[lelouch11]

Hi, I had a general question regarding intrinsics which you might be able to answer. `llvm.stacksave` and `llvm.stackrestore` are used for implementing VLAs in C (to my knowledge). Is it required to emit those for variable size allocas, or could you skip them? If skipping is possible, how is the case handled when the basic block does not post-dominate an exit block?

[woodruffw]

AFAIK, LLVM doesn’t actually require those intrinsics for VLAs — the `alloca` instruction can take a non-immediate for the stack object size. I can’t remember off the top of my head whether I’ve seen LLVM consistently include the intrinsics as well.

As for the dominator case: it’s possible that it does need them then. But for a really trivial VLA (like at the level of the function’s frame) it shouldn’t.

[jcranmer]

The only case I recall seeing @llvm.{stacksave,stackrestore} use was when LLVM inlined a function with a non-entry-block-alloca into another function, where the intrinsics got added immediately before/after what used to be the function call.