[clang] [CIR] Relative vtable layout for virtual base offset (PR #195025)

[via cfe-commits]

https://github.com/xiongzile updated https://github.com/llvm/llvm-project/pull/195025

>From e465a414191fd0468b545e52637aa5a0257eae73 Mon Sep 17 00:00:00 2001
From: Zile Xiong <xiongzile at bytedance.com>
Date: Thu, 30 Apr 2026 15:14:45 +0800
Subject: [PATCH 1/2] [CIR] Relative vtable layout for virtual base offset

---
 clang/lib/CIR/CodeGen/CIRGenItaniumCXXABI.cpp | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/clang/lib/CIR/CodeGen/CIRGenItaniumCXXABI.cpp b/clang/lib/CIR/CodeGen/CIRGenItaniumCXXABI.cpp
index 946739d4e1702..f56f291c70772 100644
--- a/clang/lib/CIR/CodeGen/CIRGenItaniumCXXABI.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenItaniumCXXABI.cpp
@@ -2057,8 +2057,10 @@ mlir::Value CIRGenItaniumCXXABI::getVirtualBaseClassOffset(
 
   mlir::Value vbaseOffset;
   if (cgm.getLangOpts().RelativeCXXABIVTables) {
-    assert(!cir::MissingFeatures::vtableRelativeLayout());
-    cgm.errorNYI(loc, "getVirtualBaseClassOffset: relative layout");
+    mlir::Value offsetPtr = builder.createBitcast(vbaseOffsetPtr, builder.getPointerTo(cgm.sInt32Ty));
+    vbaseOffset = cgf.getBuilder().createLoad(
+        loc, Address(offsetPtr, cgm.sInt32Ty,
+                     CharUnits::fromQuantity(4))); // vbase.offset
   } else {
     mlir::Value offsetPtr = builder.createBitcast(
         vbaseOffsetPtr, builder.getPointerTo(cgm.ptrDiffTy));

>From efc33ef2a9ba43432431845b5f3c0b9cbf131740 Mon Sep 17 00:00:00 2001
From: Zile Xiong <xiongzile99 at gmail.com>
Date: Thu, 30 Apr 2026 17:24:00 +0800
Subject: [PATCH 2/2] [CIR] add test for getVirtualBaseOffsetOffset:
 RelativeCXXABIVTables

---
 clang/test/CIR/CodeGenCXX/vtable-relative.cpp | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
 create mode 100644 clang/test/CIR/CodeGenCXX/vtable-relative.cpp

diff --git a/clang/test/CIR/CodeGenCXX/vtable-relative.cpp b/clang/test/CIR/CodeGenCXX/vtable-relative.cpp
new file mode 100644
index 0000000000000..8a6829a56d970
--- /dev/null
+++ b/clang/test/CIR/CodeGenCXX/vtable-relative.cpp
@@ -0,0 +1,15 @@
+// vbase-offset.cpp
+
+struct V {
+  int x;
+};
+
+struct A : virtual V {
+};
+
+struct B : A {
+};
+
+int f(B *p) {
+  return static_cast<V *>(p)->x;
+}