================
@@ -633,6 +633,50 @@ def Packed : DiagGroup<"packed", [PackedNonPod]>;
def PaddedBitField : DiagGroup<"padded-bitfield">;
def Padded : DiagGroup<"padded", [PaddedBitField]>;
def UnalignedAccess : DiagGroup<"unaligned-access">;
+def MSBitfieldCompatibility : DiagGroup<"ms-bitfield-compatibility"> {
+ code Documentation = [{
+ Under the MSVC ABI adjacemt bit-fields are not packed if the underlying
type
+ has a different storage size. This warning indicates that a pair of
adjacent
+ bit-fields may not pack in the same way due to this behavioural difference.
+
+ This can occur when mixing different types explicitly:
+
+ .. code-block:: c++
+
+ struct S {
+ uint16_t field1 : 1;
+ uint32_t field2 : 1;
+ };
+
+ or more subtly through enums
+
+ .. code-block:: c++
+
+ enum Enum1 { /* ... */ };
+ enum class Enum2 : unsigned char { /* ... */ };
+ struct S {
+ Enum1 field1 : 1;
+ Enum2 field2 : 1;
+ };
+
+ In each of these cases under the MSVC ABI the second bit-field will not be
+ packed with the preceding bit-field, and instead will be aligned as if the
+ fields were each separately defined integer fields of their respective
storage
+ size. For binary compatibility this is obviously and observably
incompatible,
+ however where bit-fields are being used solely for memory use reduction
this
+ incomplete packing may silently increase the size of objects vs what is
+ expected.
+
+ This issue can be addressed by ensuring the storage type of each bit-field
is
+ the same, either by explicitly using the same integer type, or in the case
of
+ enum types declaring the enum types with the same storage size. For enum
types
+ where you cannot specify the underlying type, the options are to either
switch
+ to int sized storage for all specifiers or to resort to declaring the
+ bit-fields with explicit integer storage types and cast in and out of the
field.
+ If such a solution is required the `preferred_type` attribute can be used
to
+ convey the actual field type to debuggers and other tooling.
----------------
ojhunt wrote:
Took a while to work out how, but done
https://github.com/llvm/llvm-project/pull/117428
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