[clang][RelativeVTables] Make the rtti_proxy LinkOnceODR instead of External linkage

PiJoules · PiJoules · commit 2607a99be986 · 2023-10-04T00:04:42.000Z
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.
diff --git a/clang/lib/CodeGen/CGVTables.cpp b/clang/lib/CodeGen/CGVTables.cpp
@@ -639,8 +639,16 @@ void CodeGenVTables::addRelativeComponent(ConstantArrayBuilder &builder,
   // want the stub/proxy to be emitted for properly calculating the offset.
   // Examples where there would be no symbol emitted are available_externally
   // and private linkages.
-  auto stubLinkage = vtableHasLocalLinkage ? llvm::GlobalValue::InternalLinkage
-                                           : llvm::GlobalValue::ExternalLinkage;
+  //
+  // `internal` linkage results in STB_LOCAL Elf binding while still manifesting a
+  // local symbol.
+  //
+  // `linkonce_odr` linkage results in a STB_DEFAULT Elf binding but also allows for
+  // the rtti_proxy to be transparently replaced with a GOTPCREL reloc by a
+  // target that supports this replacement.
+  auto stubLinkage = vtableHasLocalLinkage
+                         ? llvm::GlobalValue::InternalLinkage
+                         : llvm::GlobalValue::LinkOnceODRLinkage;
 
   llvm::Constant *target;
   if (auto *func = dyn_cast<llvm::Function>(globalVal)) {
diff --git a/clang/test/CodeGenCXX/RelativeVTablesABI/child-vtable-in-comdat.cpp b/clang/test/CodeGenCXX/RelativeVTablesABI/child-vtable-in-comdat.cpp
@@ -8,7 +8,6 @@
 // CHECK-DAG: $_ZTS1B = comdat any
 // CHECK-DAG: $_ZTI1B = comdat any
 // CHECK-DAG: $_ZTI1B.rtti_proxy = comdat any
-// CHECK-DAG: $_ZTI1A.rtti_proxy = comdat any
 
 // 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.
 // 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 @@
 // CHECK-DAG: @_ZTS1B =
 // CHECK-DAG: @_ZTI1A =
 // CHECK-DAG: @_ZTI1B =
-// CHECK-DAG: @_ZTI1B.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1B, comdat
+// CHECK-DAG: @_ZTI1B.rtti_proxy = linkonce_odr hidden unnamed_addr constant ptr @_ZTI1B, comdat
 
 // We will emit a vtable for B here, so it does have an alias, but we will not
 // emit one for A.
diff --git a/clang/test/CodeGenCXX/RelativeVTablesABI/parent-vtable-in-comdat.cpp b/clang/test/CodeGenCXX/RelativeVTablesABI/parent-vtable-in-comdat.cpp
@@ -18,7 +18,7 @@
 // CHECK: @_ZTVN10__cxxabiv117__class_type_infoE = external global [0 x ptr]
 // CHECK: @_ZTS1A = linkonce_odr constant [3 x i8] c"1A\00", comdat, align 1
 // CHECK: @_ZTI1A = linkonce_odr constant { ptr, ptr } { ptr getelementptr inbounds (i8, ptr @_ZTVN10__cxxabiv117__class_type_infoE, i32 8), ptr @_ZTS1A }, comdat, align 8
-// CHECK: @_ZTI1A.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1A, comdat
+// CHECK: @_ZTI1A.rtti_proxy = linkonce_odr hidden unnamed_addr constant ptr @_ZTI1A, comdat
 // CHECK: @_ZTV1A = linkonce_odr unnamed_addr alias { [3 x i32] }, ptr @_ZTV1A.local
 
 // CHECK:      define linkonce_odr void @_ZN1A3fooEv(ptr {{.*}}%this) unnamed_addr #{{[0-9]+}} comdat
diff --git a/clang/test/CodeGenCXX/RelativeVTablesABI/relative-vtables-hwasan.cpp b/clang/test/CodeGenCXX/RelativeVTablesABI/relative-vtables-hwasan.cpp
@@ -6,7 +6,7 @@
 /// hwasan-instrumented.
 // 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
 // CHECK-DAG: @_ZTV1A = unnamed_addr alias { [3 x i32] }, ptr @_ZTV1A.local
-// CHECK-DAG: @_ZTI1A.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1A, no_sanitize_hwaddress, comdat
+// CHECK-DAG: @_ZTI1A.rtti_proxy = linkonce_odr hidden unnamed_addr constant ptr @_ZTI1A, no_sanitize_hwaddress, comdat
 
 class A {
 public:
@@ -22,7 +22,7 @@ void A_foo(A *a) {
 /// If the vtable happens to be hidden, then the alias is not needed. In this
 /// case, the original vtable struct itself should be unsanitized.
 // 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
-// CHECK-DAG: @_ZTI1B.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1B, no_sanitize_hwaddress, comdat
+// CHECK-DAG: @_ZTI1B.rtti_proxy = linkonce_odr hidden unnamed_addr constant ptr @_ZTI1B, no_sanitize_hwaddress, comdat
 
 class __attribute__((visibility("hidden"))) B {
 public:
diff --git a/clang/test/CodeGenCXX/RelativeVTablesABI/simple-vtable-definition.cpp b/clang/test/CodeGenCXX/RelativeVTablesABI/simple-vtable-definition.cpp
@@ -14,7 +14,7 @@
 // CHECK: @_ZTI1A ={{.*}} constant { ptr, ptr } { ptr getelementptr inbounds (i8, ptr @_ZTVN10__cxxabiv117__class_type_infoE, i32 8), ptr @_ZTS1A }, align 8
 
 // The rtti should be in a comdat
-// CHECK: @_ZTI1A.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1A, comdat
+// CHECK: @_ZTI1A.rtti_proxy = {{.*}}comdat
 
 // The vtable symbol is exposed through an alias.
 // @_ZTV1A = dso_local unnamed_addr alias { [3 x i32] }, ptr @_ZTV1A.local
diff --git a/clang/test/CodeGenCXX/RelativeVTablesABI/type-info.cpp b/clang/test/CodeGenCXX/RelativeVTablesABI/type-info.cpp
@@ -13,8 +13,8 @@
 // CHECK: @_ZTVN10__cxxabiv120__si_class_type_infoE = external global [0 x ptr]
 // CHECK: @_ZTS1B ={{.*}} constant [3 x i8] c"1B\00", align 1
 // 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
-// CHECK: @_ZTI1A.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1A, comdat
-// CHECK: @_ZTI1B.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1B, comdat
+// CHECK: @_ZTI1A.rtti_proxy = linkonce_odr hidden unnamed_addr constant ptr @_ZTI1A, comdat
+// CHECK: @_ZTI1B.rtti_proxy = linkonce_odr hidden unnamed_addr constant ptr @_ZTI1B, comdat
 
 // CHECK:      define {{.*}}ptr @_Z11getTypeInfov() local_unnamed_addr
 // CHECK-NEXT: entry:
