[PATCH] D125582: [llvm-ml] Add support for extern proc

[Alan Zhao via Phabricator via llvm-commits]

ayzhao added a comment.

In D125582#3515465 <https://reviews.llvm.org/D125582#3515465>, @hans wrote:

>> Interestingly enough, under MSVC the following is allowed:
>>
>> extern foo:proc
>>
>> mov eax, foo
>>
>> MSVC will output:
>>
>> mov eax, 0
>>
>> while llvm-ml will currently output:
>>
>> mov eax, dword ptr [foo]
>>
>> (since foo is an extern)
>>
>> Arguably, llvm-ml's output makes more sense, even though it's
>>  inconsistent with MSVC ml. However, since moving an extern proc symbol
>>  to a register doesn't really make sense in the first place, we'll treat
>>  it as undefined behavior for now.
>
> Maybe I'm just not familiar enough with MASM, but both outputs seem odd to me. I would expect MSVC's output to be accompanied by a relocation, but it seems that's not the case?
>
> "mov eax, foo" reads like "move the address of foo into eax" to me, which seems like it would be a sensible thing to do. What llvm-ml does is more like "load from foo into eax" which seems odd if "foo" is a function.

It seems that the instruction for "move the address of foo into eax" should have a rip relative address anyways, just because `foo` is an extern. See: https://godbolt.org/z/Wsjq3KdvT

Based on that, I think llvm-ml is doing the right thing currently.

> In any case, if we can't support it in a sensible way, can we at least error about it? And there should be a test.




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