llvm dev - Nov 2021 - [RFC] Asynchronous unwind tables attribute

>
> On one hand, we have the `uwtable` attribute in LLVM IR, which tells
> whether to emit CFI directives. On the other hand, we have the `clang
> -cc1` command-line option `-funwind-tables=1|2 ` and the codegen
> option `VALUE_CODEGENOPT(UnwindTables, 2, 0) ///< Unwind tables (1) or
> asynchronous unwind tables (2)`.
> Thus we lose along the way the information whether we want just some
> unwind tables or asynchronous unwind tables.
>
> Asynchronous unwind tables take more space in the runtime image, I'd
> estimate something like 80-90% more, as the difference is adding
> roughly the same number of CFI directives as for prologues, only a bit
> simpler (e.g. `.cfi_offset reg, off` vs. `.cfi_restore reg`). Or even
> more, if you consider tail duplication of epilogue blocks.
> Asynchronous unwind tables could also restrict code generation to
> having only a finite number of frame pointer adjustments (an example
> of*not*  having a finite number of `SP` adjustments is on AArch64 when
> untagging the stack (MTE) in some cases the compiler can modify `SP`
> in a loop).
> Having the CFI precise up to an instruction generally also means one
> cannot bundle together CFI instructions once the prologue is done,
> they need to be interspersed with ordinary instructions, which means
> extra `DW_CFA_advance_loc` commands, further increasing the unwind
> tables size.
>
> That is to say, async unwind tables impose a non-negligible overhead,
> yet for the most common use cases (like C++ exceptions), they are not
> even needed.
>
> We could, for example, extend the `uwtable` attribute with an optional
> value, e.g.
>    -  `uwtable` (default to 2)
>    -  `uwtable(1)`, sync unwind tables
>    -  `uwtable(2)`, async unwind tables
>    -  `uwtable(3)`, async unwind tables, but tracking only a subset of
> registers (e.g. CFA and return address)
>
> Or add a new attribute `async_uwtable`.
>
> Other suggestions? Comments?
>
Yes, thanks for bringing this up. OpenVMS on x86-64 needs full async
unwind tables as our system allows for exceptions to be handled during
both the prologue and epilogue.  We are currently avoiding the poor
quality of prologue/epilogue CFI information at present until we can
move forward to a recent LLVM version (long story) where I think the
support is better.

I like just having a uwtable level (1,2,3,etc).

John
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