@honza: PING
On 10/3/18 12:53 PM, Martin Liška wrote:
> On 10/3/18 11:04 AM, Jan Hubicka wrote:
>>>
>>> That was promised to be done by Honza Hubička. He's very skilled in IPA
>>> optimizations and he's aware
>>> of optimizations that cause troubles for live-patching.
>>
>> :) I am not sure how skilful I am, but here is what I arrived to.
>
> Heh! Thanks for the analysis.
>
>>
>> We have transformations that are modeled as clonning, which are
>> - inlining (can't be disabled completely because of always inline, but
>> -fno-inline
>> does most of stuff)
>> - cloning (disabled via -fno-ipa-cp)
>> - ipa-sra (-fno-ipa-sra)
>> - splitting (-fno-partial-inlining)
>> These should play well with Martin's tracking code
>
> I hope so!
>
>>
>> We propagate info about side effects of function:
>> - function attribute discovery (pure, const, nothrow, malloc)
>> Some of this can be disabled by -fno-ipa-pure-const, but not all
>> of it.
>
> Would it be possible to add option for the remaining ones?
>
> Nothrow does not have flag but it is obviously not a concern
>> for C++
>
> s/C++/C?
>
>> - ipa-pta (disabled by default, -fno-ipa-pta)
>> - ipa-reference (list of accessed/modified global vars), disable by
>> -fno-ipa-refernece
>> - stack alignment requirements (no flag to disable)
>
> Would it be possible to add flag for it? Can you please point to a location
> where
> the optimization happen?
>
>> - inter-procedural register allocation (-fno-ipa-ra)
>>
>> We perform discovery of functions/variables with no address taken and
>> optimizations that are not valid otherwise such as duplicating them
>> or doing skipping them for alias analysis (no flag to disable)
>
> Can you be please more verbose here? What optimizations do you mean?
>
>>
>> Identical code folding merges function bodies that are semanticaly
>> equivalent
>> and thus one can't patch one without patching another, -fno-ipa-icf
>
> Agree, I recommend disabling that.
>
>>
>> Unreachable code/variable removal may be concern too (no flag to disable)
>
> For functions that should be fine and handled by my script.
> For variables can be problem when a variable becomes alive But that
> should be extremely rare for live-patching.
>
>>
>> Write only global variable discovery (no flag to dosable)
>
> Similarly.
>
>>
>> Visibility changes with -flto and/or -fwhole-program
>>
>> We also have profile propagation (discovery of cuntions used only in cold
>> regions,
>> but that I guess is only performance issue not correctness)
>> No flag to disable
>
> Hope these 2 does not happen for current Linux kernel.
>
> Martin
>
>>
>> Honza
>>
>>>
>>> Martin
>>>
>>>>
>>>> thanks.
>>>>
>>>> Qing
>>>>
>>>
>
