[llvm] [JITLink][XCOFF] Setup initial build support for XCOFF (PR #127266)

[Lang Hames via llvm-commits]

lhames wrote:

> ```
> In main resolving { (".main": 0xa00000000001000 [Callable]), ("main": 0xa00000000000000 [Data]) }
>   Flags for .main: expected 0x20 found 0x30 // expected Callabled, found Callable and Exported
>   Flags for main: expected 0x0 found 0x10 // expected None, found Exported
> ```

The initial JITSymbolFlags values should be created as part of object file interface construction [here](https://github.com/llvm/llvm-project/blob/6053ca004a58ed6a1bd10c5c470857cb6a2c0db7/llvm/lib/ExecutionEngine/Orc/ObjectFileInterface.cpp#L231), with flags for individual symbols within the interface computed [here](https://github.com/llvm/llvm-project/blob/6053ca004a58ed6a1bd10c5c470857cb6a2c0db7/llvm/lib/ExecutionEngine/RuntimeDyld/JITSymbol.cpp#L25). In particular I would expect these lines to set the `Exported` flag on every symbol that isn't explicitly hidden / local:
```
  if (!GV.hasLocalLinkage() && !GV.hasHiddenVisibility())
    Flags |= JITSymbolFlags::Exported;
```

It might be worth stepping through a test case in the debugger to see what interface is produced.

> Second, `llvm-mc` is not very well supported on AIX yet (the `.csect` directive is not supported) so it will not work for our case. I'm wondering if it's okay to have an C test case.

We can't use C test cases as llvm tests can't assume that clang is available (e.g. someone might be doing an LLVM-only build on a system with gcc as the only available compiler).

You can use obj2yaml to produce a yaml representation of your object if that helps, but if MC doesn't work then I suspect obj2yaml might also have issues.

Binary object files are allowable as a last resort.

> I'm considering leaving out the `-check` for the testcase (as we need to register some FileInfo for the checks to work, which > will come in the next patch) in favour of jit the object file directly on aix. It would be something like the following:
> 
> ```
> // REQUIRES: target=powerpc64-ibm-aix{{.*}}
> 
> // RUN: rm -rf %t && mkdir -p %t
> // RUN: clang --target=powerpc64-ibm-aix -c -O3 -fPIC -o %t/xcoff_ppc64.o %s
> // RUN: llvm-jitlink -triple=powerpc64-ibm-aix %t/xcoff_ppc64.o
> 
> int main(void) { return 0; }
> ```

That seems fine to me as a test case for the initial checkin, except that it'll need to be an assembly, yaml or binary test case rather than C.

https://github.com/llvm/llvm-project/pull/127266