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

[Martin Liška]

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
>>>
>>