[PATCH] D55483: Introduce the callback attribute and emit !callback metadata

[Johannes Doerfert via Phabricator via llvm-commits]

jdoerfert marked 2 inline comments as done.
jdoerfert added a comment.

In D55483#1326029 <https://reviews.llvm.org/D55483#1326029>, @aaron.ballman wrote:

> This is missing all of the Sema and SemaCXX tests. Should have tests for member functions, variadic functions, incorrect arguments, incorrect subjects, etc.


I will write more tests and update this revision.



================
Comment at: include/clang/Basic/Attr.td:1204
+              VariadicUnsignedArgument<"PayloadIndices">];
+  let Subjects = SubjectList<[Function]>;
+  let Documentation = [Undocumented];
----------------
aaron.ballman wrote:
> Should this also apply to Objective-C methods?
> 
> Why should the user specify this attribute on the function as opposed to on the parameter? e.g.,
> ```
> // Why this:
> __attribute__((callback (1, 2, 3)))
> void* broker0(void* (*callee)(void *), void *payload, int otherPayload) {
>   return callee(payload);
> }
> 
> // Instead of this:
> void* broker0(void* (*callee)(void *) __attribute__((callback (2, 3))), void *payload, int otherPayload) {
>   return callee(payload);
> }
> 
> // Or this:
> void* broker0(void* (*callee)(void *) __attribute__((callback (payload, otherPayload))), void *payload, int otherPayload) {
>   return callee(payload);
> }
> ```
> I ask because these "use an index" attributes are really hard for users to use in practice. They have to account for 0-vs-1 based indexing, implicit this parameters, etc and if we can avoid that, it may be worth the effort.
> Should this also apply to Objective-C methods?

I don't need it to and unless somebody does, I'd say no.


> I ask because these "use an index" attributes are really hard for users to use in practice. They have to account for 0-vs-1 based indexing, implicit this parameters, etc and if we can avoid that, it may be worth the effort.

I was thinking that the function notation makes it clear that there is *only one callback per function* allowed right now. I don't expect many manual users of this feature until we improve the middle-end support, so it is unclear to me if this requirement needs to be removed as well.

Other than that, some thoughts: 
- I do not feel strongly about this.
- The middle requirement seems not much better n the first, we would still need to deal with index numbers (callbacks without arguments are not really interesting for now). 
- The last encoding requires us to define a symbol for "unknown argument" (maybe _ or ?).


================
Comment at: include/clang/Basic/Attr.td:1205
+  let Subjects = SubjectList<[Function]>;
+  let Documentation = [Undocumented];
+}
----------------
aaron.ballman wrote:
> No new undocumented attributes, please.
Ok, I can write documentation similar to the commit message and the lang-ref documentation for the callback metadata.


Repository:
  rC Clang

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https://reviews.llvm.org/D55483