1 files changed, 100 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
index 61054e7ae46f..6acace1d9fd4 100644
--- a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
+++ b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
@@ -359,6 +359,36 @@ template <> struct DenseMapInfo<VTableSlotSummary> {
 
 namespace {
 
+// Returns true if the function must be unreachable based on ValueInfo.
+//
+// In particular, identifies a function as unreachable in the following
+// conditions
+//   1) All summaries are live.
+//   2) All function summaries indicate it's unreachable
+bool mustBeUnreachableFunction(ValueInfo TheFnVI) {
+  if ((!TheFnVI) || TheFnVI.getSummaryList().empty()) {
+    // Returns false if ValueInfo is absent, or the summary list is empty
+    // (e.g., function declarations).
+    return false;
+  }
+
+  for (auto &Summary : TheFnVI.getSummaryList()) {
+    // Conservatively returns false if any non-live functions are seen.
+    // In general either all summaries should be live or all should be dead.
+    if (!Summary->isLive())
+      return false;
+    if (auto *FS = dyn_cast<FunctionSummary>(Summary.get())) {
+      if (!FS->fflags().MustBeUnreachable)
+        return false;
+    }
+    // Do nothing if a non-function has the same GUID (which is rare).
+    // This is correct since non-function summaries are not relevant.
+  }
+  // All function summaries are live and all of them agree that the function is
+  // unreachble.
+  return true;
+}
+
 // A virtual call site. VTable is the loaded virtual table pointer, and CS is
 // the indirect virtual call.
 struct VirtualCallSite {
@@ -562,10 +592,12 @@ struct DevirtModule {
   void buildTypeIdentifierMap(
       std::vector<VTableBits> &Bits,
       DenseMap<Metadata *, std::set<TypeMemberInfo>> &TypeIdMap);
+
   bool
   tryFindVirtualCallTargets(std::vector<VirtualCallTarget> &TargetsForSlot,
                             const std::set<TypeMemberInfo> &TypeMemberInfos,
-                            uint64_t ByteOffset);
+                            uint64_t ByteOffset,
+                            ModuleSummaryIndex *ExportSummary);
 
   void applySingleImplDevirt(VTableSlotInfo &SlotInfo, Constant *TheFn,
                              bool &IsExported);
@@ -640,6 +672,23 @@ struct DevirtModule {
 
   bool run();
 
+  // Look up the corresponding ValueInfo entry of `TheFn` in `ExportSummary`.
+  //
+  // Caller guarantees that `ExportSummary` is not nullptr.
+  static ValueInfo lookUpFunctionValueInfo(Function *TheFn,
+                                           ModuleSummaryIndex *ExportSummary);
+
+  // Returns true if the function definition must be unreachable.
+  //
+  // Note if this helper function returns true, `F` is guaranteed
+  // to be unreachable; if it returns false, `F` might still
+  // be unreachable but not covered by this helper function.
+  //
+  // Implementation-wise, if function definition is present, IR is analyzed; if
+  // not, look up function flags from ExportSummary as a fallback.
+  static bool mustBeUnreachableFunction(Function *const F,
+                                        ModuleSummaryIndex *ExportSummary);
+
   // Lower the module using the action and summary passed as command line
   // arguments. For testing purposes only.
   static bool
@@ -969,7 +1018,8 @@ void DevirtModule::buildTypeIdentifierMap(
 
 bool DevirtModule::tryFindVirtualCallTargets(
     std::vector<VirtualCallTarget> &TargetsForSlot,
-    const std::set<TypeMemberInfo> &TypeMemberInfos, uint64_t ByteOffset) {
+    const std::set<TypeMemberInfo> &TypeMemberInfos, uint64_t ByteOffset,
+    ModuleSummaryIndex *ExportSummary) {
   for (const TypeMemberInfo &TM : TypeMemberInfos) {
     if (!TM.Bits->GV->isConstant())
       return false;
@@ -997,6 +1047,11 @@ bool DevirtModule::tryFindVirtualCallTargets(
     if (Fn->getName() == "__cxa_pure_virtual")
       continue;
 
+    // We can disregard unreachable functions as possible call targets, as
+    // unreachable functions shouldn't be called.
+    if (mustBeUnreachableFunction(Fn, ExportSummary))
+      continue;
+
     TargetsForSlot.push_back({Fn, &TM});
   }
 
@@ -1053,6 +1108,9 @@ bool DevirtIndex::tryFindVirtualCallTargets(
       if (VTP.VTableOffset != P.AddressPointOffset + ByteOffset)
         continue;
 
+      if (mustBeUnreachableFunction(VTP.FuncVI))
+        continue;
+
       TargetsForSlot.push_back(VTP.FuncVI);
     }
   }
@@ -1744,7 +1802,7 @@ void DevirtModule::rebuildGlobal(VTableBits &B) {
                          GlobalVariable::PrivateLinkage, NewInit, "", B.GV);
   NewGV->setSection(B.GV->getSection());
   NewGV->setComdat(B.GV->getComdat());
-  NewGV->setAlignment(MaybeAlign(B.GV->getAlignment()));
+  NewGV->setAlignment(B.GV->getAlign());
 
   // Copy the original vtable's metadata to the anonymous global, adjusting
   // offsets as required.
@@ -2014,6 +2072,44 @@ void DevirtModule::removeRedundantTypeTests() {
   }
 }
 
+ValueInfo
+DevirtModule::lookUpFunctionValueInfo(Function *TheFn,
+                                      ModuleSummaryIndex *ExportSummary) {
+  assert((ExportSummary != nullptr) &&
+         "Caller guarantees ExportSummary is not nullptr");
+
+  const auto TheFnGUID = TheFn->getGUID();
+  const auto TheFnGUIDWithExportedName = GlobalValue::getGUID(TheFn->getName());
+  // Look up ValueInfo with the GUID in the current linkage.
+  ValueInfo TheFnVI = ExportSummary->getValueInfo(TheFnGUID);
+  // If no entry is found and GUID is different from GUID computed using
+  // exported name, look up ValueInfo with the exported name unconditionally.
+  // This is a fallback.
+  //
+  // The reason to have a fallback:
+  // 1. LTO could enable global value internalization via
+  // `enable-lto-internalization`.
+  // 2. The GUID in ExportedSummary is computed using exported name.
+  if ((!TheFnVI) && (TheFnGUID != TheFnGUIDWithExportedName)) {
+    TheFnVI = ExportSummary->getValueInfo(TheFnGUIDWithExportedName);
+  }
+  return TheFnVI;
+}
+
+bool DevirtModule::mustBeUnreachableFunction(
+    Function *const F, ModuleSummaryIndex *ExportSummary) {
+  // First, learn unreachability by analyzing function IR.
+  if (!F->isDeclaration()) {
+    // A function must be unreachable if its entry block ends with an
+    // 'unreachable'.
+    return isa<UnreachableInst>(F->getEntryBlock().getTerminator());
+  }
+  // Learn unreachability from ExportSummary if ExportSummary is present.
+  return ExportSummary &&
+         ::mustBeUnreachableFunction(
+             DevirtModule::lookUpFunctionValueInfo(F, ExportSummary));
+}
+
 bool DevirtModule::run() {
   // If only some of the modules were split, we cannot correctly perform
   // this transformation. We already checked for the presense of type tests
@@ -2137,7 +2233,7 @@ bool DevirtModule::run() {
                      cast<MDString>(S.first.TypeID)->getString())
                  .WPDRes[S.first.ByteOffset];
     if (tryFindVirtualCallTargets(TargetsForSlot, TypeMemberInfos,
-                                  S.first.ByteOffset)) {
+                                  S.first.ByteOffset, ExportSummary)) {
 
       if (!trySingleImplDevirt(ExportSummary, TargetsForSlot, S.second, Res)) {
         DidVirtualConstProp |=