[LTO/WPD] Allow devirtualization to function alias in vtable

Follow on to D144209 to support single implementation devirtualization
for Regular LTO when the vtable holds a function alias.

For now I have prevented other optimizations performed in regular LTO
that need to analyze the contents of the function target when the vtable
holds an alias, as I'm not sure they are always correct to perform in
that case.

Differential Revision: https://reviews.llvm.org/D144270

Author: teresajohnson

llvm/include/llvm/Transforms/IPO/WholeProgramDevirt.h

@@ -118,14 +118,14 @@ struct TypeMemberInfo {
 
 // A virtual call target, i.e. an entry in a particular vtable.
 struct VirtualCallTarget {
-  VirtualCallTarget(Function *Fn, const TypeMemberInfo *TM);
+  VirtualCallTarget(GlobalValue *Fn, const TypeMemberInfo *TM);
 
   // For testing only.
   VirtualCallTarget(const TypeMemberInfo *TM, bool IsBigEndian)
       : Fn(nullptr), TM(TM), IsBigEndian(IsBigEndian), WasDevirt(false) {}
 
-  // The function stored in the vtable.
-  Function *Fn;
+  // The function (or an alias to a function) stored in the vtable.
+  GlobalValue *Fn;
 
   // A pointer to the type identifier member through which the pointer to Fn is
   // accessed.

llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp

@@ -313,9 +313,10 @@ void wholeprogramdevirt::setAfterReturnValues(
   }
 }
 
-VirtualCallTarget::VirtualCallTarget(Function *Fn, const TypeMemberInfo *TM)
+VirtualCallTarget::VirtualCallTarget(GlobalValue *Fn, const TypeMemberInfo *TM)
     : Fn(Fn), TM(TM),
-      IsBigEndian(Fn->getParent()->getDataLayout().isBigEndian()), WasDevirt(false) {}
+      IsBigEndian(Fn->getParent()->getDataLayout().isBigEndian()),
+      WasDevirt(false) {}
 
 namespace {
 
@@ -1009,7 +1010,13 @@ bool DevirtModule::tryFindVirtualCallTargets(
     if (!Ptr)
       return false;
 
-    auto Fn = dyn_cast<Function>(Ptr->stripPointerCasts());
+    auto C = Ptr->stripPointerCasts();
+    // Make sure this is a function or alias to a function.
+    auto Fn = dyn_cast<Function>(C);
+    auto A = dyn_cast<GlobalAlias>(C);
+    if (!Fn && A)
+      Fn = dyn_cast<Function>(A->getAliasee());
+
     if (!Fn)
       return false;
 
@@ -1026,7 +1033,11 @@ bool DevirtModule::tryFindVirtualCallTargets(
     if (mustBeUnreachableFunction(Fn, ExportSummary))
       continue;
 
-    TargetsForSlot.push_back({Fn, &TM});
+    // Save the symbol used in the vtable to use as the devirtualization
+    // target.
+    auto GV = dyn_cast<GlobalValue>(C);
+    assert(GV);
+    TargetsForSlot.push_back({GV, &TM});
   }
 
   // Give up if we couldn't find any targets.
@@ -1207,7 +1218,7 @@ bool DevirtModule::trySingleImplDevirt(
     WholeProgramDevirtResolution *Res) {
   // See if the program contains a single implementation of this virtual
   // function.
-  Function *TheFn = TargetsForSlot[0].Fn;
+  auto *TheFn = TargetsForSlot[0].Fn;
   for (auto &&Target : TargetsForSlot)
     if (TheFn != Target.Fn)
       return false;
@@ -1453,23 +1464,30 @@ bool DevirtModule::tryEvaluateFunctionsWithArgs(
   // Evaluate each function and store the result in each target's RetVal
   // field.
   for (VirtualCallTarget &Target : TargetsForSlot) {
-    if (Target.Fn->arg_size() != Args.size() + 1)
+    // TODO: Skip for now if the vtable symbol was an alias to a function,
+    // need to evaluate whether it would be correct to analyze the aliasee
+    // function for this optimization.
+    auto Fn = dyn_cast<Function>(Target.Fn);
+    if (!Fn)
+      return false;
+
+    if (Fn->arg_size() != Args.size() + 1)
       return false;
 
     Evaluator Eval(M.getDataLayout(), nullptr);
     SmallVector<Constant *, 2> EvalArgs;
     EvalArgs.push_back(
-        Constant::getNullValue(Target.Fn->getFunctionType()->getParamType(0)));
+        Constant::getNullValue(Fn->getFunctionType()->getParamType(0)));
     for (unsigned I = 0; I != Args.size(); ++I) {
-      auto *ArgTy = dyn_cast<IntegerType>(
-          Target.Fn->getFunctionType()->getParamType(I + 1));
+      auto *ArgTy =
+          dyn_cast<IntegerType>(Fn->getFunctionType()->getParamType(I + 1));
       if (!ArgTy)
         return false;
       EvalArgs.push_back(ConstantInt::get(ArgTy, Args[I]));
     }
 
     Constant *RetVal;
-    if (!Eval.EvaluateFunction(Target.Fn, RetVal, EvalArgs) ||
+    if (!Eval.EvaluateFunction(Fn, RetVal, EvalArgs) ||
         !isa<ConstantInt>(RetVal))
       return false;
     Target.RetVal = cast<ConstantInt>(RetVal)->getZExtValue();
@@ -1690,8 +1708,14 @@ void DevirtModule::applyVirtualConstProp(CallSiteInfo &CSInfo, StringRef FnName,
 bool DevirtModule::tryVirtualConstProp(
     MutableArrayRef<VirtualCallTarget> TargetsForSlot, VTableSlotInfo &SlotInfo,
     WholeProgramDevirtResolution *Res, VTableSlot Slot) {
+  // TODO: Skip for now if the vtable symbol was an alias to a function,
+  // need to evaluate whether it would be correct to analyze the aliasee
+  // function for this optimization.
+  auto Fn = dyn_cast<Function>(TargetsForSlot[0].Fn);
+  if (!Fn)
+    return false;
   // This only works if the function returns an integer.
-  auto RetType = dyn_cast<IntegerType>(TargetsForSlot[0].Fn->getReturnType());
+  auto RetType = dyn_cast<IntegerType>(Fn->getReturnType());
   if (!RetType)
     return false;
   unsigned BitWidth = RetType->getBitWidth();
@@ -1709,11 +1733,18 @@ bool DevirtModule::tryVirtualConstProp(
   // inline all implementations of the virtual function into each call site,
   // rather than using function attributes to perform local optimization.
   for (VirtualCallTarget &Target : TargetsForSlot) {
-    if (Target.Fn->isDeclaration() ||
-        !computeFunctionBodyMemoryAccess(*Target.Fn, AARGetter(*Target.Fn))
+    // TODO: Skip for now if the vtable symbol was an alias to a function,
+    // need to evaluate whether it would be correct to analyze the aliasee
+    // function for this optimization.
+    auto Fn = dyn_cast<Function>(Target.Fn);
+    if (!Fn)
+      return false;
+
+    if (Fn->isDeclaration() ||
+        !computeFunctionBodyMemoryAccess(*Fn, AARGetter(*Fn))
              .doesNotAccessMemory() ||
-        Target.Fn->arg_empty() || !Target.Fn->arg_begin()->use_empty() ||
-        Target.Fn->getReturnType() != RetType)
+        Fn->arg_empty() || !Fn->arg_begin()->use_empty() ||
+        Fn->getReturnType() != RetType)
       return false;
   }
 
@@ -2221,7 +2252,7 @@ bool DevirtModule::run() {
 
   // For each (type, offset) pair:
   bool DidVirtualConstProp = false;
-  std::map<std::string, Function*> DevirtTargets;
+  std::map<std::string, GlobalValue *> DevirtTargets;
   for (auto &S : CallSlots) {
     // Search each of the members of the type identifier for the virtual
     // function implementation at offset S.first.ByteOffset, and add to
@@ -2276,7 +2307,14 @@ bool DevirtModule::run() {
   if (RemarksEnabled) {
     // Generate remarks for each devirtualized function.
     for (const auto &DT : DevirtTargets) {
-      Function *F = DT.second;
+      GlobalValue *GV = DT.second;
+      auto F = dyn_cast<Function>(GV);
+      if (!F) {
+        auto A = dyn_cast<GlobalAlias>(GV);
+        assert(A && isa<Function>(A->getAliasee()));
+        F = dyn_cast<Function>(A->getAliasee());
+        assert(F);
+      }
 
       using namespace ore;
       OREGetter(F).emit(OptimizationRemark(DEBUG_TYPE, "Devirtualized", F)

llvm/test/ThinLTO/X86/devirt_function_alias2.ll

@@ -44,19 +44,17 @@
 ; RUN:   2>&1 | FileCheck %s --check-prefix=REMARK
 ; RUN: llvm-dis %t4.1.4.opt.bc -o - | FileCheck %s --check-prefix=CHECK-IR1
 
-;; TODO: Enable the below lines once Regular LTO support for devirtualizing
-;; aliases in the vtable is supported.
 ;; Test Regular LTO
-; RUN opt -o %t5.o %s
-; RUN llvm-lto2 run %t5.o -save-temps -pass-remarks=. \
-; RUN   -whole-program-visibility \
-; RUN   -o %t6 \
-; RUN   -r=%t5.o,test,px \
-; RUN   -r=%t5.o,_ZTV1D,px \
-; RUN   -r=%t5.o,_ZN1D1mEi,px \
-; RUN   -r=%t5.o,_ZN1D1mEiAlias,px \
-; RUN   2>&1 | FileCheck %s --check-prefix=REMARK
-; RUN llvm-dis %t6.0.4.opt.bc -o - | FileCheck %s --check-prefix=CHECK-IR1
+; RUN: opt -o %t5.o %s
+; RUN: llvm-lto2 run %t5.o -save-temps -pass-remarks=. \
+; RUN:   -whole-program-visibility \
+; RUN:   -o %t6 \
+; RUN:   -r=%t5.o,test,px \
+; RUN:   -r=%t5.o,_ZTV1D,px \
+; RUN:   -r=%t5.o,_ZN1D1mEi,px \
+; RUN:   -r=%t5.o,_ZN1D1mEiAlias,px \
+; RUN:   2>&1 | FileCheck %s --check-prefix=REMARK
+; RUN: llvm-dis %t6.0.4.opt.bc -o - | FileCheck %s --check-prefix=CHECK-IR1
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-grtev4-linux-gnu"