[llvm-dev] [RFC] Asynchronous unwind tables attribute

[Momchil Velikov via llvm-dev]

On Sat, 11 Dec 2021 at 00:47, Eric Christopher <echristo at gmail.com> wrote:
> Ultimately I think I'd like to know why you think you should do this, hence the request for use cases :)

Right, so the use cases I have in mind are:
* synchronous exceptions: these need non-instruction precise unwind
tables, correct at function calls; generally all the callee-saved
registers need to be described
   no `uwtable`, `uwtable(1)` support this case; `uwtable(2)` too, but
is needlessly precise and verbose
* debugging: best debugging experience would be instruction precise
unwind tables,
 over the whole function, however, there is a bit of a wiggle room:
   - unwind tables may not be instruction precise, essentially at the
same level of precision as the case for exceptions
   - unwind tables for epilogues might be missing
   This is what we have in most backends now. `uwtable(2)` supports
this case, `uwtable(1)` results in smaller unwind tables, at the cost
of a degraded user experience.
* sampling profilers: for best precision need instruction precise
unwind tables both in the prologue and in the epilogue (which makes
for comparatively large unwind tables)
  `uiwtable(2)` supports this case.
  In principle, there is no need to describe all the callee-saved
registers, just those needed to get a function's caller, so, whatever
  passes for  "return address, "SP", and "FP", and others.  In the
course of the discussion, we agreed that's best addressed by a
separate mechanism.

~chill