[PATCH] D135103: RFC: [AArch64] Add SEH_Nop for PAC/BTI instructions in prologues/epilogues

[Martin Storsjö via Phabricator via llvm-commits]

mstorsjo added a comment.

In D135103#3832085 <https://reviews.llvm.org/D135103#3832085>, @efriedma wrote:

> For the following example:
>
>   int f(void g(), int a) { g(); return a; }
>
> If I compile with the following command:
>
>   cl /c a.c /d2guardsignret
>
> llvm-readobj gives me the following:
>
>   File: a.obj
>   Format: COFF-ARM64
>   Arch: aarch64
>   AddressSize: 64bit
>   UnwindInformation [
>     RuntimeFunction {
>       Function: f (0x0)
>       ExceptionRecord: $unwind$f (0x0)
>       ExceptionData {
>         FunctionLength: 40
>         Version: 0
>         ExceptionData: No
>         EpiloguePacked: Yes
>         EpilogueOffset: 0
>         ByteCodeLength: 8
>         Prologue [
>           0xd600              ; stp x19, lr, [sp, #0]
>           0x01                ; sub sp, #16
>           0xfc                ; Bad opcode!
>           0xe4                ; end
>         ]
>       }
>     }
>   ]
>
> So apparently there is, in fact, a way to encode this, using the undocumented opcode 0xfc.  Why this isn't documented, I have no idea.

I tried this out - apparently this option has been around since some time in MSVC 2019 at least. And apparently it's also possible to invoke this with `/guard:signret`. This generates the `pacibsp` instruction - I wonder if there's a way to make it generate `paciasp` instead - which I presume would require a separate unwinding opcode to distinguish from `pacibsp`?

However, curiously, I don't get the output you got, if I compile with the same command - I have to build with `/O2` for it to produce this unwind info. Without `/O2`, I get packed unwind, with `CR=2` (which is defined as reserved) - which apparently is a way to handle signed return addresses in packed unwind info too.

I filed https://github.com/MicrosoftDocs/cpp-docs/pull/4202 as an initial attempt to document this (and hopefully poke the right people to document it publicly).


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