[OpenMP] Unused external variables are emitted when declared on the target

[jhuber6]

The following code snippet will cause the external variable a to be emitted in the IR even though it is not used. See also the godbolt link, https://godbolt.org/z/M5MsrnbnM.

extern int a;
#pragma omp declare target to(a) device_type(host)

int main() { return 0; }

The standard semantics maintain that unused extern variables will not be emitted. This is important for linking as linking in a static library that defines a will be extracted even though it's unused.

[llvmbot]

@llvm/issue-subscribers-openmp

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[h-vetinari]

I think this issue needs to be reopened an then use the /cherry-pick syntax for 141c4e7

[EugeneZelenko]

/cherry-pick 141c4e7

[llvmbot]

/branch llvm/llvm-project-release-prs/issue64133

[llvmbot]

/pull-request llvm/llvm-project-release-prs#502