[WebAssembly] `__has_attribute(musttail)` evaluates to true even when `musttail` is unsupported

[ahatanak]

$ cat test.cpp

#if __has_attribute(musttail)
#warning HAS IT
#else
#warning DOES NOT HAVE
#endif
void F(void) { [[clang::musttail]] return F(); }

$ clang++ -std=c++20  -target wasm32-unknown-unknown -c test.cpp
test.cpp:2:2: warning: HAS IT [-W#warnings]
    2 | #warning HAS IT
      |  ^
test.cpp:6:6: error: WebAssembly 'tail-call' feature not enabled
    6 | void F(void) { [[clang::musttail]] return F(); }
      |      ^
1 warning and 1 error generated.

[llvmbot]

@llvm/issue-subscribers-backend-webassembly

Author: Akira Hatanaka (ahatanak)

$ cat test.cpp ``` #if __has_attribute(musttail) #warning HAS IT #else #warning DOES NOT HAVE #endif void F(void) { [[clang::musttail]] return F(); } ```

$ clang++ -std=c++20 -target wasm32-unknown-unknown -c test.cpp
test.cpp:2:2: warning: HAS IT [-W#warnings]
2 | #warning HAS IT
| ^
test.cpp:6:6: error: WebAssembly 'tail-call' feature not enabled
6 | void F(void) { [[clang::musttail]] return F(); }
| ^
1 warning and 1 error generated.

[dschuff]

Yeah, probably this should evaluate to true only when the tail-call feature is enabled.

[AaronBallman]

Yeah, nothing here is looking at the target to see if it supports the attribute:

llvm-project/clang/lib/Sema/SemaStmt.cpp

Line 685 in b358af1

bool Sema::checkMustTailAttr(const Stmt *St, const Attr &MTA) {

We should add something to TargetInfo.h in Clang that different targets can override and then check that from SemaStmt.cpp. That handles the attribute being accepted at all. Then Attr.td should be modified to make MustTail a TargetSpecificAttr so that __has_attribute support behaves according to the target. This should be pretty straightforward, so marking as a good first issue. However, this might be the first target-specific statement attribute we support, so there could be some extra work involved.

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor is working on this issue. If someone is assigned to the issue or claimed to be working on it, ping the person. After one week without a response, the assignee may be changed.
2. Leave a comment indicating that you are working on the issue, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Akira Hatanaka (ahatanak)

$ cat test.cpp ```c #if __has_attribute(musttail) #warning HAS IT #else #warning DOES NOT HAVE #endif void F(void) { [[clang::musttail]] return F(); } ```

$ clang++ -std=c++20  -target wasm32-unknown-unknown -c test.cpp
test.cpp:2:2: warning: HAS IT [-W#warnings]
    2 | #warning HAS IT
      |  ^
test.cpp:6:6: error: WebAssembly 'tail-call' feature not enabled
    6 | void F(void) { [[clang::musttail]] return F(); }
      |      ^
1 warning and 1 error generated.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Akira Hatanaka (ahatanak)

$ cat test.cpp ```c #if __has_attribute(musttail) #warning HAS IT #else #warning DOES NOT HAVE #endif void F(void) { [[clang::musttail]] return F(); } ```

$ clang++ -std=c++20  -target wasm32-unknown-unknown -c test.cpp
test.cpp:2:2: warning: HAS IT [-W#warnings]
    2 | #warning HAS IT
      |  ^
test.cpp:6:6: error: WebAssembly 'tail-call' feature not enabled
    6 | void F(void) { [[clang::musttail]] return F(); }
      |      ^
1 warning and 1 error generated.

[ahatanak]

Can/should clang take into consideration whether the target supports musttail when evaluating __has_attribute(musttail)?

The user who reported this issue wanted to use __has_attribute(musttail) like this:

void F(void) {
#if __has_attribute(musttail)
  [[clang::musttail]]
#endif
  return F();
}

[AaronBallman]

Can/should clang take into consideration whether the target supports musttail when evaluating __has_attribute(musttail)?

I think it should.

The user who reported this issue wanted to use __has_attribute(musttail) like this:

void F(void) {
#if __has_attribute(musttail)
  [[clang::musttail]]
#endif
  return F();
}

Yeah, we'd want that code to not diagnose as using an unknown attribute.