[llvm-dev] LLVM bc converter from LLVM 3.9 to LLVM 3.1

Tue Aug 2 22:07:49 PDT 2016

Thanks Steve and Mehdi for the explanation.

Steve,

I am a little be confused by looking at the code in
https://android.googlesource.com/platform/frameworks/compile/libbcc/+/master/bcinfo/BitReader_3_0/BitcodeReader.cpp.
Looks like the BitcodeReader do some translation while reading the bitcode.
If LLVM ToT can read the bitcode of LLVM 3.0, while can't we just use the
bitcode reader in LLVM ToT?

Also, the bitcode "downgrade" is done by :
// Use the LLVM 3.2 bitcode writer, instead of the top-of-tree version.
llvm_3_2::WriteBitcodeToFile(module, OS);

Is the corresponding source code also available? if so could you point me
to the corresponding file?

Thanks again
Hongbin

On Tue, Aug 2, 2016 at 9:29 PM, Stephen Hines <srhines at google.com> wrote:

> Ah, I had missed that. That is great news to hear, actually. The RS team
> had already started planning how to adapt their translator to have a 4.0
> (really 3.x) reader for when the dreaded 4.1/5.0 release would make things
> difficult. I guess they can reclaim some of their time now. ;) Thank you so
> much for clarifying this.
>
> Thanks,
> Steve
>
> On Tue, Aug 2, 2016 at 8:56 PM, Mehdi Amini <mehdi.amini at apple.com> wrote:
>
>>
>> On Aug 2, 2016, at 8:49 PM, Stephen Hines <srhines at google.com> wrote:
>>
>>
>>
>> On Tue, Aug 2, 2016 at 8:44 PM, Mehdi Amini <mehdi.amini at apple.com>
>> wrote:
>>
>>>
>>> On Aug 2, 2016, at 8:38 PM, Stephen Hines via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>> Hi Hongbin,
>>>
>>> On Tue, Aug 2, 2016 at 8:26 PM, Hongbin Zheng <etherzhhb at gmail.com>
>>> wrote:
>>>
>>>> Hi Steve,
>>>>
>>>> Several people told me that LLVM TOT bcreader can read odder version of
>>>> bitcode without any problem. Do you know the oldest version of bitcode that
>>>> the TOT bitcode reader supports?
>>>>
>>>
>>> The current policy is that LLVM can read bitcode for any prior version
>>> with the same major version number. Thus 3.9svn can read all the way back
>>> to 3.0 (and also 3.1/3.2/...). It cannot read 2.9 or earlier bitcode. It
>>> was also policy that the next major release (but only the .0 version) will
>>> be able to read the prior version of bitcode too. Thus 4.0 is expected to
>>> read 3.x bitcode as well. 4.1 is not expected to read 3.x bitcode, and it
>>> will be an error to try to do so. With the new shift to version numbering
>>> (i.e. major versions are coming out faster - check the recent archives
>>> here), I am not sure how reliable bitcode will be as a storage format. This
>>> is something that the RenderScript team is also planning for, since 4.0
>>> will be the last supported release that can easily read the 3.2 IR that we
>>> currently generate.
>>>
>>>
>>> No this is not the current situation: with the version numbering change,
>>> the policy changed as well. The new and current policy is: we support
>>> bitcode since 3.0 till we changed our mind.
>>> Note also that LLVM 4.1 will be a minor “patch” release (like 3.8.1),
>>> you can expect LLVM 5.0 to be released ~6 months after LLVM 4.0.
>>>
>>
>> Isn't that consistent with what I wrote
>>
>>
>> Sorry if mis-explained, let me try again.
>>
>> (i.e. 4.0 will read 3.x,
>>
>>
>> This is correct.
>>
>> but anything afterwards will not)?
>>
>>
>> This isn’t. The policy was till 3.8:: "The bitcode format produced by a
>> X.Y release will be readable by all following X.Z releases and the (X+1).0
>> release.” (Source
>> http://llvm.org/releases/3.8.1/docs/DeveloperPolicy.html#ir-backwards-compatibility
>>  )
>> This policy is supporting your claim. However with the version numbering
>> change, the policy is now:  "The current LLVM version supports loading any
>> bitcode since version 3.0.” (Source:
>> http://llvm.org/docs/DeveloperPolicy.html#ir-backwards-compatibility )
>>
>> Unless we change the policy (and I expect it would need a strong
>> motivation), we will support the 3.0 bitcode in LLVM 5.0, LLVM 6.0, etc.
>>
>> The intent is that the new version numbering does not lower the bar on
>> the bitcode backward compatibility.
>>
>>
>> Is 4.0 not going to be able to read 3.x at all? If so, that seems like a
>> big surprise for quite a few out-of-tree clients of LLVM. I didn't see
>> anything altering the policy in that email thread, and only assumed that
>> the more rapid major release cycle was going to make things worse (but
>> still consistent with the old policy in theory).
>>
>>
>> I hope it is more clear now.
>>
>> —
>> Mehdi
>>
>>
>>
>>
>>>
>>>
>>>
>>>
>>>>
>>>> Also, why do you generate bitcode instead of the textural
>>>> representation, i.e. the "ll" file, for older version of LLVM? I guess
>>>> generating "ll" code is simpler.
>>>>
>>>
>>> Our project has been using the binary bitcode format for about 6 years
>>> now (starting with 2.9). We finally stabilized on 3.2 bitcode a few years
>>> ago, rather than continuing to add new versions. The text version of LLVM
>>> IR has no compatibility guarantees (i.e. 3.9svn can't read 3.8 textual IR).
>>> It also takes up more space than the binary representation, which was an
>>> initial concern for our product too.
>>>
>>>
>>>>
>>>> Another approach I am thinking is linking to two different version of
>>>> LLVM at the same time, and use the old version of IRBuilder to build
>>>> bitcode from new version of LLVM IR. How do you think about this?
>>>>
>>>
>>> That is exactly what our bitcode translator is essentially doing. We use
>>> the TOT IRBuilder and also link in (forked) past readers/writers. This does
>>> mean that we have to adapt the APIs for the past readers/writers when
>>> IRBuilder changes, but that doesn't happen all that often.
>>>
>>> Thanks,
>>> Steve
>>>
>>>
>>>>
>>>> At last, we also do not support exception handling:)
>>>>
>>>>
>>>> Thanks.
>>>> Hongbin
>>>>
>>>> On Tue, Aug 2, 2016 at 7:26 PM, Stephen Hines <srhines at google.com>
>>>> wrote:
>>>>
>>>>> Hi Hongbin,
>>>>>
>>>>> On Tue, Aug 2, 2016 at 5:42 PM, Jakub Kuderski <
>>>>> kubakuderski+llvm at gmail.com> wrote:
>>>>>
>>>>>> I also have a look at the code, looks like it directly parse the
>>>>>>> bitcode and build in memory representation in a different LLVM version than
>>>>>>> the bitcode. Is this correct?
>>>>>>
>>>>>> According to your description, I guess the BitCodeWriter should be
>>>>>>> the one to do the bitcode version downgrade, right?
>>>>>>
>>>>>> I think so.
>>>>>>
>>>>>> I don't know much about it and don't want to give you misleading
>>>>>> information; I'm cc-ing Stephen.
>>>>>>
>>>>>>
>>>>>> On 2 August 2016 at 16:53, Hongbin Zheng <etherzhhb at gmail.com> wrote:
>>>>>>
>>>>>>> Thanks Jakub. Looks Like I didn't rely all.
>>>>>>>
>>>>>>> I also have a look at the code, looks like it directly parse the
>>>>>>> bitcode and build in memory representation in a different LLVM version than
>>>>>>> the bitcode. Is this correct?
>>>>>>>
>>>>>>
>>>>> Yes, it uses a TOT reader (which can support reading all 3.x Bitcode),
>>>>> and then lowers the in-memory representation back through a forked version
>>>>> of the 3.2 BitcodeWriter.
>>>>>
>>>>>
>>>>>>
>>>>>>> According to your description, I guess the BitCodeWriter should be
>>>>>>> the one to do the bitcode version downgrade, right?
>>>>>>>
>>>>>>
>>>>> Yes, we use a separately-maintained LLVM 3.2 BticodeWriter to generate
>>>>> the downgraded form. We also have 2 other BitcodeWriters based on roughly
>>>>> LLVM 2.9 (but not exactly 2.9, so I would steer you away from those).
>>>>> https://android.googlesource.com/platform/frameworks/compile/slang/+/refs/heads/master/BitWriter_3_2/
>>>>> is a link to the actual primary writer we use.
>>>>>
>>>>>
>>>>>>
>>>>>>>
>>>>>>> Does this took work on LLVM 3.9svn?
>>>>>>>
>>>>>>
>>>>> It currently only works up to LLVM r256229. We will most likely be
>>>>> updating it to closer to TOT LLVM in the next few months, but it isn't an
>>>>> extreme priority for us right now. Usually, maintenance requires us to fix
>>>>> up any internal API differences that might have changed for
>>>>> walking/manipulating the in-memory representation.
>>>>>
>>>>>
>>>>>>
>>>>>>> And could you give some hints about how you test the bitcode
>>>>>>> translator?
>>>>>>>
>>>>>>
>>>>> We have very few tests for this, and mostly rely on older Android
>>>>> devices running applications that we build using the translated bitcode. If
>>>>> the older device (which only has LLVM 3.2) passes those tests, we consider
>>>>> it a success. I suppose that we could take a 3.2-era BitcodeReader and wash
>>>>> bitcode back and forth through it, but I don't work directly on
>>>>> RenderScript these days.
>>>>>
>>>>> One word of caution about reusing our BitcodeWriter passes - we have
>>>>> only ever cared about C99, so we definitely removed some code that relates
>>>>> to exception handling (and possibly some other C++-specific features). We
>>>>> also only expect IR constructs that come about from normal C99
>>>>> compilation/optimization, so if you have handwritten IR, there may be
>>>>> unsupported instructions that we don't know how to generate legacy bitcode
>>>>> for.
>>>>>
>>>>> Thanks,
>>>>> Steve
>>>>>
>>>>>
>>>>>>
>>>>>>> Thanks
>>>>>>> Hongbin
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Tue, Aug 2, 2016 at 3:25 PM, Jakub Kuderski <
>>>>>>> kubakuderski+llvm at gmail.com> wrote:
>>>>>>>
>>>>>>>> I'm not sure if it was intended, but I think that you responded
>>>>>>>> only to me (not to the list).
>>>>>>>>
>>>>>>>> Anyway, RenderScript currently uses llvm 3.2 IR as binary
>>>>>>>> representation/exchangeable format, but the llvm version is higher
>>>>>>>> internally. Essentially, there it reads old bitcode and operates on the
>>>>>>>> newer one in memory. When it's done, it's also able to output bitcode in
>>>>>>>> the old format. The corresponding classes should be something like
>>>>>>>> BitCodeReader and BitCodeWriter.
>>>>>>>> The reader/writer is maintained to work with Android's llvm
>>>>>>>> version, which follows upstream llvm version.
>>>>>>>>
>>>>>>>> I haven't worked on it myself, but if you have some specific
>>>>>>>> questions, I should be able to point you to appropriate people.
>>>>>>>>
>>>>>>>> Regards
>>>>>>>>
>>>>>>>> On 2 August 2016 at 15:17, Hongbin Zheng <etherzhhb at gmail.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> Hi Jakub,
>>>>>>>>>
>>>>>>>>> Thanks a lot. Do you have any document about this tool? or could
>>>>>>>>> you gave me a rough idea what it do?
>>>>>>>>>
>>>>>>>>> Thanks
>>>>>>>>> Hongbin
>>>>>>>>>
>>>>>>>>> On Tue, Aug 2, 2016 at 3:06 PM, Jakub Kuderski <
>>>>>>>>> kubakuderski+llvm at gmail.com> wrote:
>>>>>>>>>
>>>>>>>>>> Hi Hongbin,
>>>>>>>>>>
>>>>>>>>>> Android's RenderScript uses a tool like that. You can find the
>>>>>>>>>> sources here:
>>>>>>>>>> https://android.googlesource.com/platform/frameworks/compile/libbcc/+/master/bcinfo/
>>>>>>>>>>  .
>>>>>>>>>>
>>>>>>>>>> Best,
>>>>>>>>>> Jakub
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On 2 August 2016 at 11:10, Hongbin Zheng via llvm-dev <
>>>>>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>>>>>
>>>>>>>>>>> Hi mailing list,
>>>>>>>>>>>
>>>>>>>>>>> I has been working on a large project that is based on LLVM 3.1.
>>>>>>>>>>> Recently we are thinking to introduce an LLVM bc converter from LLVM 3.9 to
>>>>>>>>>>> LLVM 3.1, such that we can introduce some of the newest LLVM analyses and
>>>>>>>>>>> optimizations to our  LLVM 3.1 based project.
>>>>>>>>>>>
>>>>>>>>>>> Have you worked on similar things that converting LLVM bc
>>>>>>>>>>> downward and has anything to share?
>>>>>>>>>>>
>>>>>>>>>>> Thanks
>>>>>>>>>>> Hongbin
>>>>>>>>>>>
>>>>>>>>>>> _______________________________________________
>>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>>
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160802/eb21730b/attachment-0001.html>