llvm dev - Sep 2020 - [RFC] New Feature Proposal: De-Optimizing Cold Functions using PGO Info

On Wed, 9 Sep 2020 at 14:27, Nemanja Ivanovic <nemanja.i.ibm at gmail.com>
wrote:
> A more aesthetic comment I have is that personally, I would prefer a
> single option with a default percentage (say 0%) rather than having to
> specify two options.
>
0% doesn't mean "don't do it", just means "only do that
to functions I
didn't see running at all", which could be misrepresented in the
profiling
run.

If we agree this should be *always* enabled, then only one option is
needed. Otherwise, we'd need negative percentages to mean "don't do
that"
and that would be weird. :)


> Also, it might be useful to add an option to dump the names of functions
> that are decorated so the user can track an execution count of such
> functions when running their code. But of course, the debug messages may be
> adequate for this purpose.
>
Remark options should be enough for that.

--renato
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200909/71858aca/attachment.html>

On Wed, Sep 9, 2020 at 10:11 AM Renato Golin <rengolin at gmail.com>
wrote:
> On Wed, 9 Sep 2020 at 14:27, Nemanja Ivanovic <nemanja.i.ibm at
gmail.com>
> wrote:
>
>> A more aesthetic comment I have is that personally, I would prefer a
>> single option with a default percentage (say 0%) rather than having to
>> specify two options.
>>
>
> 0% doesn't mean "don't do it", just means "only do
that to functions I
> didn't see running at all", which could be misrepresented in the
profiling
> run.
>
> If we agree this should be *always* enabled, then only one option is
> needed. Otherwise, we'd need negative percentages to mean
"don't do that"
> and that would be weird. :)
>
> I am not sure I follow. My suggestion was to have one option that would
give you a default of 0% (i.e. only add the attribute on functions that
were never called). So the semantics would be fairly straightforward:
- Default (i.e. no -profile-deopt-cold): do nothing
- Option with no arg (i.e. -profile-deopt-cold): add attribute only to
functions that have an execution count of zero
- Option with an arg (i.e. -profile-deopt-cold=<N>): add attribute to
functions that account for <N>% of total execution counts

>
>> Also, it might be useful to add an option to dump the names of
functions
>> that are decorated so the user can track an execution count of such
>> functions when running their code. But of course, the debug messages
may be
>> adequate for this purpose.
>>
>
> Remark options should be enough for that.
>
> --renato
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200909/4b2a852f/attachment-0001.html>

On Wed, 9 Sep 2020 at 19:26, Nemanja Ivanovic <nemanja.i.ibm at gmail.com>
wrote:
> - Default (i.e. no -profile-deopt-cold): do nothing
> - Option with no arg (i.e. -profile-deopt-cold): add attribute only to
> functions that have an execution count of zero
> - Option with an arg (i.e. -profile-deopt-cold=<N>): add attribute to
> functions that account for <N>% of total execution counts
>
I see. This looks confusing to me, but perhaps it's just me.

Though, I'm not sure we can get this behaviour from the same flag, as you
need to provide a default value if the flag isn't passed (usually boolean
or integer, not both).
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200909/9f840692/attachment.html>