Re: [GSoC]Bypass assembler when generating LTO object files

[Iain Sandoe <idsandoe@googlemail.com>]

> On 12 Apr 2022, at 13:31, Martin Liška <mliska@suse.cz> wrote:
> 
> On 4/12/22 11:58, Richard Biener wrote:
>> On Tue, Apr 12, 2022 at 11:20 AM Jan Hubicka via Gcc <gcc@gcc.gnu.org> wrote:
>>> 
>>> Hi,
>>>> 
>>>> 
>>>>> On 08-Apr-2022, at 6:32 PM, Jan Hubicka <hubicka@kam.mff.cuni.cz> wrote:
>>>>> 
>>>>> Ankur,
>>>>>> I was browsing the list of submitted GSoC projects this year and the
>>>>>> project regarding bypassing assembler when generating LTO object files
>>>>>> caught my eye.
>>>>> I apologize for late reply.  I would be very happy to mentor this
>>>>> project.
>>>> 
>>>> Thanks for the reply, but unfortunately, due to some reasons, I would not
>>>> be able to take part in GSoC this year.
>>>> But the project seems interesting and would be amazing opportunity to
>>>> learn a lot more things for me, so would it be okay if I try to give it a
>>>> go outside GSoC if no-one else picks it as their GSoC project this year ?
>>> 
>>> I would be still very happy to help with that! However it would be also
>>> pity to not take part of GSoC, so if there is something I can help with
>>> on that let me know.
>>>> 
>>>>>> 
>>>>>> I already have a gcc built from source (sync-ed with trunk/master) and
>>>>>> launched the test-suite on it.
>>>>>> 
>>>>>> I am currently in process of understanding the primilary patch
>>>>>> (https://gcc.gnu.org/legacy-ml/gcc/2014-09/msg00340.html), and
>>>>>> experimenting with it.
>>>>>> 
>>>>>> are there any other things I should be aware of (useful Doc/blog or a
>>>>>> bug tracking the project) before proceeding further ?
>>>>> 
>>>>> I think it is pretty much all that exists.  Basically we will need to
>>>>> implement everything that is necessary to stream out valid object file
>>>>> directly from GCC rather than going through gas.  The experimental
>>>>> prototype sort of worked but it was lacking few things.
>>>> 
>>>> When I try to apply that patch on my local branch ( branched from trunk ),
>>>> it seem to be incompatible with the current working tree. Is there a
>>>> specific branch that I have to apply it to ? or is it due to the recent
>>>> file rename patch ( changing extensions from .c to .cc ) ?
>>>> 
>>>> ```
>>>> $ git apply --check bypass_asm_patch
>>>> 
>>>> error: patch failed: Makefile.in:1300
>>>> error: Makefile.in: patch does not apply
>>>> error: common.opt: No such file or directory
>>>> error: langhooks.c: No such file or directory
>>>> error: lto/Make-lang.in: No such file or directory
>>>> error: lto/lto-object.c: No such file or directory
>>>> error: lto/lto.c: No such file or directory
>>>> error: lto/lto.h: No such file or directory
>>>> error: lto-streamer.h: No such file or directory
>>>> error: toplev.c: No such file or directory
>>>> ```
>>> 
>>> I can try to update the patch, or it probably should apply to trunk
>>> checked out around the date I sent the patch.  Indeed we need to change
>>> c to cc but there are likely more changes since then - most importnatly
>>> the early debug info.
>>> At I will see how easy/hard is to make the patch build with current
>>> trunk.
>> We do have ideas for the early debug with the asm-out abstraction to
>> also solve a different issue (missing simple-object support for mingw/darwin).
> 
> Note the debug info will be stored to a different .s file, then the file
> will be converted .o by GAS and then the bytecode will be stored to an ELF
> section of a compiled object. I've got also binutils patch when we'll
> be able to directly use the embedded section with compile.o@offset.

Which will not work, as written, for Darwin since that is neither ELF nor does it
use GAS - but hopefully, we can find some equivalent mechanism (there is already
some support in the Darwin backend for switching asm context for LTO output).

>> Namely assemble the early debug in a different file and include the resulting
>> native object in binary form in the compile output - not needing to write
>> assembly .data for that would be a good way to make this more viable.
> 
> Btw. do you have any estimation how slow is GAS when we speak about debug info?
> How much time can we save since the latest speed-up achieved by GAS?
> 
>> You might want to talk to Martin Liska for this who I think had some
>> prototype on this?
> 
> I can provide a prototype if needed.

I’d like to be in to loop from the Darwin PoV..
thanks
Iain

> 
> Cheers,
> Martin
> 
>> Richard.
>>> Honza
>>>> 
>>>> Thanks
>>>> - Ankur