[GlobalDCE] [VFE] Avoid dropping vfunc dependencies when an invalid vtable entry is present

[kubamracek]

Summary

See the attached (pre-committed) test case. Looks like when we scan vtables for a particular vload in ScanVTableLoad and an entry in one possible vtable is invalid (null or non-fptr), we bail in a wrong way -- we completely stop the scanning of vtables with a return and this results in dropped dependencies because we don't perform GVDependencies[Caller].insert(Callee); on following valid vtable entries. Let's fix that.

[fhahn]

LGTM, thanks! It looks like the test only occurs the 'not function pointer' case. Could you also add a case covering the not a pointer case?

llvm/lib/Transforms/IPO/GlobalDCE.cpp

for (auto &VTableInfo : TypeIdMap[TypeId]) {
     uint64_t VTableOffset = VTableInfo.second;
 
     Constant *Ptr =
         getPointerAtOffset(VTable->getInitializer(), VTableOffset + CallOffset,
                            *Caller->getParent(), VTable);
     if (!Ptr) {
       LLVM_DEBUG(dbgs() << "can't find pointer in vtable!\n");
       VFESafeVTables.erase(VTable);
-      return;
+      continue;
     }
 
     auto Callee = dyn_cast<Function>(Ptr->stripPointerCasts());
     if (!Callee) {
       LLVM_DEBUG(dbgs() << "vtable entry is not function pointer!\n");
       VFESafeVTables.erase(VTable);
-      return;
+      continue;
     }
 
     LLVM_DEBUG(dbgs() << "vfunc dep " << Caller->getName() << " -> "
                       << Callee->getName() << "\n");
     GVDependencies[Caller].insert(Callee);
   }
 }
 

llvm/test/Transforms/GlobalDCE/virtual-functions-null.ll

 ; CHECK-SAME: ] }, align 8
 
 @vtableB = internal unnamed_addr constant { [2 x i8*] } { [2 x i8*] [
   i8* bitcast (void ()* @vfunc1 to i8*),
   i8* bitcast (void ()* @vfunc2 to i8*)
 ]}, align 8, !type !{i64 0, !"vfunc1.type"}, !type !{i64 8, !"vfunc2.type"}, !vcall_visibility !{i64 2}
 
 ; CHECK:      @vtableB = internal unnamed_addr constant { [2 x i8*] } { [2 x i8*] [
-; CHECK-SAME:   i8* null,
+; CHECK-SAME:   i8* bitcast (void ()* @vfunc1 to i8*),
 ; CHECK-SAME:   i8* bitcast (void ()* @vfunc2 to i8*)
 ; CHECK-SAME: ] }, align 8
 
 define internal void @vfunc1() {
   ret void
 }
 
 define internal void @vfunc2() {