[CIR] Upstream handling for FunctionToPointerDecay casts

[andykaylor]

Overview

Upstream support for handling implicit FunctionToPointerDecay casts in ClangIR.

Suggested minimal test case

int f1();
void f2() {
  int (*t)() = f1;
}

Existing incubator tests

clang/test/CIR/CodeGen/function-to-pointer-decay.c

[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: Andy Kaylor (andykaylor)

### Overview Upstream support for handling implicit FunctionToPointerDecay casts in ClangIR.

Suggested minimal test case

int f1();
void f2() {
  int (*t)() = f1;
}

Existing incubator tests

clang/test/CIR/CodeGen/function-to-pointer-decay.c

[andykaylor]

I have marked this as a "good first issue" but interested contributors should be aware of some special handling needed for working on ClangIR issues.

First, in order to even reproduce the problem you will need to build clang with ClangIR enabled. By default, clang does not build ClangIR support. You can find instructions for building with ClangIR enabled here.

Second, after building but before starting any other work, you should verify that you see an error when compiling the suggested minimal test case with ClangIR enabled. You should see an error (in this case, error: ClangIR code gen Not Yet Implemented: CastExpr: : FunctionToPointerDecay) when you compile the test case using the following options: -fclangir -emit-cir. The error may be followed by other errors or a stack dump. If you do not see an error, either someone else has already implemented this feature, or you are using the wrong repository (perhaps you built the ClangIR incubator repository?).

Third, the code to implement this feature is already present in the ClangIR incubator repository. The task here is to find the correct code, migrate it to the main LLVM repository, and creating a pull request to merge it there. As part of the migration process, you may need to update the code to conform to the coding standards used in the LLVM project. You will also need to create or migrate a LIT test that exercises the new code.

Don't hesitate to ask for help if you need it. The entire point of "good first issue" is to help new people get involved with our project.

[Arghnews]

I would like to work on this issue, please assign it to me

Thank you @andykaylor for your very helpful description on how to go about this