Re: Update LTO plugin interface

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Fri, Dec 3, 2010 at 4:49 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Fri, Dec 3, 2010 at 11:16 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> 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.
>>
>
> I checked the first patch into hjl/lto branch at
>
> http://git.kernel.org/?p=devel/binutils/hjl/x86.git;a=summary
>
> Now I can collect stage 2 command line.
>

I checked in a patch to implement stage 2 linking. Everything
seems to work, including "gcc -static ... -lm".


-- 
H.J.