Re: [PATCH v3 2/8] lib: collect library files in an archive

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.12.2020 12:37, Bertrand Marquis wrote:
>> On 23 Nov 2020, at 15:21, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> In order to (subsequently) drop odd things like CONFIG_NEEDS_LIST_SORT
>> just to avoid bloating binaries when only some arch-es and/or
>> configurations need generic library routines, combine objects under lib/
>> into an archive, which the linker then can pick the necessary objects
>> out of.
>>
>> Note that we can't use thin archives just yet, until we've raised the
>> minimum required binutils version suitably.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>

Thanks.

>> @@ -60,7 +64,14 @@ include Makefile
>> # ---------------------------------------------------------------------------
>>
>> quiet_cmd_ld = LD      $@
>> -cmd_ld = $(LD) $(XEN_LDFLAGS) -r -o $@ $(real-prereqs)
>> +cmd_ld = $(LD) $(XEN_LDFLAGS) -r -o $@ $(filter-out %.a,$(real-prereqs)) \
>> +               --start-group $(filter %.a,$(real-prereqs)) --end-group
> 
> This might be a good idea to add a comment to explain why the start/end-group
> is needed so that someone does not change this back in the future.

Since we're trying to inherit Linux'es build system, I did look
there and iirc there was no comment, so I didn't see a basis for
us to have one.

> Something like: put libraries between start/end group to have unused symbols 
> removed.

Now that's not the reason - why you describe is the default
behavior for archives, and there is something like a "whole
archive" option iirc to change to a mode where all objects
get pulled out. Instead this is a symbol resolution thing
aiui - by default earlier archives can't resolve undefined
symbols first referenced by objects pulled out of later
archives.

Jan