llvm dev - Jan 2021 - [RFC] Backend for Motorola 6800 series CPU (M68k)

Hi All,

Another update on this topic: Currently three patches have been approved, thanks
for all reviewers’ time and efforts :-)

Here are the remaining patches:
 - Patch 1/8 D88385: Got one LGTM, however I believe Jessica Clarke (jrtc27)
(CC-ed here) is listed as blocker. Does anyone here know the best way to contact
Jessica? Since the patch has been blocked for quite a long time and pinging
@jrtc27 on Phab didn’t seem to work
 - Patch 3/8 D88389   (Basic .td files)
 - Patch 4/8 D88390   (MC layer)
 - Patch 5/8 D88391   (Target lowering)
 - Patch 6/8 D88392   (IR Tests)

Except Patch 6, which only has minor formatting issues to fix, I didn’t see any
major objections or concerns on other patches at this time point.

In addition to the patch review, tomorrow Dec 18th 19:00 CET (10:00 AM PST),
Nick Desaulniers is going to give a talk on compiling Linux Kernel for M68k
using LLVM.
Please checkout http://m68k.info/ <http://m68k.info/> for more info about
the live stream

Best,
-Min
> On Dec 4, 2020, at 1:03 AM, John Paul Adrian Glaubitz <glaubitz at
physik.fu-berlin.de> wrote:
> 
> Hi Min!
> 
> On 12/4/20 8:32 AM, Min-Yih Hsu wrote:
>> Another major update on this topic: currently I’ve addressed most of
the feedbacks
>> on the Phabricator patches. (I think only one was left and that’s
related to Windows,
>> but since our own buildbots only run non-Windows system I think it’s
fine for now)
>> 
>> More importantly, I’ve fixed rest of the test failures, so now except
one XFAIL test
>> case, which takes more time to fix it, M68k passes all of its tests now
:-)
>> 
>> Personally, I think the M68k backend is in good shape to become an
experimental LLVM backend.
>> 
>> Currently only Patch 7 (D88393) has Renato’s semi-LGTM, and he was also
asking other
>> people’s opinion and LGTM. It would be great if anyone can help
reviewing and approving
>> these patches.
>> 
>> The journey of getting M68k backend upstream has gone pretty far and
again, I really
>> appreciate feedbacks and helps from reviewers and everyone on this
thread!
> 
> Congratulations on this important milestone and the fantastic work you
delivered.
> 
> I would have never expected that this effort would ever get this far, so
I'm more
> than impressed and happy about the news.
> 
> The backend is also already actively being used in another project, namely
a compiler
> explorer instance which features various compilers for m68k including your
LLVM
> port [1].
> 
> And, secondly, Nick Desaulniers will be giving a short talk on December
18th, on our
> monthly m68k online meeting [2] where he will be talking about compiling
the Linux
> kernel for m68k using your new LLVM backend :-).
> 
> Adrian
> 
>> [1] http://brownbot.mooo.com/
>> [2] http://m68k.info/
> 
> -- 
> .''`.  John Paul Adrian Glaubitz
> : :' :  Debian Developer - glaubitz at debian.org
> `. `'   Freie Universitaet Berlin - glaubitz at physik.fu-berlin.de
>  `-    GPG: 62FF 8A75 84E0 2956 9546  0006 7426 3B37 F5B5 F913
> 
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Hi Min!

On 12/18/20 1:21 AM, Min-Yih Hsu wrote:
> Another update on this topic: Currently three patches have been approved,
thanks for all reviewers’ time and efforts :-)
I've read the reviews. Awesome progress.
> Here are the remaining patches:
>  - Patch 1/8 D88385: Got one LGTM, however I believe Jessica Clarke
(jrtc27) (CC-ed here) is listed as blocker.
     Does anyone here know the best way to contact Jessica? Since the patch has
been blocked for quite a long time
     and pinging @jrtc27 on Phab didn’t seem to work

I pinged jrtc27 on IRC (OFTC) and will keep pinging until we get an answer.
>  - Patch 3/8 D88389   (Basic .td files)
>  - Patch 4/8 D88390   (MC layer)
>  - Patch 5/8 D88391   (Target lowering)
>  - Patch 6/8 D88392   (IR Tests)
> 
> Except Patch 6, which only has minor formatting issues to fix, I didn’t see
any major
> objections or concerns on other patches at this time point.
Sounds great.
> In addition to the patch review, tomorrow Dec 18th 19:00 CET (10:00 AM
PST), Nick Desaulniers
> is going to give a talk on compiling Linux Kernel for M68k using LLVM.
> Please checkout http://m68k.info/ <http://m68k.info/> for more info
about the live stream
Yep. I'm very exciting for this talk! And we're planning to put it on
Youtube later.

Adrian

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer - glaubitz at debian.org
`. `'   Freie Universitaet Berlin - glaubitz at physik.fu-berlin.de
  `-    GPG: 62FF 8A75 84E0 2956 9546  0006 7426 3B37 F5B5 F913

Hi All,

Happy New Year!

This is another update on the M68k backend.

Currently we still have 3 patches to review:
1. (1/8) TableGen changes https://reviews.llvm.org/D88385
<https://reviews.llvm.org/D88385>
2. (4/8) MC layer changes https://reviews.llvm.org/D88390
<https://reviews.llvm.org/D88390>
3. (5/8) Target Lowering https://reviews.llvm.org/D88391
<https://reviews.llvm.org/D88391>

4th (D88390) and 5th (D88391) patches only have minor formatting issues. 

The biggest challenge now is in the first patch, D88385. Currently the patch
proposed to add a new TableGen backend (CodeBeads) to generate auxiliary data
structures for variable length encoding as well as complex addressing mode
encodings. This is equivalent to X86’s usage of `MCInstrDesc::TSFlags` to carry
their instruction encoding. However, TSFlags only has 64 bits, where M68k’s
current design needs at least 24 bytes to encode all the instructions. While we
might able to find an optimal way to fit M68k’s info into 64 bits in the future,
the refactoring efforts to do it now is huge, especially on changing
instructions definitions in TG files.

Jessica, who is listed in the blocker list of this patch, raised concerns about
the necessity of this specialized TG backend, and proposed to re-use X86’s way
to handle this problem. That is, put the M68k’s encoding info into MCInstrDesc
with modifications in the InstrInfoEmitter TG backend.

Simon thought special handling in the InstrInfoEmitter/MCInstrDesc for M68k
doesn’t have a huge difference with adding a specialized CodeBeads backend like
the patch is doing now.

Renado preferred to keep the CodeBeads backend since it works well for M68k and
later refactoring, if there is any, will probably only affect M68k. He also
thought “where” to put this implementation - whether a TableGen or MCInstrDesc
change - should come before asking “why” doing this change.

Personally, I also prefer to keep the current CodeBeads implementation and
optimize it in the future. Since:
1. CodeBeads is an optional TG backend. It will not affect any existing Target. 
2. As mentioned earlier, it works well on M68k now. And if there is any changes
on it or even removal on this TG backend in the future, the refactoring cost is
pretty low.
3. We don’t have many resources to conduct large scale fixes in a short time
(the longer this patch series diverged from upstream, the higher the rebasing
cost for other parts of M68k backend) on this issue right now.

I would happy to hear if there is any suggestion on solving this challenge.

Thank you for the all the reviewing efforts!

Best,
-Min
> On Dec 17, 2020, at 11:42 PM, John Paul Adrian Glaubitz <glaubitz at
physik.fu-berlin.de> wrote:
> 
> Hi Min!
> 
> On 12/18/20 1:21 AM, Min-Yih Hsu wrote:
>> Another update on this topic: Currently three patches have been
approved, thanks for all reviewers’ time and efforts :-)
> 
> I've read the reviews. Awesome progress.
> 
>> Here are the remaining patches:
>> - Patch 1/8 D88385: Got one LGTM, however I believe Jessica Clarke
(jrtc27) (CC-ed here) is listed as blocker.
>     Does anyone here know the best way to contact Jessica? Since the patch
has been blocked for quite a long time
>     and pinging @jrtc27 on Phab didn’t seem to work
> 
> I pinged jrtc27 on IRC (OFTC) and will keep pinging until we get an answer.
> 
>> - Patch 3/8 D88389   (Basic .td files)
>> - Patch 4/8 D88390   (MC layer)
>> - Patch 5/8 D88391   (Target lowering)
>> - Patch 6/8 D88392   (IR Tests)
>> 
>> Except Patch 6, which only has minor formatting issues to fix, I didn’t
see any major
>> objections or concerns on other patches at this time point.
> 
> Sounds great.
> 
>> In addition to the patch review, tomorrow Dec 18th 19:00 CET (10:00 AM
PST), Nick Desaulniers
>> is going to give a talk on compiling Linux Kernel for M68k using LLVM.
>> Please checkout http://m68k.info/ <http://m68k.info/> for more
info about the live stream
> 
> Yep. I'm very exciting for this talk! And we're planning to put it
on Youtube later.
> 
> Adrian
> 
> -- 
> .''`.  John Paul Adrian Glaubitz
> : :' :  Debian Developer - glaubitz at debian.org
> `. `'   Freie Universitaet Berlin - glaubitz at physik.fu-berlin.de
>  `-    GPG: 62FF 8A75 84E0 2956 9546  0006 7426 3B37 F5B5 F913
> 
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