[PATCH] D125645: [ARM SEH 3] [ARM] [MC] Add support for writing ARM WinEH unwind info

[Martin Storsjö via Phabricator via llvm-commits]

mstorsjo added a comment.

In D125645#3516878 <https://reviews.llvm.org/D125645#3516878>, @efriedma wrote:

> It looks like this doesn't include any directive to set the "F" bit in xdata,

That's true. If we'd have unwind info splitting implemented, that bit would be set on the follow-up fragments, implicitly. Maybe there's a usecase for manually creating such unwind data though? I think that one can be added without much troubles later, if there's a concrete demand for it though...

(On that note, when rereading the docs about the "F" bit, I'm left wondering how that's supposed to work. If we have a huge function, and describe a prologue-less fragment in the middle of the function - how will unwinding from it work, when there's no prologue opcodes in that fragment to use for unwinding, if unwinding starts from outside of any potential epilogue in it, as there also can be fragments without epilogues?)

> or the "Condition" field in epilogue scopes.  Is the omission intentional?  (I assume your LLVM patch won't emit sequences that would require either of those, but that might change, and people writing assembly might want them.)

Right, that's probably a bit bigger omission. Yeah it's not needed by our generated code, but it would probably be good to be able to express it if we wanted to. Maybe `.seh_startepilogue_cond <cond>` or something like that? (We'd probably want to have the default `.seh_startepilogue` as is - I think.)



================
Comment at: llvm/include/llvm/Support/Win64EH.h:79
+  UOP_WideSaveRegMask,
+  // Using UOP_SetFP from above
+  UOP_SaveRegsR4R7LR,
----------------
efriedma wrote:
> Not sure I like calling this operation UOP_SetFP, given it can set an arbitrary register.
Oh, right, I guess I brought over the design a bit too much from the aarch64 case here, while it's notably different here.

Should we call it `UOP_SaveSP` and `.seh_save_sp` then?


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