[clang] [CIR] Add support for bitfields in unions (PR #145096)

[via cfe-commits]

https://github.com/Andres-Salamanca created https://github.com/llvm/llvm-project/pull/145096

This PR introduces support for bitfields inside unions in CIR.

>From 51aa8f274cbf8f7044e3c9dbd27b1e26b09a22ed Mon Sep 17 00:00:00 2001
From: Andres Salamanca <andrealebarbaritos at gmail.com>
Date: Fri, 20 Jun 2025 14:53:25 -0500
Subject: [PATCH] Add support for bitfields in unions

---
 .../CIR/CodeGen/CIRGenRecordLayoutBuilder.cpp | 11 +++++++----
 clang/test/CIR/CodeGen/bitfield-union.c       | 19 +++++++++++++++++++
 2 files changed, 26 insertions(+), 4 deletions(-)
 create mode 100644 clang/test/CIR/CodeGen/bitfield-union.c

diff --git a/clang/lib/CIR/CodeGen/CIRGenRecordLayoutBuilder.cpp b/clang/lib/CIR/CodeGen/CIRGenRecordLayoutBuilder.cpp
index 8dbf1b36a93b2..3d50a537e676b 100644
--- a/clang/lib/CIR/CodeGen/CIRGenRecordLayoutBuilder.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenRecordLayoutBuilder.cpp
@@ -655,11 +655,14 @@ void CIRRecordLowering::lowerUnion() {
   // locate the "most appropriate" storage type.
   for (const FieldDecl *field : recordDecl->fields()) {
     mlir::Type fieldType;
-    if (field->isBitField())
-      cirGenTypes.getCGModule().errorNYI(recordDecl->getSourceRange(),
-                                         "bitfields in lowerUnion");
-    else
+    if (field->isBitField()) {
+      if (field->isZeroLengthBitField())
+        continue;
+      fieldType = getBitfieldStorageType(field->getBitWidthValue());
+      setBitFieldInfo(field, CharUnits::Zero(), fieldType);
+    } else {
       fieldType = getStorageType(field);
+    }
 
     // This maps a field to its index. For unions, the index is always 0.
     fieldIdxMap[field->getCanonicalDecl()] = 0;
diff --git a/clang/test/CIR/CodeGen/bitfield-union.c b/clang/test/CIR/CodeGen/bitfield-union.c
new file mode 100644
index 0000000000000..72e14c5fb71f3
--- /dev/null
+++ b/clang/test/CIR/CodeGen/bitfield-union.c
@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o %t.cir
+// RUN: FileCheck --input-file=%t.cir %s --check-prefix=CIR
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o %t-cir.ll
+// RUN: FileCheck --input-file=%t-cir.ll %s --check-prefix=LLVM
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-llvm %s -o %t.ll
+// RUN: FileCheck --input-file=%t.ll %s --check-prefix=OGCG
+
+typedef union {
+  int x;
+  int y : 4;
+  int z : 8;
+} demo;
+
+// CIR:  !rec_demo = !cir.record<union "demo" {!s32i, !u8i, !u8i}>
+// LLVM: %union.demo = type { i32 }
+// OGCG: %union.demo = type { i32 }
+demo d; 
+
+