================
@@ -7219,6 +7219,31 @@ its underlying representation to be a WebAssembly
``funcref``.
}];
}
+def PreferredTypeDocumentation : Documentation {
+ let Category = DocCatField;
+ let Content = [{
+This attribute allows adjusting the type of a bit-field in debug information.
+This can be helpful when a bit-field is intended to store an enumeration value,
+but has to be specified as having the enumeration's underlying type in order to
+facilitate compiler optimizations or bit-field packing behavior. Normally, the
+underlying type is what is emitted in debug information, which can make it hard
+for debuggers to know to map a bit-field's value back to a particular
enumeration.
+
+.. code-block:: c++
+
+ enum Colors { Red, Green, Blue };
+
+ struct S {
+ [[clang::preferred_type(Colors)]] unsigned ColorVal : 2;
+ [[clang::preferred_type(bool)]] unsigned UseAlternateColorSpace : 1;
+ } s = { Green, false };
+
+Without the attribute, a debugger is likely to display the value `1` for
`ColorVal`
+and `0` for `UseAlternateColorSpace`. With the attribute, the debugger may now
+display `Green` and `false` instead.
+ }];
----------------
AaronBallman wrote:
We should add additional documentation now that we've not limited this to just
enumeration types. Maybe something along the lines of:
```
This can be used to map a bit-field to an arbitrary type that isn't integral or
an enumeration type. For example:
.. code-block:: c++
struct A {
short a1;
short a2;
};
struct B {
[[clang::preferred_type(A)]] unsigned b1 : 32 = 0x000F'000C;
};
will associate the type ``A`` with the ``b1`` bit-field and is intended to
display something like this in the debugger:
.. code-block:: text
Process 2755547 stopped
* thread #1, name = 'test-preferred-', stop reason = step in
frame #0: 0x0000555555555148 test-preferred-type`main at test.cxx:13:14
10 int main()
11 {
12 B b;
-> 13 return b.b1;
14 }
(lldb) v -T
(B) b = {
(A:32) b1 = {
(short) a1 = 12
(short) a2 = 15
}
}
Note that debuggers may not be able to handle more complex mappings and so this
usage is debugger-dependent.
```
https://github.com/llvm/llvm-project/pull/69104
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