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

[Rishi Raj <rishiraj45035@gmail.com>]

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---------- Forwarded message ---------
From: Rishi Raj <rishiraj45035@gmail.com>
Date: Tue, 4 Apr 2023 at 05:57
Subject: Re: [GSOC] Submission of draft proposal.
To: Jan Hubicka <hubicka@ucw.cz>
Cc: <gcc@gcc.gnu.org>, <mjambor@suse.cz>
oops, I forgot to change the subject in previous email :(

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

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
> > >
> > >
> > >
> > >
>