Reland: Dead Virtual Function Elimination

Remove dead virtual functions from vtables with
replaceNonMetadataUsesWith, so that CGProfile metadata gets cleaned up
correctly.

Original commit message:

Currently, it is hard for the compiler to remove unused C++ virtual
functions, because they are all referenced from vtables, which are referenced
by constructors. This means that if the constructor is called from any live
code, then we keep every virtual function in the final link, even if there
are no call sites which can use it.

This patch allows unused virtual functions to be removed during LTO (and
regular compilation in limited circumstances) by using type metadata to match
virtual function call sites to the vtable slots they might load from. This
information can then be used in the global dead code elimination pass instead
of the references from vtables to virtual functions, to more accurately
determine which functions are reachable.

To make this transformation safe, I have changed clang's code-generation to
always load virtual function pointers using the llvm.type.checked.load
intrinsic, instead of regular load instructions. I originally tried writing
this using clang's existing code-generation, which uses the llvm.type.test
and llvm.assume intrinsics after doing a normal load. However, it is possible
for optimisations to obscure the relationship between the GEP, load and
llvm.type.test, causing GlobalDCE to fail to find virtual function call
sites.

The existing linkage and visibility types don't accurately describe the scope
in which a virtual call could be made which uses a given vtable. This is
wider than the visibility of the type itself, because a virtual function call
could be made using a more-visible base class. I've added a new
!vcall_visibility metadata type to represent this, described in
TypeMetadata.rst. The internalization pass and libLTO have been updated to
change this metadata when linking is performed.

This doesn't currently work with ThinLTO, because it needs to see every call
to llvm.type.checked.load in the linkage unit. It might be possible to
extend this optimisation to be able to use the ThinLTO summary, as was done
for devirtualization, but until then that combination is rejected in the
clang driver.

To test this, I've written a fuzzer which generates random C++ programs with
complex class inheritance graphs, and virtual functions called through object
and function pointers of different types. The programs are spread across
multiple translation units and DSOs to test the different visibility
restrictions.

I've also tried doing bootstrap builds of LLVM to test this. This isn't
ideal, because only classes in anonymous namespaces can be optimised with
-fvisibility=default, and some parts of LLVM (plugins and bugpoint) do not
work correctly with -fvisibility=hidden. However, there are only 12 test
failures when building with -fvisibility=hidden (and an unmodified compiler),
and this change does not cause any new failures for either value of
-fvisibility.

On the 7 C++ sub-benchmarks of SPEC2006, this gives a geomean code-size
reduction of ~6%, over a baseline compiled with "-O2 -flto
-fvisibility=hidden -fwhole-program-vtables". The best cases are reductions
of ~14% in 450.soplex and 483.xalancbmk, and there are no code size
increases.

I've also run this on a set of 8 mbed-os examples compiled for Armv7M, which
show a geomean size reduction of ~3%, again with no size increases.

I had hoped that this would have no effect on performance, which would allow
it to awlays be enabled (when using -fwhole-program-vtables). However, the
changes in clang to use the llvm.type.checked.load intrinsic are causing ~1%
performance regression in the C++ parts of SPEC2006. It should be possible to
recover some of this perf loss by teaching optimisations about the
llvm.type.checked.load intrinsic, which would make it worth turning this on
by default (though it's still dependent on -fwhole-program-vtables).

Differential revision: https://reviews.llvm.org/D63932

llvm-svn: 375094

Author: ostannard

clang/include/clang/Basic/CodeGenOptions.def

@@ -278,6 +278,10 @@ CODEGENOPT(EmitLLVMUseLists, 1, 0) ///< Control whether to serialize use-lists.
 CODEGENOPT(WholeProgramVTables, 1, 0) ///< Whether to apply whole-program
                                       ///  vtable optimization.
 
+CODEGENOPT(VirtualFunctionElimination, 1, 0) ///< Whether to apply the dead
+                                             /// virtual function elimination
+                                             /// optimization.
+
 /// Whether to use public LTO visibility for entities in std and stdext
 /// namespaces. This is enabled by clang-cl's /MT and /MTd flags.
 CODEGENOPT(LTOVisibilityPublicStd, 1, 0)

clang/include/clang/Driver/Options.td

@@ -1885,6 +1885,13 @@ def fforce_emit_vtables : Flag<["-"], "fforce-emit-vtables">, Group<f_Group>,
     HelpText<"Emits more virtual tables to improve devirtualization">;
 def fno_force_emit_vtables : Flag<["-"], "fno-force-emit-vtables">, Group<f_Group>,
   Flags<[CoreOption]>;
+
+def fvirtual_function_elimination : Flag<["-"], "fvirtual-function-elimination">, Group<f_Group>,
+  Flags<[CoreOption, CC1Option]>,
+  HelpText<"Enables dead virtual function elimination optimization. Requires -flto=full">;
+def fno_virtual_function_elimination : Flag<["-"], "fno-virtual-function_elimination">, Group<f_Group>,
+  Flags<[CoreOption]>;
+
 def fwrapv : Flag<["-"], "fwrapv">, Group<f_Group>, Flags<[CC1Option]>,
   HelpText<"Treat signed integer overflow as two's complement">;
 def fwritable_strings : Flag<["-"], "fwritable-strings">, Group<f_Group>, Flags<[CC1Option]>,

clang/lib/CodeGen/CGClass.cpp

@@ -2784,11 +2784,16 @@ void CodeGenFunction::EmitVTablePtrCheck(const CXXRecordDecl *RD,
 
 bool CodeGenFunction::ShouldEmitVTableTypeCheckedLoad(const CXXRecordDecl *RD) {
   if (!CGM.getCodeGenOpts().WholeProgramVTables ||
-      !SanOpts.has(SanitizerKind::CFIVCall) ||
-      !CGM.getCodeGenOpts().SanitizeTrap.has(SanitizerKind::CFIVCall) ||
       !CGM.HasHiddenLTOVisibility(RD))
     return false;
 
+  if (CGM.getCodeGenOpts().VirtualFunctionElimination)
+    return true;
+
+  if (!SanOpts.has(SanitizerKind::CFIVCall) ||
+      !CGM.getCodeGenOpts().SanitizeTrap.has(SanitizerKind::CFIVCall))
+    return false;
+
   std::string TypeName = RD->getQualifiedNameAsString();
   return !getContext().getSanitizerBlacklist().isBlacklistedType(
       SanitizerKind::CFIVCall, TypeName);
@@ -2811,8 +2816,13 @@ llvm::Value *CodeGenFunction::EmitVTableTypeCheckedLoad(
        TypeId});
   llvm::Value *CheckResult = Builder.CreateExtractValue(CheckedLoad, 1);
 
-  EmitCheck(std::make_pair(CheckResult, SanitizerKind::CFIVCall),
-            SanitizerHandler::CFICheckFail, nullptr, nullptr);
+  std::string TypeName = RD->getQualifiedNameAsString();
+  if (SanOpts.has(SanitizerKind::CFIVCall) &&
+      !getContext().getSanitizerBlacklist().isBlacklistedType(
+          SanitizerKind::CFIVCall, TypeName)) {
+    EmitCheck(std::make_pair(CheckResult, SanitizerKind::CFIVCall),
+              SanitizerHandler::CFICheckFail, {}, {});
+  }
 
   return Builder.CreateBitCast(
       Builder.CreateExtractValue(CheckedLoad, 0),

clang/lib/CodeGen/CGVTables.cpp

@@ -808,7 +808,7 @@ CodeGenVTables::GenerateConstructionVTable(const CXXRecordDecl *RD,
   assert(!VTable->isDeclaration() && "Shouldn't set properties on declaration");
   CGM.setGVProperties(VTable, RD);
 
-  CGM.EmitVTableTypeMetadata(VTable, *VTLayout.get());
+  CGM.EmitVTableTypeMetadata(RD, VTable, *VTLayout.get());
 
   return VTable;
 }
@@ -1039,7 +1039,32 @@ bool CodeGenModule::HasHiddenLTOVisibility(const CXXRecordDecl *RD) {
   return true;
 }
 
-void CodeGenModule::EmitVTableTypeMetadata(llvm::GlobalVariable *VTable,
+llvm::GlobalObject::VCallVisibility
+CodeGenModule::GetVCallVisibilityLevel(const CXXRecordDecl *RD) {
+  LinkageInfo LV = RD->getLinkageAndVisibility();
+  llvm::GlobalObject::VCallVisibility TypeVis;
+  if (!isExternallyVisible(LV.getLinkage()))
+    TypeVis = llvm::GlobalObject::VCallVisibilityTranslationUnit;
+  else if (HasHiddenLTOVisibility(RD))
+    TypeVis = llvm::GlobalObject::VCallVisibilityLinkageUnit;
+  else
+    TypeVis = llvm::GlobalObject::VCallVisibilityPublic;
+
+  for (auto B : RD->bases())
+    if (B.getType()->getAsCXXRecordDecl()->isDynamicClass())
+      TypeVis = std::min(TypeVis,
+                    GetVCallVisibilityLevel(B.getType()->getAsCXXRecordDecl()));
+
+  for (auto B : RD->vbases())
+    if (B.getType()->getAsCXXRecordDecl()->isDynamicClass())
+      TypeVis = std::min(TypeVis,
+                    GetVCallVisibilityLevel(B.getType()->getAsCXXRecordDecl()));
+
+  return TypeVis;
+}
+
+void CodeGenModule::EmitVTableTypeMetadata(const CXXRecordDecl *RD,
+                                           llvm::GlobalVariable *VTable,
                                            const VTableLayout &VTLayout) {
   if (!getCodeGenOpts().LTOUnit)
     return;
@@ -1099,4 +1124,10 @@ void CodeGenModule::EmitVTableTypeMetadata(llvm::GlobalVariable *VTable,
       VTable->addTypeMetadata((PointerWidth * I).getQuantity(), MD);
     }
   }
+
+  if (getCodeGenOpts().VirtualFunctionElimination) {
+    llvm::GlobalObject::VCallVisibility TypeVis = GetVCallVisibilityLevel(RD);
+    if (TypeVis != llvm::GlobalObject::VCallVisibilityPublic)
+      VTable->addVCallVisibilityMetadata(TypeVis);
+  }
 }

clang/lib/CodeGen/CodeGenModule.h

@@ -1280,8 +1280,16 @@ class CodeGenModule : public CodeGenTypeCache {
   /// optimization.
   bool HasHiddenLTOVisibility(const CXXRecordDecl *RD);
 
+  /// Returns the vcall visibility of the given type. This is the scope in which
+  /// a virtual function call could be made which ends up being dispatched to a
+  /// member function of this class. This scope can be wider than the visibility
+  /// of the class itself when the class has a more-visible dynamic base class.
+  llvm::GlobalObject::VCallVisibility
+  GetVCallVisibilityLevel(const CXXRecordDecl *RD);
+
   /// Emit type metadata for the given vtable using the given layout.
-  void EmitVTableTypeMetadata(llvm::GlobalVariable *VTable,
+  void EmitVTableTypeMetadata(const CXXRecordDecl *RD,
+                              llvm::GlobalVariable *VTable,
                               const VTableLayout &VTLayout);
 
   /// Generate a cross-DSO type identifier for MD.

clang/lib/CodeGen/ItaniumCXXABI.cpp

@@ -644,53 +644,88 @@ CGCallee ItaniumCXXABI::EmitLoadOfMemberFunctionPointer(
     VTableOffset = Builder.CreateTrunc(VTableOffset, CGF.Int32Ty);
     VTableOffset = Builder.CreateZExt(VTableOffset, CGM.PtrDiffTy);
   }
-  // Compute the address of the virtual function pointer.
-  llvm::Value *VFPAddr = Builder.CreateGEP(VTable, VTableOffset);
 
   // Check the address of the function pointer if CFI on member function
   // pointers is enabled.
   llvm::Constant *CheckSourceLocation;
   llvm::Constant *CheckTypeDesc;
   bool ShouldEmitCFICheck = CGF.SanOpts.has(SanitizerKind::CFIMFCall) &&
                             CGM.HasHiddenLTOVisibility(RD);
-  if (ShouldEmitCFICheck) {
-    CodeGenFunction::SanitizerScope SanScope(&CGF);
-
-    CheckSourceLocation = CGF.EmitCheckSourceLocation(E->getBeginLoc());
-    CheckTypeDesc = CGF.EmitCheckTypeDescriptor(QualType(MPT, 0));
-    llvm::Constant *StaticData[] = {
-        llvm::ConstantInt::get(CGF.Int8Ty, CodeGenFunction::CFITCK_VMFCall),
-        CheckSourceLocation,
-        CheckTypeDesc,
-    };
-
-    llvm::Metadata *MD =
-        CGM.CreateMetadataIdentifierForVirtualMemPtrType(QualType(MPT, 0));
-    llvm::Value *TypeId = llvm::MetadataAsValue::get(CGF.getLLVMContext(), MD);
+  bool ShouldEmitVFEInfo = CGM.getCodeGenOpts().VirtualFunctionElimination &&
+                           CGM.HasHiddenLTOVisibility(RD);
+  llvm::Value *VirtualFn = nullptr;
 
-    llvm::Value *TypeTest = Builder.CreateCall(
-        CGM.getIntrinsic(llvm::Intrinsic::type_test), {VFPAddr, TypeId});
+  {
+    CodeGenFunction::SanitizerScope SanScope(&CGF);
+    llvm::Value *TypeId = nullptr;
+    llvm::Value *CheckResult = nullptr;
+
+    if (ShouldEmitCFICheck || ShouldEmitVFEInfo) {
+      // If doing CFI or VFE, we will need the metadata node to check against.
+      llvm::Metadata *MD =
+          CGM.CreateMetadataIdentifierForVirtualMemPtrType(QualType(MPT, 0));
+      TypeId = llvm::MetadataAsValue::get(CGF.getLLVMContext(), MD);
+    }
 
-    if (CGM.getCodeGenOpts().SanitizeTrap.has(SanitizerKind::CFIMFCall)) {
-      CGF.EmitTrapCheck(TypeTest);
+    llvm::Value *VFPAddr = Builder.CreateGEP(VTable, VTableOffset);
+
+    if (ShouldEmitVFEInfo) {
+      // If doing VFE, load from the vtable with a type.checked.load intrinsic
+      // call. Note that we use the GEP to calculate the address to load from
+      // and pass 0 as the offset to the intrinsic. This is because every
+      // vtable slot of the correct type is marked with matching metadata, and
+      // we know that the load must be from one of these slots.
+      llvm::Value *CheckedLoad = Builder.CreateCall(
+          CGM.getIntrinsic(llvm::Intrinsic::type_checked_load),
+          {VFPAddr, llvm::ConstantInt::get(CGM.Int32Ty, 0), TypeId});
+      CheckResult = Builder.CreateExtractValue(CheckedLoad, 1);
+      VirtualFn = Builder.CreateExtractValue(CheckedLoad, 0);
+      VirtualFn = Builder.CreateBitCast(VirtualFn, FTy->getPointerTo(),
+                                        "memptr.virtualfn");
     } else {
-      llvm::Value *AllVtables = llvm::MetadataAsValue::get(
-          CGM.getLLVMContext(),
-          llvm::MDString::get(CGM.getLLVMContext(), "all-vtables"));
-      llvm::Value *ValidVtable = Builder.CreateCall(
-          CGM.getIntrinsic(llvm::Intrinsic::type_test), {VTable, AllVtables});
-      CGF.EmitCheck(std::make_pair(TypeTest, SanitizerKind::CFIMFCall),
-                    SanitizerHandler::CFICheckFail, StaticData,
-                    {VTable, ValidVtable});
+      // When not doing VFE, emit a normal load, as it allows more
+      // optimisations than type.checked.load.
+      if (ShouldEmitCFICheck) {
+        CheckResult = Builder.CreateCall(
+            CGM.getIntrinsic(llvm::Intrinsic::type_test),
+            {Builder.CreateBitCast(VFPAddr, CGF.Int8PtrTy), TypeId});
+      }
+      VFPAddr =
+          Builder.CreateBitCast(VFPAddr, FTy->getPointerTo()->getPointerTo());
+      VirtualFn = Builder.CreateAlignedLoad(VFPAddr, CGF.getPointerAlign(),
+                                            "memptr.virtualfn");
     }
+    assert(VirtualFn && "Virtual fuction pointer not created!");
+    assert((!ShouldEmitCFICheck || !ShouldEmitVFEInfo || CheckResult) &&
+           "Check result required but not created!");
+
+    if (ShouldEmitCFICheck) {
+      // If doing CFI, emit the check.
+      CheckSourceLocation = CGF.EmitCheckSourceLocation(E->getBeginLoc());
+      CheckTypeDesc = CGF.EmitCheckTypeDescriptor(QualType(MPT, 0));
+      llvm::Constant *StaticData[] = {
+          llvm::ConstantInt::get(CGF.Int8Ty, CodeGenFunction::CFITCK_VMFCall),
+          CheckSourceLocation,
+          CheckTypeDesc,
+      };
 
-    FnVirtual = Builder.GetInsertBlock();
-  }
+      if (CGM.getCodeGenOpts().SanitizeTrap.has(SanitizerKind::CFIMFCall)) {
+        CGF.EmitTrapCheck(CheckResult);
+      } else {
+        llvm::Value *AllVtables = llvm::MetadataAsValue::get(
+            CGM.getLLVMContext(),
+            llvm::MDString::get(CGM.getLLVMContext(), "all-vtables"));
+        llvm::Value *ValidVtable = Builder.CreateCall(
+            CGM.getIntrinsic(llvm::Intrinsic::type_test), {VTable, AllVtables});
+        CGF.EmitCheck(std::make_pair(CheckResult, SanitizerKind::CFIMFCall),
+                      SanitizerHandler::CFICheckFail, StaticData,
+                      {VTable, ValidVtable});
+      }
+
+      FnVirtual = Builder.GetInsertBlock();
+    }
+  } // End of sanitizer scope
 
-  // Load the virtual function to call.
-  VFPAddr = Builder.CreateBitCast(VFPAddr, FTy->getPointerTo()->getPointerTo());
-  llvm::Value *VirtualFn = Builder.CreateAlignedLoad(
-      VFPAddr, CGF.getPointerAlign(), "memptr.virtualfn");
   CGF.EmitBranch(FnEnd);
 
   // In the non-virtual path, the function pointer is actually a
@@ -1634,7 +1669,7 @@ void ItaniumCXXABI::emitVTableDefinitions(CodeGenVTables &CGVT,
     EmitFundamentalRTTIDescriptors(RD);
 
   if (!VTable->isDeclarationForLinker())
-    CGM.EmitVTableTypeMetadata(VTable, VTLayout);
+    CGM.EmitVTableTypeMetadata(RD, VTable, VTLayout);
 }
 
 bool ItaniumCXXABI::isVirtualOffsetNeededForVTableField(

clang/lib/Driver/ToolChains/Clang.cpp

@@ -5369,9 +5369,30 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     CmdArgs.push_back(Args.MakeArgString(TargetInfo.str()));
   }
 
-  bool WholeProgramVTables =
-      Args.hasFlag(options::OPT_fwhole_program_vtables,
-                   options::OPT_fno_whole_program_vtables, false);
+  bool VirtualFunctionElimination =
+      Args.hasFlag(options::OPT_fvirtual_function_elimination,
+                   options::OPT_fno_virtual_function_elimination, false);
+  if (VirtualFunctionElimination) {
+    // VFE requires full LTO (currently, this might be relaxed to allow ThinLTO
+    // in the future).
+    if (D.getLTOMode() != LTOK_Full)
+      D.Diag(diag::err_drv_argument_only_allowed_with)
+          << "-fvirtual-function-elimination"
+          << "-flto=full";
+
+    CmdArgs.push_back("-fvirtual-function-elimination");
+  }
+
+  // VFE requires whole-program-vtables, and enables it by default.
+  bool WholeProgramVTables = Args.hasFlag(
+      options::OPT_fwhole_program_vtables,
+      options::OPT_fno_whole_program_vtables, VirtualFunctionElimination);
+  if (VirtualFunctionElimination && !WholeProgramVTables) {
+    D.Diag(diag::err_drv_argument_not_allowed_with)
+        << "-fno-whole-program-vtables"
+        << "-fvirtual-function-elimination";
+  }
+
   if (WholeProgramVTables) {
     if (!D.isUsingLTO())
       D.Diag(diag::err_drv_argument_only_allowed_with)

clang/lib/Frontend/CompilerInvocation.cpp

@@ -758,6 +758,8 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
   Opts.CodeViewGHash = Args.hasArg(OPT_gcodeview_ghash);
   Opts.MacroDebugInfo = Args.hasArg(OPT_debug_info_macro);
   Opts.WholeProgramVTables = Args.hasArg(OPT_fwhole_program_vtables);
+  Opts.VirtualFunctionElimination =
+      Args.hasArg(OPT_fvirtual_function_elimination);
   Opts.LTOVisibilityPublicStd = Args.hasArg(OPT_flto_visibility_public_std);
   Opts.SplitDwarfFile = Args.getLastArgValue(OPT_split_dwarf_file);
   Opts.SplitDwarfOutput = Args.getLastArgValue(OPT_split_dwarf_output);

clang/test/CodeGenCXX/vcall-visibility-metadata.cpp

@@ -0,0 +1,88 @@
+// RUN: %clang_cc1 -flto -flto-unit -triple x86_64-unknown-linux -emit-llvm -fvirtual-function-elimination -fwhole-program-vtables -o - %s | FileCheck %s
+
+
+// Anonymous namespace.
+namespace {
+// CHECK: @_ZTVN12_GLOBAL__N_11AE = {{.*}} !vcall_visibility [[VIS_TU:![0-9]+]]
+struct A {
+  A() {}
+  virtual int f() { return 1; }
+};
+}
+void *construct_A() {
+  return new A();
+}
+
+
+// Hidden visibility.
+// CHECK: @_ZTV1B = {{.*}} !vcall_visibility [[VIS_DSO:![0-9]+]]
+struct __attribute__((visibility("hidden"))) B {
+  B() {}
+  virtual int f() { return 1; }
+};
+B *construct_B() {
+  return new B();
+}
+
+
+// Default visibility.
+// CHECK-NOT: @_ZTV1C = {{.*}} !vcall_visibility
+struct __attribute__((visibility("default"))) C {
+  C() {}
+  virtual int f() { return 1; }
+};
+C *construct_C() {
+  return new C();
+}
+
+
+// Hidden visibility, public LTO visibility.
+// CHECK-NOT: @_ZTV1D = {{.*}} !vcall_visibility
+struct __attribute__((visibility("hidden"))) [[clang::lto_visibility_public]] D {
+  D() {}
+  virtual int f() { return 1; }
+};
+D *construct_D() {
+  return new D();
+}
+
+
+// Hidden visibility, but inherits from class with default visibility.
+// CHECK-NOT: @_ZTV1E = {{.*}} !vcall_visibility
+struct __attribute__((visibility("hidden"))) E : C {
+  E() {}
+  virtual int f() { return 1; }
+};
+E *construct_E() {
+  return new E();
+}
+
+
+// Anonymous namespace, but inherits from class with default visibility.
+// CHECK-NOT: @_ZTVN12_GLOBAL__N_11FE = {{.*}} !vcall_visibility
+namespace {
+struct __attribute__((visibility("hidden"))) F : C {
+  F() {}
+  virtual int f() { return 1; }
+};
+}
+void *construct_F() {
+  return new F();
+}
+
+
+// Anonymous namespace, but inherits from class with hidden visibility.
+// CHECK: @_ZTVN12_GLOBAL__N_11GE = {{.*}} !vcall_visibility [[VIS_DSO:![0-9]+]]
+namespace {
+struct __attribute__((visibility("hidden"))) G : B {
+  G() {}
+  virtual int f() { return 1; }
+};
+}
+void *construct_G() {
+  return new G();
+}
+
+
+// CHECK-DAG: [[VIS_DSO]] = !{i64 1}
+// CHECK-DAG: [[VIS_TU]] = !{i64 2}