Re: GCC options for kernel live-patching (Was: Add a new option to control inlining only on static functions)

["Martin Liška" <mliska@suse.cz>]

@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 Hubièka. 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
>>>>
>>>
>