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

On Wed, 9 Sep 2020 at 18:15, Min-Yih Hsu via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> David mentioned in D87337 that LLVM has used similar techniques on code
> size (not sure what he was referencing, my guess will be something related
> to hot-cold code splitting).
>
IIUC, it's just using optsize instead of optnone. The idea is that, if the
code really doesn't run often/at all, then the performance impact of
reducing the size is negligible, but the size impact is considerable.

I'd wager that optsize could even be faster than optnone, as it would
delete a lot of useless code... but not noticeable, as it wouldn't run much.

This is an idea that we (Verona Language) are interested in, too.
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200909/e397f06d/attachment.html>

Would it make sense to have a flag to select optnone or optsize? We would
probably also do the tradeoff for a smaller binary.

On Wed, Sep 9, 2020, 19:28 Renato Golin <rengolin at gmail.com> wrote:
> On Wed, 9 Sep 2020 at 18:15, Min-Yih Hsu via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> David mentioned in D87337 that LLVM has used similar techniques on code
>> size (not sure what he was referencing, my guess will be something
related
>> to hot-cold code splitting).
>>
>
> IIUC, it's just using optsize instead of optnone. The idea is that, if
the
> code really doesn't run often/at all, then the performance impact of
> reducing the size is negligible, but the size impact is considerable.
>
> I'd wager that optsize could even be faster than optnone, as it would
> delete a lot of useless code... but not noticeable, as it wouldn't run
much.
>
> This is an idea that we (Verona Language) are interested in, too.
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200909/37ccd617/attachment.html>

FYI David is referring to PGSO (profile-guided size optimization) as it exists
directly under that name, see: https://reviews.llvm.org/D67120. And yeah using
PGSO is selecting optsize while this change is selecting optnone.

On 9/9/20, 10:58 AM, "llvm-dev on behalf of Tobias Hieta via llvm-dev"
<llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces at
lists.llvm.org> on behalf of llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>> wrote:

Would it make sense to have a flag to select optnone or optsize? We would
probably also do the tradeoff for a smaller binary.

On Wed, Sep 9, 2020, 19:28 Renato Golin <rengolin at
gmail.com<mailto:rengolin at gmail.com>> wrote:
On Wed, 9 Sep 2020 at 18:15, Min-Yih Hsu via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
David mentioned in D87337 that LLVM has used similar techniques on code size
(not sure what he was referencing, my guess will be something related to
hot-cold code splitting).

IIUC, it's just using optsize instead of optnone. The idea is that, if the
code really doesn't run often/at all, then the performance impact of
reducing the size is negligible, but the size impact is considerable.

I'd wager that optsize could even be faster than optnone, as it would delete
a lot of useless code... but not noticeable, as it wouldn't run much.

This is an idea that we (Verona Language) are interested in, too.
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200910/ffc47f4d/attachment.html>