https://github.com/el-ev created
https://github.com/llvm/llvm-project/pull/196544
Resolves #196476.
When a class is marked final, typeid on references to that type can be resolved
at compile time since the most-derived type is statically known.
Example:
<details>
```cpp
// clang -cc1 -std=c++11 -emit-llvm -o - example.cpp
namespace std {
class type_info {};
}
struct Poly {
virtual ~Poly();
};
struct Final final : Poly {};
void final_ref(Final &f) { typeid(f); }
void final_deref(Final *f) { typeid(*f); }
```
Before:
```
define void @_Z9final_refR5Final(ptr %f) {
entry:
%f.addr = alloca ptr, align 8
store ptr %f, ptr %f.addr, align 8
%0 = load ptr, ptr %f.addr, align 8, !nonnull !2, !align !3
%vtable = load ptr, ptr %0, align 8
%1 = getelementptr inbounds ptr, ptr %vtable, i64 -1
%2 = load ptr, ptr %1, align 8
ret void
}
define void @_Z11final_derefP5Final(ptr %f) {
entry:
%f.addr = alloca ptr, align 8
store ptr %f, ptr %f.addr, align 8
%0 = load ptr, ptr %f.addr, align 8
%1 = icmp eq ptr %0, null
br i1 %1, label %typeid.bad_typeid, label %typeid.end
typeid.bad_typeid: ; preds = %entry
call void @__cxa_bad_typeid() #1
unreachable
typeid.end: ; preds = %entry
%vtable = load ptr, ptr %0, align 8
%2 = getelementptr inbounds ptr, ptr %vtable, i64 -1
%3 = load ptr, ptr %2, align 8
ret void
}
```
After:
```
define void @_Z9final_refR5Final(ptr %f) {
entry:
%f.addr = alloca ptr, align 8
store ptr %f, ptr %f.addr, align 8
ret void
}
define void @_Z11final_derefP5Final(ptr %f) {
entry:
%f.addr = alloca ptr, align 8
store ptr %f, ptr %f.addr, align 8
ret void
}
```
</details>
>From 1e5ed20100a2d42b3daaf76db426e77698aba6bb Mon Sep 17 00:00:00 2001
From: Iris Shi <[email protected]>
Date: Fri, 8 May 2026 22:18:54 +0800
Subject: [PATCH] [clang][AST] Teach `CXXTypeidExpr::isMostDerived` to use
`isEffectivelyFinal`
---
clang/lib/AST/ExprCXX.cpp | 5 ++
clang/test/CodeGenCXX/typeid-most-derived.cpp | 57 +++++++++++++++++++
2 files changed, 62 insertions(+)
create mode 100644 clang/test/CodeGenCXX/typeid-most-derived.cpp
diff --git a/clang/lib/AST/ExprCXX.cpp b/clang/lib/AST/ExprCXX.cpp
index be1bd3ba913ea..40e129d03dcea 100644
--- a/clang/lib/AST/ExprCXX.cpp
+++ b/clang/lib/AST/ExprCXX.cpp
@@ -149,6 +149,11 @@ bool CXXTypeidExpr::isPotentiallyEvaluated() const {
bool CXXTypeidExpr::isMostDerived(const ASTContext &Context) const {
assert(!isTypeOperand() && "Cannot call isMostDerived for typeid(type)");
const Expr *E = getExprOperand()->IgnoreParenNoopCasts(Context);
+
+ if (const CXXRecordDecl *RD = E->getType()->getAsCXXRecordDecl())
+ if (RD->isEffectivelyFinal())
+ return true;
+
if (const auto *DRE = dyn_cast<DeclRefExpr>(E)) {
QualType Ty = DRE->getDecl()->getType();
if (!Ty->isPointerOrReferenceType())
diff --git a/clang/test/CodeGenCXX/typeid-most-derived.cpp
b/clang/test/CodeGenCXX/typeid-most-derived.cpp
new file mode 100644
index 0000000000000..a2d9f2e2bc6eb
--- /dev/null
+++ b/clang/test/CodeGenCXX/typeid-most-derived.cpp
@@ -0,0 +1,57 @@
+// RUN: %clang_cc1 %s -triple %itanium_abi_triple -Wno-unused-value -emit-llvm
-o - -std=c++11 | FileCheck %s
+
+namespace std {
+ class type_info {};
+}
+
+struct Poly {
+ virtual int foo() { return 42; }
+ virtual ~Poly();
+};
+
+struct Derived : Poly {};
+struct Final final : Poly {
+ int foo() override { return 84; }
+};
+
+// Most derived
+void value(Poly p) { typeid(p); }
+// CHECK-LABEL: define {{.*}}void @_Z5value4Poly
+// CHECK-NOT: %vtable
+// CHECK: ret void
+
+// Most derived
+void final_ref(Final &f) { typeid(f); }
+// CHECK-LABEL: define {{.*}}void @_Z9final_refR5Final
+// CHECK-NOT: %vtable
+// CHECK: ret void
+
+// Most derived
+void final_deref(Final *f) { typeid(*f); }
+// CHECK-LABEL: define {{.*}}void @_Z11final_derefP5Final
+// CHECK-NOT: %vtable
+// CHECK: ret void
+
+// Not most derived
+void poly_ref(Poly &p) { typeid(p); }
+// CHECK-LABEL: define {{.*}}void @_Z8poly_refR4Poly
+// CHECK: %vtable
+// CHECK: ret void
+
+// Not most derived
+void poly_deref(Poly *p) { typeid(*p); }
+// CHECK-LABEL: define {{.*}}void @_Z10poly_derefP4Poly
+// CHECK: %vtable
+// CHECK: ret void
+
+// Not most derived
+void derived_ref(Derived &d) { typeid(d); }
+// CHECK-LABEL: define {{.*}}void @_Z11derived_refR7Derived
+// CHECK: %vtable
+// CHECK: ret void
+
+// Not most derived
+void derived_deref(Derived *d) { typeid(*d); }
+// CHECK-LABEL: define {{.*}}void @_Z13derived_derefP7Derived
+// CHECK: %vtable
+// CHECK: ret void
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