Re: [PATCH] IPA: support -flto + -flive-patching=inline-clone

["Martin Liška" <mliska@suse.cz>]

On 10/6/22 10:29, Richard Biener wrote:
> On Wed, Oct 5, 2022 at 8:18 PM Qing Zhao via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>>
>>
>>> On Oct 5, 2022, at 1:36 PM, Martin Liška <mliska@suse.cz> wrote:
>>>
>>> On 10/5/22 16:50, Qing Zhao wrote:
>>>> I have two questions on this:
>>>
>>> Hello.
>>>
>>>>
>>>> 1.  What’s the motivation to enable -flive-patching with -flto? Is there any application that will try -flive-patching with -flto now?
>>>
>>> We're planning supporting GCC LTO Linux kernel support, so that's one motivation. And the second one is a possible
>>> use in user-space livepatching. Note majority of modern distros default to -flto (openSUSE, Fedora, Debian, Ubuntu, ...).
>>
>> Okay, I see. That’s reasonable.
>>>
>>>>
>>>> 2. Why only enable -flive-patching=inline-clone with -flto?
>>>
>>> Because the inline-only-static level (which you added/requested) would have to properly
>>> block inter-procedural inlining that happens in LTO (can_inline_edge_by_limits_p) and
>>> I'm not sure it would be properly blocked. So, feel free to extend my patch if you want?
>>
>> -flive-patching=inline-only-static
>>
>> Only enable static functions inlining,  all the inlining of external visible functions are blocked, So, LTO should be compatible with this naturally without any issue, I think.
>>
>> i.e, when "-flive-patching=inline-only-static -flto"  present together, all the inter-procedural inlining should be automatically blocked by -flive-patching=inline-only-static already.
>>
>> Do I miss anything here?
> 
> WPA will promote externally visible functions static when all accesses
> are from LTO IR, I don't think we preserve
> the "original" visibility for IPA inlining heuristics.

That said, can we go with my original patch?

> 
> OTOH inline-only-static could disable WPA inlining and do all inlining early ...

And this can be extended by Oracle folks if they are interested in -flive-patching=inline-only-static
combined with LTO.

Martin

> 
> Richard,
> 
>>
>> thanks.
>>
>> Qing
>>
>>>
>>> Martin
>>>
>>>>
>>>> thanks.
>>>
>>