[clang] Add an off-by-default warning to complain about MSVC bitfield padding (PR #117428)

[Oliver Hunt via cfe-commits]

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@@ -0,0 +1,180 @@
+
+// RUN: %clang_cc1 -fsyntax-only -Wms-bitfield-compatibility -verify -triple armv8 -std=c++23 %s
+// RUN: %clang_cc1 -fsyntax-only -DMS_BITFIELDS -mms-bitfields -verify=msbitfields -triple armv8-apple-macos10.15 -std=c++23 %s
+
+// msbitfields-no-diagnostics
+
+enum Enum1 { Enum1_A, Enum1_B };
+enum Enum2 { Enum2_A, Enum2_B };
+
+enum class EnumU32_1 : unsigned { A, B };
+enum class EnumU32_2 : unsigned { A, B };
+enum class EnumU64 : unsigned long long { A, B };
+enum class EnumI32 : int { A, B };
+enum class EnumU8 : unsigned char { A, B };
+enum class EnumI8 : char { A, B };
+enum class EnumU16 : unsigned short { A, B };
+enum class EnumI16 : short { A, B };
+
+struct A {
+  unsigned int a : 15;
+  unsigned int b : 15;
+};
+static_assert(sizeof(A) == 4);
+
+struct B {
+  unsigned int a : 15;
+           int b : 15;
+};
+static_assert(sizeof(B) == 4);
+
+struct C {
+  unsigned int a : 15;
+           int b : 15;
+};
+static_assert(sizeof(C) == 4);
+
+struct D {
+  Enum1 a : 15;
+  Enum1 b : 15;
+};
+static_assert(sizeof(D) == 4);
+
+struct E {
+  Enum1 a : 15;
+  Enum2 b : 15;
+};
+static_assert(sizeof(E) == 4);
+
+struct F {
+  EnumU32_1 a : 15;
+  EnumU32_2 b : 15;
+};
+static_assert(sizeof(F) == 4);
+
+struct G {
+  EnumU32_1 a : 15;
+  EnumU64 b : 15;
+  // expected-warning at -1 {{bit-field 'b' of type 'EnumU64' has a different storage size than the preceding bit-field (8 vs 4 bytes) and will not be packed under the MSVC ABI}}
+  // expected-note at -3 {{preceding bit-field 'a' declared here with type 'EnumU32_1'}}
+};
+
+#ifdef MS_BITFIELDS
+  static_assert(sizeof(G) == 16);
+#else
+  static_assert(sizeof(G) == 8);
+#endif
+
+struct H {
+  EnumU32_1 a : 10;
+  EnumI32 b : 10;
+  EnumU32_1 c : 10;
+};
+static_assert(sizeof(H) == 4);
+
+struct I {
+  EnumU8 a : 3;
+  EnumI8 b : 5;
+  EnumU32_1 c : 10;
+  // expected-warning at -1 {{bit-field 'c' of type 'EnumU32_1' has a different storage size than the preceding bit-field (4 vs 1 bytes) and will not be packed under the MSVC ABI}}
+  // expected-note at -3 {{preceding bit-field 'b' declared here with type 'EnumI8'}}
+};
+#ifdef MS_BITFIELDS
+static_assert(sizeof(I) == 8);
+#else
+static_assert(sizeof(I) == 4);
+#endif
+
+struct J {
+  EnumU8 : 0;
+  EnumU8 b : 4;
+};
+static_assert(sizeof(J) == 1);
+
+struct K {
+  EnumU8 a : 4;
+  EnumU8 : 0;
+};
+static_assert(sizeof(K) == 1);
+
+struct L {
+  EnumU32_1 a : 10;
+  EnumU32_2 b : 10;
+  EnumU32_1 c : 10;
+};
+
+static_assert(sizeof(L) == 4);
+
+struct M {
+  EnumU32_1 a : 10;
+  EnumI32 b : 10;
+  EnumU32_1 c : 10;
+};
+
+static_assert(sizeof(M) == 4);
+
----------------
ojhunt wrote:

@rnk thoughts?

https://github.com/llvm/llvm-project/pull/117428