[CIR] Add support for discrete bit-field ABI (#1860)

This PR adds support for the **discrete bit-field layout**, which is
used in the Microsoft bit-field ABI.

Since ClangIR does not currently support the Microsoft ABI, this uses
the compiler flag
```bash
-mms-bitfields          Set the default structure layout to be compatible with the Microsoft compiler standard
```
Fixes #1798

Author: Andres-Salamanca

clang/include/clang/CIR/Dialect/IR/CIRDataLayout.h

@@ -95,6 +95,18 @@ class CIRDataLayout {
     return llvm::alignTo(getTypeStoreSize(Ty), getABITypeAlign(Ty).value());
   }
 
+  /// Returns the offset in bits between successive objects of the
+  /// specified type, including alignment padding; always a multiple of 8.
+  ///
+  /// If Ty is a scalable vector type, the scalable property will be set and
+  /// the runtime size will be a positive integer multiple of the base size.
+  ///
+  /// This is the amount that alloca reserves for this type. For example,
+  /// returns 96 or 128 for x86_fp80, depending on alignment.
+  llvm::TypeSize getTypeAllocSizeInBits(mlir::Type ty) const {
+    return 8 * getTypeAllocSize(ty);
+  }
+
   llvm::TypeSize getPointerTypeSizeInBits(mlir::Type Ty) const {
     assert(mlir::isa<cir::PointerType>(Ty) &&
            "This should only be called with a pointer type");

clang/lib/CIR/CodeGen/CIRRecordLayoutBuilder.cpp

@@ -513,8 +513,44 @@ RecordDecl::field_iterator
 CIRRecordLowering::accumulateBitFields(RecordDecl::field_iterator Field,
                                        RecordDecl::field_iterator FieldEnd) {
 
-  if (isDiscreteBitFieldABI())
-    llvm_unreachable("NYI");
+  if (isDiscreteBitFieldABI()) {
+    // run stores the first element of the current run of bitfields. FieldEnd is
+    // used as a special value to note that we don't have a current run. A
+    // bitfield run is a contiguous collection of bitfields that can be stored
+    // in the same storage block. Zero-sized bitfields and bitfields that would
+    // cross an alignment boundary break a run and start a new one.
+    RecordDecl::field_iterator run = FieldEnd;
+    // tail is the offset of the first bit off the end of the current run. It's
+    // used to determine if the ASTRecordLayout is treating these two bitfields
+    // as contiguous. StartBitOffset is offset of the beginning of the Run.
+    uint64_t startBitOffset, tail = 0;
+    for (; Field != FieldEnd && Field->isBitField(); ++Field) {
+      // Zero-width bitfields end runs.
+      if (Field->isZeroLengthBitField()) {
+        run = FieldEnd;
+        continue;
+      }
+      uint64_t bitOffset = getFieldBitOffset(*Field);
+      mlir::Type type = cirGenTypes.convertTypeForMem(Field->getType());
+      // If we don't have a run yet, or don't live within the previous run's
+      // allocated storage then we allocate some storage and start a new run.
+      if (run == FieldEnd || bitOffset >= tail) {
+        run = Field;
+        startBitOffset = bitOffset;
+        tail = startBitOffset + dataLayout.getTypeAllocSizeInBits(type);
+        // Add the storage member to the record.  This must be added to the
+        // record before the bitfield members so that it gets laid out before
+        // the bitfields it contains get laid out.
+        members.push_back(StorageInfo(bitsToCharUnits(startBitOffset), type));
+      }
+      // Bitfields get the offset of their storage but come afterward and remain
+      // there after a stable sort.
+      members.push_back(MemberInfo(bitsToCharUnits(startBitOffset),
+                                   MemberInfo::InfoKind::Field, nullptr,
+                                   *Field));
+    }
+    return Field;
+  }
 
   CharUnits RegSize =
       bitsToCharUnits(astContext.getTargetInfo().getRegisterWidth());

clang/test/CIR/CodeGen/mms-bitfields.c

@@ -0,0 +1,66 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -mms-bitfields -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 -mms-bitfields -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 -mms-bitfields -emit-llvm %s -o %t.ll
+// RUN: FileCheck --input-file=%t.ll %s --check-prefix=OGCG
+
+struct s1 {
+  int       f32 : 2;
+  long long f64 : 30;
+} s1;
+
+// CIR-DAG: !rec_s1 = !cir.record<struct "s1" {!s32i, !s64i} #cir.record.decl.ast>
+// LLVM-DAG: %struct.s1 = type { i32, i64 }
+// OGCG-DAG: %struct.s1 = type { i32, i64 }
+
+struct s2 {
+    int  a : 24;
+    char b;
+    int  c : 30;
+} Clip;
+
+// CIR-DAG: !rec_s2 = !cir.record<struct "s2" {!s32i, !s8i, !s32i} #cir.record.decl.ast>
+// LLVM-DAG: %struct.s2 = type { i32, i8, i32 }
+// OGCG-DAG: %struct.s2 = type { i32, i8, i32 }
+
+#pragma pack (push,1)
+
+struct Inner {
+  unsigned int    A    :  1;
+  unsigned int    B    :  1;
+  unsigned int    C    :  1;
+  unsigned int    D    : 30;
+} Inner;
+
+#pragma pack (pop)
+
+// CIR-DAG: !rec_Inner = !cir.record<struct "Inner" {!u32i, !u32i} #cir.record.decl.ast>
+// LLVM-DAG: %struct.Inner = type { i32, i32 }
+// OGCG-DAG: %struct.Inner = type { i32, i32 }
+
+#pragma pack(push, 1)
+
+union HEADER {
+  struct A {
+    int                                         :  3;  // Bits 2:0
+    int a                                       :  9;  // Bits 11:3
+    int                                         :  12;  // Bits 23:12
+    int b                                       :  17;  // Bits 40:24
+    int                                         :  7;  // Bits 47:41
+    int c                                       :  4;  // Bits 51:48
+    int                                         :  4;  // Bits 55:52
+    int d                                       :  3;  // Bits 58:56
+    int                                         :  5;  // Bits 63:59
+  } Bits;
+} HEADER;
+
+#pragma pack(pop)
+
+// CIR-DAG: !rec_A = !cir.record<struct "A" {!s32i, !s32i, !s32i} #cir.record.decl.ast>
+// CIR-DAG: !rec_HEADER = !cir.record<union "HEADER" {!rec_A} #cir.record.decl.ast>
+// LLVM-DAG: %struct.A = type { i32, i32, i32 }
+// LLVM-DAG: %union.HEADER = type { %struct.A }
+// OGCG-DAG: %struct.A = type { i32, i32, i32 }
+// OGCG-DAG: %union.HEADER = type { %struct.A }
+