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

[via cfe-commits]

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

Refer: https://github.com/llvm/llvm-project/issues/192330

>From f743d0ae5fbedd926567c678fa9d0baf91575000 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] [CIR] Relative vtable layout for virtual base offset

---
 clang/include/clang/CIR/MissingFeatures.h     | 1 -
 clang/lib/CIR/CodeGen/CIRGenItaniumCXXABI.cpp | 5 +++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/clang/include/clang/CIR/MissingFeatures.h b/clang/include/clang/CIR/MissingFeatures.h
index 645fae3e4404d..36093ca35ec01 100644
--- a/clang/include/clang/CIR/MissingFeatures.h
+++ b/clang/include/clang/CIR/MissingFeatures.h
@@ -339,7 +339,6 @@ struct MissingFeatures {
   static bool vlas() { return false; }
   static bool vtableInitialization() { return false; }
   static bool vtableEmitMetadata() { return false; }
-  static bool vtableRelativeLayout() { return false; }
   static bool weakRefReference() { return false; }
   static bool writebacks() { return false; }
   static bool msvcCXXPersonality() { return false; }
diff --git a/clang/lib/CIR/CodeGen/CIRGenItaniumCXXABI.cpp b/clang/lib/CIR/CodeGen/CIRGenItaniumCXXABI.cpp
index 946739d4e1702..48519ddac1a03 100644
--- a/clang/lib/CIR/CodeGen/CIRGenItaniumCXXABI.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenItaniumCXXABI.cpp
@@ -2057,8 +2057,9 @@ mlir::Value CIRGenItaniumCXXABI::getVirtualBaseClassOffset(
 
   mlir::Value vbaseOffset;
   if (cgm.getLangOpts().RelativeCXXABIVTables) {
-    assert(!cir::MissingFeatures::vtableRelativeLayout());
-    cgm.errorNYI(loc, "getVirtualBaseClassOffset: relative layout");
+    vbaseOffset = cgf.getBuilder().createLoad(
+        loc, Address(vbaseOffsetPtr, cgm.sInt32Ty,
+                     CharUnits::fromQuantity(4))); // vbase.offset
   } else {
     mlir::Value offsetPtr = builder.createBitcast(
         vbaseOffsetPtr, builder.getPointerTo(cgm.ptrDiffTy));