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

This PR introduces support for bitfields inside unions in CIR.

Author: Andres-Salamanca

clang/lib/CIR/CodeGen/CIRGenRecordLayoutBuilder.cpp

@@ -704,11 +704,14 @@ 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;

clang/test/CIR/CodeGen/bitfield-union.c

@@ -0,0 +1,30 @@
+// 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 }
+
+typedef union {
+  int x;
+  int y : 3;
+  int   : 0;
+  int z : 2;
+} zero_bit;
+
+// CIR:  !rec_zero_bit = !cir.record<union "zero_bit" {!s32i, !u8i, !u8i}>
+// LLVM: %union.zero_bit = type { i32 }
+// OGCG: %union.zero_bit = type { i32 }
+
+demo d;
+zero_bit z;