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[clang][RelativeVTables] Make the rtti_proxy LinkOnceODR instead of External linkage
This way, it the rtti_proxies can be candidates for being replaced altogether with GOTPCREL relocations because they are discardable. Functionally, this shouldn't change the final ELF linkage of the proxies.
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6 files changed

+17
-10
lines changed

6 files changed

+17
-10
lines changed

clang/lib/CodeGen/CGVTables.cpp

Lines changed: 10 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -639,8 +639,16 @@ void CodeGenVTables::addRelativeComponent(ConstantArrayBuilder &builder,
639639
// want the stub/proxy to be emitted for properly calculating the offset.
640640
// Examples where there would be no symbol emitted are available_externally
641641
// and private linkages.
642-
auto stubLinkage = vtableHasLocalLinkage ? llvm::GlobalValue::InternalLinkage
643-
: llvm::GlobalValue::ExternalLinkage;
642+
//
643+
// `internal` linkage results in STB_LOCAL Elf binding while still manifesting a
644+
// local symbol.
645+
//
646+
// `linkonce_odr` linkage results in a STB_DEFAULT Elf binding but also allows for
647+
// the rtti_proxy to be transparently replaced with a GOTPCREL reloc by a
648+
// target that supports this replacement.
649+
auto stubLinkage = vtableHasLocalLinkage
650+
? llvm::GlobalValue::InternalLinkage
651+
: llvm::GlobalValue::LinkOnceODRLinkage;
644652

645653
llvm::Constant *target;
646654
if (auto *func = dyn_cast<llvm::Function>(globalVal)) {

clang/test/CodeGenCXX/RelativeVTablesABI/child-vtable-in-comdat.cpp

Lines changed: 1 addition & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,6 @@
88
// CHECK-DAG: $_ZTS1B = comdat any
99
// CHECK-DAG: $_ZTI1B = comdat any
1010
// CHECK-DAG: $_ZTI1B.rtti_proxy = comdat any
11-
// CHECK-DAG: $_ZTI1A.rtti_proxy = comdat any
1211

1312
// VTable for B is emitted here since we access it when creating an instance of B. The VTable is also linkonce_odr and in its own comdat.
1413
// CHECK-DAG: @_ZTV1B.local = linkonce_odr hidden unnamed_addr constant { [3 x i32] } { [3 x i32] [i32 0, i32 trunc (i64 sub (i64 ptrtoint (ptr @_ZTI1B.rtti_proxy to i64), i64 ptrtoint (ptr getelementptr inbounds ({ [3 x i32] }, ptr @_ZTV1B.local, i32 0, i32 0, i32 2) to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint (ptr dso_local_equivalent @_ZN1B3fooEv to i64), i64 ptrtoint (ptr getelementptr inbounds ({ [3 x i32] }, ptr @_ZTV1B.local, i32 0, i32 0, i32 2) to i64)) to i32)] }, comdat($_ZTV1B), align 4
@@ -18,7 +17,7 @@
1817
// CHECK-DAG: @_ZTS1B =
1918
// CHECK-DAG: @_ZTI1A =
2019
// CHECK-DAG: @_ZTI1B =
21-
// CHECK-DAG: @_ZTI1B.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1B, comdat
20+
// CHECK-DAG: @_ZTI1B.rtti_proxy = linkonce_odr hidden unnamed_addr constant ptr @_ZTI1B, comdat
2221

2322
// We will emit a vtable for B here, so it does have an alias, but we will not
2423
// emit one for A.

clang/test/CodeGenCXX/RelativeVTablesABI/parent-vtable-in-comdat.cpp

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -18,7 +18,7 @@
1818
// CHECK: @_ZTVN10__cxxabiv117__class_type_infoE = external global [0 x ptr]
1919
// CHECK: @_ZTS1A = linkonce_odr constant [3 x i8] c"1A\00", comdat, align 1
2020
// CHECK: @_ZTI1A = linkonce_odr constant { ptr, ptr } { ptr getelementptr inbounds (i8, ptr @_ZTVN10__cxxabiv117__class_type_infoE, i32 8), ptr @_ZTS1A }, comdat, align 8
21-
// CHECK: @_ZTI1A.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1A, comdat
21+
// CHECK: @_ZTI1A.rtti_proxy = linkonce_odr hidden unnamed_addr constant ptr @_ZTI1A, comdat
2222
// CHECK: @_ZTV1A = linkonce_odr unnamed_addr alias { [3 x i32] }, ptr @_ZTV1A.local
2323

2424
// CHECK: define linkonce_odr void @_ZN1A3fooEv(ptr {{.*}}%this) unnamed_addr #{{[0-9]+}} comdat

clang/test/CodeGenCXX/RelativeVTablesABI/relative-vtables-hwasan.cpp

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -6,7 +6,7 @@
66
/// hwasan-instrumented.
77
// CHECK-DAG: @_ZTV1A.local = private unnamed_addr constant { [3 x i32] } { [3 x i32] [i32 0, i32 trunc (i64 sub (i64 ptrtoint (ptr @_ZTI1A.rtti_proxy to i64), i64 ptrtoint (ptr getelementptr inbounds ({ [3 x i32] }, ptr @_ZTV1A.local, i32 0, i32 0, i32 2) to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint (ptr dso_local_equivalent @_ZN1A3fooEv to i64), i64 ptrtoint (ptr getelementptr inbounds ({ [3 x i32] }, ptr @_ZTV1A.local, i32 0, i32 0, i32 2) to i64)) to i32)] }, no_sanitize_hwaddress, align 4
88
// CHECK-DAG: @_ZTV1A = unnamed_addr alias { [3 x i32] }, ptr @_ZTV1A.local
9-
// CHECK-DAG: @_ZTI1A.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1A, no_sanitize_hwaddress, comdat
9+
// CHECK-DAG: @_ZTI1A.rtti_proxy = linkonce_odr hidden unnamed_addr constant ptr @_ZTI1A, no_sanitize_hwaddress, comdat
1010

1111
class A {
1212
public:
@@ -22,7 +22,7 @@ void A_foo(A *a) {
2222
/// If the vtable happens to be hidden, then the alias is not needed. In this
2323
/// case, the original vtable struct itself should be unsanitized.
2424
// CHECK-DAG: @_ZTV1B = hidden unnamed_addr constant { [3 x i32] } { [3 x i32] [i32 0, i32 trunc (i64 sub (i64 ptrtoint (ptr @_ZTI1B.rtti_proxy to i64), i64 ptrtoint (ptr getelementptr inbounds ({ [3 x i32] }, ptr @_ZTV1B, i32 0, i32 0, i32 2) to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint (ptr dso_local_equivalent @_ZN1B3fooEv to i64), i64 ptrtoint (ptr getelementptr inbounds ({ [3 x i32] }, ptr @_ZTV1B, i32 0, i32 0, i32 2) to i64)) to i32)] }, no_sanitize_hwaddress, align 4
25-
// CHECK-DAG: @_ZTI1B.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1B, no_sanitize_hwaddress, comdat
25+
// CHECK-DAG: @_ZTI1B.rtti_proxy = linkonce_odr hidden unnamed_addr constant ptr @_ZTI1B, no_sanitize_hwaddress, comdat
2626

2727
class __attribute__((visibility("hidden"))) B {
2828
public:

clang/test/CodeGenCXX/RelativeVTablesABI/simple-vtable-definition.cpp

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -14,7 +14,7 @@
1414
// CHECK: @_ZTI1A ={{.*}} constant { ptr, ptr } { ptr getelementptr inbounds (i8, ptr @_ZTVN10__cxxabiv117__class_type_infoE, i32 8), ptr @_ZTS1A }, align 8
1515

1616
// The rtti should be in a comdat
17-
// CHECK: @_ZTI1A.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1A, comdat
17+
// CHECK: @_ZTI1A.rtti_proxy = {{.*}}comdat
1818

1919
// The vtable symbol is exposed through an alias.
2020
// @_ZTV1A = dso_local unnamed_addr alias { [3 x i32] }, ptr @_ZTV1A.local

clang/test/CodeGenCXX/RelativeVTablesABI/type-info.cpp

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -13,8 +13,8 @@
1313
// CHECK: @_ZTVN10__cxxabiv120__si_class_type_infoE = external global [0 x ptr]
1414
// CHECK: @_ZTS1B ={{.*}} constant [3 x i8] c"1B\00", align 1
1515
// CHECK: @_ZTI1B ={{.*}} constant { ptr, ptr, ptr } { ptr getelementptr inbounds (i8, ptr @_ZTVN10__cxxabiv120__si_class_type_infoE, i32 8), ptr @_ZTS1B, ptr @_ZTI1A }, align 8
16-
// CHECK: @_ZTI1A.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1A, comdat
17-
// CHECK: @_ZTI1B.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1B, comdat
16+
// CHECK: @_ZTI1A.rtti_proxy = linkonce_odr hidden unnamed_addr constant ptr @_ZTI1A, comdat
17+
// CHECK: @_ZTI1B.rtti_proxy = linkonce_odr hidden unnamed_addr constant ptr @_ZTI1B, comdat
1818

1919
// CHECK: define {{.*}}ptr @_Z11getTypeInfov() local_unnamed_addr
2020
// CHECK-NEXT: entry:

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