Update LTO plugin interface

H.J. Lu hjl.tools@gmail.com
Fri Dec 3 19:17:00 GMT 2010


On Fri, Dec 3, 2010 at 11:10 AM, Cary Coutant <ccoutant@google.com> wrote:
>>>> Another way to do this would be to put a marker in the command line
>>>> that identifies where those libraries begin, and the linker could just
>>>> go back and rescan those libraries if needed, before the final layout
>>>> of the endcaps.
>>>
>>> I like that idea in general, but the difficulty is knowing where to put
>>> the marker.  E.g., the user is going to specify -lm, and we are going to
>>> need to rescan it.  If the user writes -lm -lmylib1 -lmylib2 we want to
>>> rescan -lm but we don't really need to rescan mylib1 and mylib2.
>>
>> All those complexities make 2 stage linking more attractive.  I
>> think I can implement it in GNU linker with the current plugin API.
>>
>> Linker just needs to remember the command line options, including
>>
>> --start-group/--end-group
>> -as-needed/--no-as-needed
>> --copy-dt-needed-entries/--no-copy-dt-needed-entries
>>
>> in stage 1.
>>
>> In stage 2, it will place LTO trans files before the first IR file
>> claimed by plugin and process the command line options.
>>
>> --whole-archive may need some special handling.  Archives
>> after --whole-archive will be ignored in stage 2.
>
> It seems to me that we just need to add a few more libraries as
> pass-through libraries, being careful to add a pass-through option
> only for libraries that are already on the command line. How does that
> add up to "all those complexities"?
>
> With what you've written here, you've just added to the complexity of
> your proposed solution, which makes it a much bigger change --
> especially since what you're proposing will require changes in both
> linkers. Adding pass-through options is a gcc driver change only.
>

I will implement 2 stage linking in GNU linker with the current plugin API.
The change shouldn't be too big.  pass-through isn't needed.  If we
keep it in GCC driver, my linker will simply ignore it.

One benefit is everything will just work, with or without LTO.

-- 
H.J.



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