[llvm] [llvm][Tablegen][llvm-tblgen] Added keyword #undef to llvm-tblgen (PR #69135)

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

> I have some concerns right now because this would technically allow to include the same TableGen file multiple times

That's already true if you don't have an include guard?

> which is something we've been recently discussing of taking advantage of forbidding, and making the #include basically #include_once.

I would have concerns about that; including a file multiple times can be useful, and diverging from C's preprocessor sounds like a bad idea and will lead to confusion.

But regardless, I do not see why that is relevant here. #undef is only relevant in the sense that #ifdef *can* be used to write an include guard, but not always.

https://github.com/llvm/llvm-project/pull/69135