tejohnson created this revision.
tejohnson added reviewers: pcc, ostannard, evgeny777, steven_wu.
Herald added subscribers: dexonsmith, hiraditya, Prazek, mehdi_amini.
Herald added projects: clang, LLVM.
First patch to support Safe Whole Program Devirtualization Enablement,
see RFC here: http://lists.llvm.org/pipermail/llvm-dev/2019-December/137543.html
Always emit !vcall_visibility metadata under -fwhole-program-vtables,
and not just for -fvirtual-function-elimination. The vcall visibility
metadata will (in a subsequent patch) be used to communicate to WPD
which vtables are safe to devirtualize, and we will optionally convert
the metadata to hidden visibility at link time. Subsequent follow on
patches will help enable this by adding vcall_visibility metadata to the
ThinLTO summaries, and always emit type test intrinsics under
-fwhole-program-vtables (and not just for vtables with hidden
visibility).
In order to do this safely with VFE, since for VFE all vtable loads must
be type checked loads which will no longer be the case, this patch adds
a new "Virtual Function Elim" module flag to communicate to GlobalDCE
whether to perform VFE using the vcall_visibility metadata.
One additional advantage of using the vcall_visibility metadata to drive
more WPD at LTO link time is that we can use the same mechanism to
enable more aggressive VFE at LTO link time as well. The link time
option proposed in the RFC will convert vcall_visibility metadata to
hidden (aka linkage unit visibility), which combined with
-fvirtual-function-elimination will allow it to be done more
aggressively at LTO link time under the same conditions.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D71907
Files:
clang/lib/CodeGen/CGVTables.cpp
clang/lib/CodeGen/CodeGenModule.cpp
clang/lib/CodeGen/ItaniumCXXABI.cpp
clang/test/CodeGenCXX/vcall-visibility-metadata.cpp
llvm/include/llvm/IR/GlobalObject.h
llvm/lib/IR/Metadata.cpp
llvm/lib/Transforms/IPO/GlobalDCE.cpp
llvm/lib/Transforms/IPO/GlobalSplit.cpp
llvm/test/Transforms/GlobalDCE/virtual-functions-base-call.ll
llvm/test/Transforms/GlobalDCE/virtual-functions-base-pointer-call.ll
llvm/test/Transforms/GlobalDCE/virtual-functions-derived-call.ll
llvm/test/Transforms/GlobalDCE/virtual-functions-derived-pointer-call.ll
llvm/test/Transforms/GlobalDCE/virtual-functions-novfe.ll
llvm/test/Transforms/GlobalDCE/virtual-functions-visibility-post-lto.ll
llvm/test/Transforms/GlobalDCE/virtual-functions-visibility-pre-lto.ll
llvm/test/Transforms/GlobalDCE/virtual-functions.ll
llvm/test/Transforms/GlobalDCE/vtable-rtti.ll
Index: llvm/test/Transforms/GlobalDCE/vtable-rtti.ll
===================================================================
--- llvm/test/Transforms/GlobalDCE/vtable-rtti.ll
+++ llvm/test/Transforms/GlobalDCE/vtable-rtti.ll
@@ -39,9 +39,10 @@
declare dso_local noalias nonnull i8* @_Znwm(i64)
@_ZTVN10__cxxabiv117__class_type_infoE = external dso_local global i8*
-!llvm.module.flags = !{!3}
+!llvm.module.flags = !{!3, !4}
!0 = !{i64 16, !"_ZTS1A"}
!1 = !{i64 16, !"_ZTSM1AFvvE.virtual"}
!2 = !{i64 2} ; translation-unit vcall visibility
!3 = !{i32 1, !"LTOPostLink", i32 1}
+!4 = !{i32 1, !"Virtual Function Elim", i32 1}
Index: llvm/test/Transforms/GlobalDCE/virtual-functions-visibility-pre-lto.ll
===================================================================
--- llvm/test/Transforms/GlobalDCE/virtual-functions-visibility-pre-lto.ll
+++ llvm/test/Transforms/GlobalDCE/virtual-functions-visibility-pre-lto.ll
@@ -85,10 +85,11 @@
declare dso_local noalias nonnull i8* @_Znwm(i64)
-!llvm.module.flags = !{}
+!llvm.module.flags = !{!5}
!0 = !{i64 16, !"_ZTS1A"}
!1 = !{i64 16, !"_ZTSM1AFvvE.virtual"}
!2 = !{i64 0} ; public vcall visibility
!3 = !{i64 1} ; linkage-unit vcall visibility
!4 = !{i64 2} ; translation-unit vcall visibility
+!5 = !{i32 1, !"Virtual Function Elim", i32 1}
Index: llvm/test/Transforms/GlobalDCE/virtual-functions-visibility-post-lto.ll
===================================================================
--- llvm/test/Transforms/GlobalDCE/virtual-functions-visibility-post-lto.ll
+++ llvm/test/Transforms/GlobalDCE/virtual-functions-visibility-post-lto.ll
@@ -85,7 +85,7 @@
declare dso_local noalias nonnull i8* @_Znwm(i64)
-!llvm.module.flags = !{!5}
+!llvm.module.flags = !{!5, !6}
!0 = !{i64 16, !"_ZTS1A"}
!1 = !{i64 16, !"_ZTSM1AFvvE.virtual"}
@@ -93,3 +93,4 @@
!3 = !{i64 1} ; linkage-unit vcall visibility
!4 = !{i64 2} ; translation-unit vcall visibility
!5 = !{i32 1, !"LTOPostLink", i32 1}
+!6 = !{i32 1, !"Virtual Function Elim", i32 1}
Index: llvm/test/Transforms/GlobalDCE/virtual-functions.ll
===================================================================
--- llvm/test/Transforms/GlobalDCE/virtual-functions.ll
+++ llvm/test/Transforms/GlobalDCE/virtual-functions.ll
@@ -48,8 +48,11 @@
ret i32 %call1
}
+!llvm.module.flags = !{!4}
+
!0 = !{i64 16, !"_ZTS1A"}
!1 = !{i64 16, !"_ZTSM1AFivE.virtual"}
!2 = !{i64 24, !"_ZTSM1AFivE.virtual"}
!3 = !{i64 2}
+!4 = !{i32 1, !"Virtual Function Elim", i32 1}
!9 = !{}
Index: llvm/test/Transforms/GlobalDCE/virtual-functions-novfe.ll
===================================================================
--- llvm/test/Transforms/GlobalDCE/virtual-functions-novfe.ll
+++ llvm/test/Transforms/GlobalDCE/virtual-functions-novfe.ll
@@ -1,3 +1,5 @@
+; Tests that VFE is not performed when the Virtual Function Elim metadata set
+; to 0. This is the same as virtual-functions.ll otherwise.
; RUN: opt < %s -globaldce -S | FileCheck %s
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
@@ -11,14 +13,12 @@
; intrinsic. Function test_A makes a call to A::foo, but there is no call to
; A::bar anywhere, so A::bar can be deleted, and its vtable slot replaced with
; null.
+; However, with the metadata set to 0 we should not perform this VFE.
%struct.A = type { i32 (...)** }
-; The pointer to A::bar in the vtable can be removed, because it will never be
-; loaded. We replace it with null to keep the layout the same. Because it is at
-; the end of the vtable we could potentially shrink the vtable, but don't
-; currently do that.
-; CHECK: @_ZTV1A = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.A*)* @_ZN1A3fooEv to i8*), i8* null] }
+; We should retain @_ZN1A3barEv in the vtable.
+; CHECK: @_ZTV1A = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.A*)* @_ZN1A3fooEv to i8*), i8* bitcast (i32 (%struct.A*)* @_ZN1A3barEv to i8*)] }
@_ZTV1A = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.A*)* @_ZN1A3fooEv to i8*), i8* bitcast (i32 (%struct.A*)* @_ZN1A3barEv to i8*)] }, align 8, !type !0, !type !1, !type !2, !vcall_visibility !3
; A::foo is called, so must be retained.
@@ -28,8 +28,9 @@
ret i32 42
}
-; A::bar is not used, so can be deleted.
-; CHECK-NOT: define internal i32 @_ZN1A3barEv(
+; A::bar is not used, so can be deleted with VFE, however, we should not be
+; performing that elimination here.
+; CHECK: define internal i32 @_ZN1A3barEv(
define internal i32 @_ZN1A3barEv(%struct.A* nocapture readnone %this) {
entry:
ret i32 1337
@@ -48,8 +49,11 @@
ret i32 %call1
}
+!llvm.module.flags = !{!4}
+
!0 = !{i64 16, !"_ZTS1A"}
!1 = !{i64 16, !"_ZTSM1AFivE.virtual"}
!2 = !{i64 24, !"_ZTSM1AFivE.virtual"}
!3 = !{i64 2}
+!4 = !{i32 1, !"Virtual Function Elim", i32 0}
!9 = !{}
Index: llvm/test/Transforms/GlobalDCE/virtual-functions-derived-pointer-call.ll
===================================================================
--- llvm/test/Transforms/GlobalDCE/virtual-functions-derived-pointer-call.ll
+++ llvm/test/Transforms/GlobalDCE/virtual-functions-derived-pointer-call.ll
@@ -110,6 +110,8 @@
declare { i8*, i1 } @llvm.type.checked.load(i8*, i32, metadata)
+!llvm.module.flags = !{!7}
+
!0 = !{i64 16, !"_ZTS1A"}
!1 = !{i64 16, !"_ZTSM1AFiiE.virtual"}
!2 = !{i64 24, !"_ZTSM1AFifE.virtual"}
@@ -117,4 +119,5 @@
!4 = !{i64 16, !"_ZTS1B"}
!5 = !{i64 16, !"_ZTSM1BFiiE.virtual"}
!6 = !{i64 24, !"_ZTSM1BFifE.virtual"}
+!7 = !{i32 1, !"Virtual Function Elim", i32 1}
!12 = !{}
Index: llvm/test/Transforms/GlobalDCE/virtual-functions-derived-call.ll
===================================================================
--- llvm/test/Transforms/GlobalDCE/virtual-functions-derived-call.ll
+++ llvm/test/Transforms/GlobalDCE/virtual-functions-derived-call.ll
@@ -70,9 +70,12 @@
declare { i8*, i1 } @llvm.type.checked.load(i8*, i32, metadata) #2
+!llvm.module.flags = !{!5}
+
!0 = !{i64 16, !"_ZTS1A"}
!1 = !{i64 16, !"_ZTSM1AFivE.virtual"}
!2 = !{i64 2}
!3 = !{i64 16, !"_ZTS1B"}
!4 = !{i64 16, !"_ZTSM1BFivE.virtual"}
+!5 = !{i32 1, !"Virtual Function Elim", i32 1}
!10 = !{}
Index: llvm/test/Transforms/GlobalDCE/virtual-functions-base-pointer-call.ll
===================================================================
--- llvm/test/Transforms/GlobalDCE/virtual-functions-base-pointer-call.ll
+++ llvm/test/Transforms/GlobalDCE/virtual-functions-base-pointer-call.ll
@@ -108,6 +108,8 @@
declare { i8*, i1 } @llvm.type.checked.load(i8*, i32, metadata)
+!llvm.module.flags = !{!7}
+
!0 = !{i64 16, !"_ZTS1A"}
!1 = !{i64 16, !"_ZTSM1AFiiE.virtual"}
!2 = !{i64 24, !"_ZTSM1AFifE.virtual"}
@@ -115,4 +117,5 @@
!4 = !{i64 16, !"_ZTS1B"}
!5 = !{i64 16, !"_ZTSM1BFiiE.virtual"}
!6 = !{i64 24, !"_ZTSM1BFifE.virtual"}
+!7 = !{i32 1, !"Virtual Function Elim", i32 1}
!12 = !{}
Index: llvm/test/Transforms/GlobalDCE/virtual-functions-base-call.ll
===================================================================
--- llvm/test/Transforms/GlobalDCE/virtual-functions-base-call.ll
+++ llvm/test/Transforms/GlobalDCE/virtual-functions-base-call.ll
@@ -70,9 +70,12 @@
declare { i8*, i1 } @llvm.type.checked.load(i8*, i32, metadata) #2
+!llvm.module.flags = !{!5}
+
!0 = !{i64 16, !"_ZTS1A"}
!1 = !{i64 16, !"_ZTSM1AFivE.virtual"}
!2 = !{i64 2}
!3 = !{i64 16, !"_ZTS1B"}
!4 = !{i64 16, !"_ZTSM1BFivE.virtual"}
+!5 = !{i32 1, !"Virtual Function Elim", i32 1}
!10 = !{}
Index: llvm/lib/Transforms/IPO/GlobalSplit.cpp
===================================================================
--- llvm/lib/Transforms/IPO/GlobalSplit.cpp
+++ llvm/lib/Transforms/IPO/GlobalSplit.cpp
@@ -111,6 +111,9 @@
ConstantInt::get(Int32Ty, ByteOffset - SplitBegin)),
Type->getOperand(1)}));
}
+
+ if (GV.hasMetadata(LLVMContext::MD_vcall_visibility))
+ SplitGV->setVCallVisibilityMetadata(GV.getVCallVisibility());
}
for (User *U : GV.users()) {
Index: llvm/lib/Transforms/IPO/GlobalDCE.cpp
===================================================================
--- llvm/lib/Transforms/IPO/GlobalDCE.cpp
+++ llvm/lib/Transforms/IPO/GlobalDCE.cpp
@@ -263,6 +263,15 @@
if (!ClEnableVFE)
return;
+ // If the Virtual Function Elim module flag is present and set to zero, then
+ // the vcall_visibility metadata was inserted for another optimization (WPD)
+ // and we may not have type checked loads on all accesses to the vtable.
+ // Don't attempt VFE in that case.
+ auto *Val = mdconst::dyn_extract_or_null<ConstantInt>(
+ M.getModuleFlag("Virtual Function Elim"));
+ if (!Val || Val->getZExtValue() == 0)
+ return;
+
ScanVTables(M);
if (VFESafeVTables.empty())
Index: llvm/lib/IR/Metadata.cpp
===================================================================
--- llvm/lib/IR/Metadata.cpp
+++ llvm/lib/IR/Metadata.cpp
@@ -1499,7 +1499,10 @@
TypeID}));
}
-void GlobalObject::addVCallVisibilityMetadata(VCallVisibility Visibility) {
+void GlobalObject::setVCallVisibilityMetadata(VCallVisibility Visibility) {
+ // Remove any existing vcall visibility metadata first in case we are
+ // updating.
+ eraseMetadata(LLVMContext::MD_vcall_visibility);
addMetadata(LLVMContext::MD_vcall_visibility,
*MDNode::get(getContext(),
{ConstantAsMetadata::get(ConstantInt::get(
Index: llvm/include/llvm/IR/GlobalObject.h
===================================================================
--- llvm/include/llvm/IR/GlobalObject.h
+++ llvm/include/llvm/IR/GlobalObject.h
@@ -178,7 +178,7 @@
void copyMetadata(const GlobalObject *Src, unsigned Offset);
void addTypeMetadata(unsigned Offset, Metadata *TypeID);
- void addVCallVisibilityMetadata(VCallVisibility Visibility);
+ void setVCallVisibilityMetadata(VCallVisibility Visibility);
VCallVisibility getVCallVisibility() const;
protected:
Index: clang/test/CodeGenCXX/vcall-visibility-metadata.cpp
===================================================================
--- clang/test/CodeGenCXX/vcall-visibility-metadata.cpp
+++ clang/test/CodeGenCXX/vcall-visibility-metadata.cpp
@@ -1,4 +1,5 @@
-// RUN: %clang_cc1 -flto -flto-unit -triple x86_64-unknown-linux -emit-llvm -fvirtual-function-elimination -fwhole-program-vtables -o - %s | FileCheck %s
+// RUN: %clang_cc1 -flto -flto-unit -triple x86_64-unknown-linux -emit-llvm -fvirtual-function-elimination -fwhole-program-vtables -o - %s | FileCheck %s --check-prefix=CHECK --check-prefix=CHECK-VFE
+// RUN: %clang_cc1 -flto -flto-unit -triple x86_64-unknown-linux -emit-llvm -fwhole-program-vtables -o - %s | FileCheck %s --check-prefix=CHECK --check-prefix=CHECK-NOVFE
// Anonymous namespace.
@@ -83,6 +84,7 @@
return new G();
}
-
// CHECK-DAG: [[VIS_DSO]] = !{i64 1}
// CHECK-DAG: [[VIS_TU]] = !{i64 2}
+// CHECK-VFE-DAG: !{i32 1, !"Virtual Function Elim", i32 1}
+// CHECK-NOVFE-DAG: !{i32 1, !"Virtual Function Elim", i32 0}
Index: clang/lib/CodeGen/ItaniumCXXABI.cpp
===================================================================
--- clang/lib/CodeGen/ItaniumCXXABI.cpp
+++ clang/lib/CodeGen/ItaniumCXXABI.cpp
@@ -670,6 +670,10 @@
CGM.HasHiddenLTOVisibility(RD);
bool ShouldEmitVFEInfo = CGM.getCodeGenOpts().VirtualFunctionElimination &&
CGM.HasHiddenLTOVisibility(RD);
+ // Emit type test when WPD enabled, as we need to ignore vcall_visibility
+ // used without type checked loads when attempting VFE.
+ bool ShouldEmitWPDInfo = CGM.getCodeGenOpts().WholeProgramVTables &&
+ CGM.HasHiddenLTOVisibility(RD);
llvm::Value *VirtualFn = nullptr;
{
@@ -677,8 +681,9 @@
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.
+ if (ShouldEmitCFICheck || ShouldEmitVFEInfo || ShouldEmitWPDInfo) {
+ // If doing CFI, VFE or WPD, we will need the metadata node to check
+ // against.
llvm::Metadata *MD =
CGM.CreateMetadataIdentifierForVirtualMemPtrType(QualType(MPT, 0));
TypeId = llvm::MetadataAsValue::get(CGF.getLLVMContext(), MD);
@@ -702,7 +707,7 @@
} else {
// When not doing VFE, emit a normal load, as it allows more
// optimisations than type.checked.load.
- if (ShouldEmitCFICheck) {
+ if (ShouldEmitCFICheck || ShouldEmitWPDInfo) {
CheckResult = Builder.CreateCall(
CGM.getIntrinsic(llvm::Intrinsic::type_test),
{Builder.CreateBitCast(VFPAddr, CGF.Int8PtrTy), TypeId});
@@ -713,7 +718,8 @@
"memptr.virtualfn");
}
assert(VirtualFn && "Virtual fuction pointer not created!");
- assert((!ShouldEmitCFICheck || !ShouldEmitVFEInfo || CheckResult) &&
+ assert((!ShouldEmitCFICheck || !ShouldEmitVFEInfo || !ShouldEmitWPDInfo ||
+ CheckResult) &&
"Check result required but not created!");
if (ShouldEmitCFICheck) {
Index: clang/lib/CodeGen/CodeGenModule.cpp
===================================================================
--- clang/lib/CodeGen/CodeGenModule.cpp
+++ clang/lib/CodeGen/CodeGenModule.cpp
@@ -542,6 +542,14 @@
getModule().addModuleFlag(llvm::Module::Override, "Cross-DSO CFI", 1);
}
+ if (CodeGenOpts.WholeProgramVTables) {
+ // Indicate whether VFE was enabled for this module, so that the
+ // vcall_visibility metadata added under whole program vtables is handled
+ // appropriately in the optimizer.
+ getModule().addModuleFlag(llvm::Module::Error, "Virtual Function Elim",
+ CodeGenOpts.VirtualFunctionElimination);
+ }
+
if (LangOpts.Sanitize.has(SanitizerKind::CFIICall)) {
getModule().addModuleFlag(llvm::Module::Override,
"CFI Canonical Jump Tables",
Index: clang/lib/CodeGen/CGVTables.cpp
===================================================================
--- clang/lib/CodeGen/CGVTables.cpp
+++ clang/lib/CodeGen/CGVTables.cpp
@@ -1126,9 +1126,10 @@
}
}
- if (getCodeGenOpts().VirtualFunctionElimination) {
+ if (getCodeGenOpts().VirtualFunctionElimination ||
+ getCodeGenOpts().WholeProgramVTables) {
llvm::GlobalObject::VCallVisibility TypeVis = GetVCallVisibilityLevel(RD);
if (TypeVis != llvm::GlobalObject::VCallVisibilityPublic)
- VTable->addVCallVisibilityMetadata(TypeVis);
+ VTable->setVCallVisibilityMetadata(TypeVis);
}
}
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