Re: [GSOC] few question about Bypass assembler when generating LTO object files

[Martin Jambor <mjambor@suse.cz>]

Hi,

On Tue, Apr 04 2023, Rishi Raj wrote:
> Thanks, Jan for the Reply! I have completed a draft proposal for this
> project. I will appreciate your's, Martin's, or anybody else feedback on
> the same.
> Here is the link to my proposal
> https://docs.google.com/document/d/1r9kzsU96kOYfIhWZx62jx4ALG-J_aJs5U0sDpwFUtts/edit?usp=sharing
>

in my opinion, the proposal looks rather good.  I don't know how to
significantly improve it, not in the remaining nine hours before the
deadline (just maybe fix the spelling of Honza's surname, it is Hubicka
:-). So please go ahead and submit it. (How the selection goes will then
depend on how many slots we get from Google).

Thanks for putting in the effort,

Martin


> On Tue, 4 Apr 2023 at 04:35, Jan Hubicka <hubicka@ucw.cz> wrote:
>
>> Hello,
>> > While going through the patch and simple-object.c I understood that the
>> > file simple-object.c is used to handle the object file format. However,
>> > this file does not contain all the architecture information required for
>> > LTO object files, so the workaround used in the patch is to read the
>> > crtbegin.o file and merge the missing attributes. While this workaround
>> is
>> > functional, it is not optimal, and the ideal solution would be to extend
>> > simple-object.c to include the missing information.
>>
>> Yes, simple-object.c simply uses architecture settings it read earlier
>> which is problem since at compile time we do not read any object files,
>> just parse sources). In my original patch the architecture flags were
>> simply left blank.  I am not sure if there is a version reading
>> crtbeing.o which would probably not a be that bad workaround, at least
>> for the start.  Having a way to specify this from the machine descriptions
>> would be better.
>>
>
>
>>
>> Besides the architecture bits, for simple-object files to work we need
>> to add the symbol table. For practically useful information we also need
>> to stream the debug info.
>>
>>
>> > Regarding the phrase "Support in the driver to properly execute *1
>> binary",
>> > it is not entirely clear what it refers to. My interpretation is that the
>> > compiler driver (the program that coordinates the compilation process)
>> > needs to be modified to correctly output LTO object files instead of
>> > assembler files (the current approach involves passing the -S and -o
>> > <obj_file_name>.o options) and also skip the assembler option while using
>> > -fbypass-asm option but I am not sure. Can Jan or Martin please shed some
>> > light on this?
>> Yes, compiler drivers decides what to do and it needs to know that with
>> -flto it does not need to produce assembly file and then invoke gas.  If
>> we go the way of reading in crtbegin.o it will also need to pass correct
>> crtbegin to *1 binary.  This is generally not that hard to do, just
>> needs to be done :)
>>
> Honza
>> >
>> > Thanks & Regards
>> >
>> > Rishi Raj
>> >
>> > On Sun, 2 Apr 2023 at 03:05, Rishi Raj <rishiraj45035@gmail.com> wrote:
>> >
>> > > Hii Everyone,
>> > > I had already expressed my interest in the " Bypass assembler when
>> > > generating LTO object files" project and making a proposal for the
>> same. I
>> > > know I should have done it earlier but I was admitted to the hospital
>> for
>> > > past few days :(.
>> > > I have a few doubts.
>> > > 1)
>> > >
>> > > "One problem is that the object files produced by
>> libiberty/simple-object.c
>> > > (which is the low-level API used by the LTO code)
>> > > are missing some information (such as the architecture info and symbol
>> > > table) and API of the simple object will need to be extended to handle
>> > > that" I found this in the previous mailing list discussion. So who
>> output this information currently in the object file, is it assembler?
>> > >
>> > > Also in the current patch for this project by Jan Hubica, from where
>> are we getting these information from? Is it from crtbegin.o?
>> > >
>> > > 2)
>> > > "Support in driver to properly execute *1 binary." I found this on Jan
>> original patch's email. what does it mean
>> > >
>> > > exactly?
>> > >
>> > > Regards
>> > >
>> > > Rishi Raj
>> > >
>> > >
>> > >
>> > >
>>