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

On Sun, 21 Nov 2021 at 00:33, Fāng-ruì Sòng <maskray at google.com> wrote:
> On 2021-11-21, Momchil Velikov wrote:
> >         | nounwind 0  |  nounwind 1
> >----------+-------------+--------------
> >uwtable 0 | <full,no>   |  <no,no>
> >----------+-------------+--------------
> >uwtable 1 | <full,no>   |  <full,no>
> >----------+-------------+--------------
> >uwtable 2 | <full,min> | <min, min>
> >----------+-------------+--------------
> >uwtable 3 | <full,full> |  <full,full>
> >
> >where
> > - "full" means full unwind info - CFA, CSRs, return address
> > - "min" is minimal, sans CSRs,
> > - "no" is, well, no unwind info and
> > - "<p,e>" is the kind generated for prologues and
epilogues,
> respectively.
>
> uwtable 1/nounwind 1: <full,no>
> uwtable 2/nounwind 1: <min,min>
>
> Why is there a full->min transition for the generated prologue?
>
Because for a synchronous unwind table it makes only sense for the prologue
to be full, <min, no> is
unusable combination, whereas <full, no> is usable for a debugger
(it's
basically what we have now for most backends).

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On 2021-11-21, Momchil Velikov wrote:
>On Sun, 21 Nov 2021 at 00:33, Fāng-ruì Sòng <maskray at google.com>
wrote:
>
>> On 2021-11-21, Momchil Velikov wrote:
>> >         | nounwind 0  |  nounwind 1
>> >----------+-------------+--------------
>> >uwtable 0 | <full,no>   |  <no,no>
>> >----------+-------------+--------------
>> >uwtable 1 | <full,no>   |  <full,no>
>> >----------+-------------+--------------
>> >uwtable 2 | <full,min> | <min, min>
>> >----------+-------------+--------------
>> >uwtable 3 | <full,full> |  <full,full>
>> >
>> >where
>> > - "full" means full unwind info - CFA, CSRs, return
address
>> > - "min" is minimal, sans CSRs,
>> > - "no" is, well, no unwind info and
>> > - "<p,e>" is the kind generated for prologues and
epilogues,
>> respectively.
>>
>> uwtable 1/nounwind 1: <full,no>
>> uwtable 2/nounwind 1: <min,min>
>>
>> Why is there a full->min transition for the generated prologue?
>>
>
>Because for a synchronous unwind table it makes only sense for the prologue
>to be full, <min, no> is
>unusable combination, whereas <full, no> is usable for a debugger
(it's
>basically what we have now for most backends).
I wanted to ask why the prologue information has degraded from full to
min when transiting from uwtable 1 to uwtable 2.

I do not understand why moving from uwtable 1 to uwtable 2 is not monotonic.