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

[Mehdi Amini via llvm-commits]

joker-eph wrote:

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

Technically true of course, and it would work with an empty file. But practically speaking without `undef` it's not clear to me how you can actually include multiple times a file to do anything useful?

> 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.

TableGen is not C, and lucky for us... It's pretty rare that people actually want the C preprocessor in a language! Not a model of design in general...

> But regardless, I do not see why that is relevant here. 

I don't quite get what you mean about relevance considering what I wrote above.

> #undef is only relevant in the sense that #ifdef can be used to write an include guard, but not always.

(I didn't understand this sentence)



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