[PATCH][RFC] Come up with -flive-patching master option.

[Qing Zhao]

Hi,

> On Nov 13, 2018, at 1:18 PM, Miroslav Benes <mbenes@suse.cz> wrote:
> 
>> Attached is the patch for new -flive-patching=[inline-only-static | inline-clone] master option.
>> 
>> '-flive-patching=LEVEL'
>>     Control GCC's optimizations to provide a safe compilation for
>>     live-patching.  Provides multiple-level control on how many of the
>>     optimizations are enabled by users' request.  The LEVEL argument
>>     should be one of the following:
>> 
>>     'inline-only-static'
>> 
>>          Only enable inlining of static functions, disable all other
>>          ipa optimizations/analyses.  As a result, when patching a
>>          static routine, all its callers need to be patches as well.
>> 
>>     'inline-clone'
>> 
>>          Only enable inlining and all optimizations that internally
>>          create clone, for example, cloning, ipa-sra, partial inlining,
>>          etc.; disable all other ipa optimizations/analyses.  As a
>>          result, when patching a routine, all its callers and its
>>          clones' callers need to be patched as well.
> 
> Based on our previous discussion I assume that "clone" optimizations are 
> safe (for LP) and the others are not. Anyway I'd welcome a note mentioning 
> that disabled optimizations are dangerous for LP.

actually, I don’t think that those disabled optimizations are “dangerous” for live-patching. one of the major reasons we disable them
is because that currently the compiler does NOT provide a good way to compute the impacted function list for those optimizations.
therefore, we disable them at this time. 

many of them could be enabled too if the compiler can report the impacted function list accurately in the future.



> 
> I know it may be the same for you, but it is not for me as a GCC user. 
> "internally create clone" sounds very... well, internal. It does not 
> describe the option much for ordinary user whow has no knowledge about GCC 
> internals.
> 
> So could you rephrase it a bit, please?

I tried to make this clear. please see the following:

'-flive-patching=LEVEL'
     Control GCC's optimizations to provide a safe compilation for
     live-patching.

     If the compiler's optimization uses a function's body or
     information extracted from its body to optimize/change another
     function, the latter is called an impacted function of the former.
     If a function is patched, its impacted functions should be patched
     too.

     The impacted functions are decided by the compiler's
     interprocedural optimizations.  For example, inlining a function
     into its caller, cloning a function and changing its caller to call
     this new clone, or extracting a function's pureness/constness
     information to optimize its direct or indirect callers, etc.

     Usually, the more ipa optimizations enabled, the larger the number
     of impacted functions for each function.  In order to control the
     number of impacted functions and computed the list of impacted
     function easily, we provide control to partially enable ipa
     optimizations on two different levels.

     The LEVEL argument should be one of the following:

     'inline-only-static'

          Only enable inlining of static functions, disable all other
          interprocedural optimizations/analyses.  As a result, when
          patching a static routine, all its callers need to be patches
          as well.

     'inline-clone'

          Only enable inlining and cloning optimizations, which includes
          inlining, cloning, interprocedural scalar replacement of
          aggregates and partial inlining.  Disable all other
          interprocedural optimizations/analyses.  As a result, when
          patching a routine, all its callers and its clones' callers
          need to be patched as well.

     When -flive-patching specified without any value, the default value
     is "inline-clone".

     This flag is disabled by default.


> 
>>     When -flive-patching specified without any value, the default value
>>     is "inline-clone".
>> 
>>     This flag is disabled by default.
>> 
>> let me know your comments and suggestions on the implementation.
> 
> I compared it to Martin's patch and ipa-icf-variables is not covered in 
> yours (I may have missed something).

Yes, you are right. I added this into my patch.

I am attaching the new patch here.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: flive-patching.patch
Type: application/octet-stream
Size: 10735 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/gcc-patches/attachments/20181113/2cee1a67/attachment.obj>
-------------- next part --------------


> 
> Thanks,
> Miroslav