &GV, Constant *InitPrefix,
+  void scheduleMapAppendingVariable(GlobalVariable &GV, GlobalVariable *OldGV,
                                     bool IsOldCtorDtor,
                                     ArrayRef<Constant *> NewMembers,
                                     unsigned MCID);
@@ -173,7 +173,7 @@ class Mapper {
   void flush();
 
 private:
-  void mapAppendingVariable(GlobalVariable &GV, Constant *InitPrefix,
+  void mapAppendingVariable(GlobalVariable &GV, GlobalVariable *OldGV,
                             bool IsOldCtorDtor,
                             ArrayRef<Constant *> NewMembers);
 
@@ -944,7 +944,7 @@ void Mapper::flush() {
           drop_begin(AppendingInits, PrefixSize));
       AppendingInits.resize(PrefixSize);
       mapAppendingVariable(*E.Data.AppendingGV.GV,
-                           E.Data.AppendingGV.InitPrefix,
+                           E.Data.AppendingGV.OldGV,
                            E.AppendingGVIsOldCtorDtor, ArrayRef(NewInits));
       break;
     }
@@ -1094,15 +1094,21 @@ void Mapper::remapFunction(Function &F) {
   }
 }
 
-void Mapper::mapAppendingVariable(GlobalVariable &GV, Constant *InitPrefix,
+void Mapper::mapAppendingVariable(GlobalVariable &GV, GlobalVariable *OldGV,
                                   bool IsOldCtorDtor,
                                   ArrayRef<Constant *> NewMembers) {
+  Constant *InitPrefix =
+      (OldGV && !OldGV->isDeclaration()) ? OldGV->getInitializer() : nullptr;
+
   SmallVector<Constant *, 16> Elements;
   if (InitPrefix) {
     unsigned NumElements =
         cast<ArrayType>(InitPrefix->getType())->getNumElements();
     for (unsigned I = 0; I != NumElements; ++I)
       Elements.push_back(InitPrefix->getAggregateElement(I));
+    OldGV->setInitializer(nullptr);
+    if (InitPrefix->hasUseList() && InitPrefix->use_empty())
+      InitPrefix->destroyConstant();
   }
 
   PointerType *VoidPtrTy;
@@ -1148,7 +1154,7 @@ void Mapper::scheduleMapGlobalInitializer(GlobalVariable &GV, Constant &Init,
 }
 
 void Mapper::scheduleMapAppendingVariable(GlobalVariable &GV,
-                                          Constant *InitPrefix,
+                                          GlobalVariable *OldGV,
                                           bool IsOldCtorDtor,
                                           ArrayRef<Constant *> NewMembers,
                                           unsigned MCID) {
@@ -1159,7 +1165,7 @@ void Mapper::scheduleMapAppendingVariable(GlobalVariable &GV,
   WE.Kind = WorklistEntry::MapAppendingVar;
   WE.MCID = MCID;
   WE.Data.AppendingGV.GV = &GV;
-  WE.Data.AppendingGV.InitPrefix = InitPrefix;
+  WE.Data.AppendingGV.OldGV = OldGV;
   WE.AppendingGVIsOldCtorDtor = IsOldCtorDtor;
   WE.AppendingGVNumNewMembers = NewMembers.size();
   Worklist.push_back(WE);
@@ -1282,12 +1288,12 @@ void ValueMapper::scheduleMapGlobalInitializer(GlobalVariable &GV,
 }
 
 void ValueMapper::scheduleMapAppendingVariable(GlobalVariable &GV,
-                                               Constant *InitPrefix,
+                                               GlobalVariable *OldGV,
                                                bool IsOldCtorDtor,
                                                ArrayRef<Constant *> NewMembers,
                                                unsigned MCID) {
   getAsMapper(pImpl)->scheduleMapAppendingVariable(
-      GV, InitPrefix, IsOldCtorDtor, NewMembers, MCID);
+      GV, OldGV, IsOldCtorDtor, NewMembers, MCID);
 }
 
 void ValueMapper::scheduleMapGlobalAlias(GlobalAlias &GA, Constant &Aliasee,