llvm dev - Oct 2014 - [LLVMdev] [cfe-dev] Proposal to add Bitcode version field to bitcode file wrapper

On Sat, Sep 27, 2014 at 11:35 PM, Alex Rosenberg <alexr at leftfield.org>
wrote:
> How is this use case different from the LTO-supported toolchains shipped
> by other vendors such as Apple? Do they have this theoretical problem too?
>
> If the issue is solely constrained to debug info metadata, then why not
> use metadata to describe the format/version of the debug info?
>
FWIW (I haven't followed the rest of this thread) - that's what we/Apple
have done. There's a module flag metadata that specifies the debug info
metadata schema version, then the verifier (or some other pass, I forget
how it's phrased) can check if the version matches the current LLVM's
debug
info metadata version, and if it doesn't match, it strips out all the debug
info metadata.

>
> Alex
>
> On Sep 27, 2014, at 3:19 AM, Greg Bedwell <gregbedwell at gmail.com>
wrote:
>
> As I understand it, the bitcode compatibility promise doesn't extend as
> far as debug info metadata (happy to be wrong here!).  I think we have a
> usecase where need to guarantee that debug information from any two
> arbitrary bitcode files going into an LTO link will result in the
> expected/correct debug information going into the resulting ELF file;
> unless we can be sure that this will always work between bitcode files
> generated by different versions we'd need some way of flagging up an
> incompatibility and providing useful information on the reason to the user.
>
> --Greg Bedwell
> SN Systems - Sony Computer Entertainment Group
>
> On 27 September 2014 02:24, Sean Silva <chisophugis at gmail.com>
wrote:
>
>>
>>
>> On Fri, Sep 26, 2014 at 5:18 PM, Yung, Douglas <
>> douglas_yung at playstation.sony.com> wrote:
>>
>>>  Sorry if I was unclear. There are currently no “known
>>> incompatibilities” that I am aware of, although I fully admit to
not being
>>> an expert on the topic. The idea is that we add versioning
information to
>>> the bitcode so that if an issue were discovered, it could be easily
>>> detected and dealt with.
>>>
>>
>> It sounds like time would be better invested in improving the testing
of
>> our bitcode compatibility promise.
>>
>> -- Sean Silva
>>
>>
>>>
>>>
>>> Douglas Yung
>>>
>>>
>>>
>>> *From:* Bob Wilson [mailto:bob.wilson at apple.com]
>>> *Sent:* Friday, September 26, 2014 16:39
>>> *To:* Yung, Douglas
>>> *Cc:* llvmdev at cs.uiuc.edu; cfe-dev at cs.uiuc.edu
>>> *Subject:* Re: [LLVMdev] Proposal to add Bitcode version field to
>>> bitcode file wrapper
>>>
>>>
>>>
>>> Bitcode backward compatibility, at least for the current major
version,
>>> is supposed to make this unnecessary. Can you provide more
information
>>> about what “known incompatibilities” you’re seeing?
>>>
>>>
>>>
>>>  On Sep 26, 2014, at 2:40 PM, Yung, Douglas <
>>> douglas_yung at playstation.sony.com> wrote:
>>>
>>>
>>>
>>> Hi,
>>>
>>>
>>>
>>> We would like to add a version number to the bitcode wrapper. This
>>> feature would allow easier identification of what compiler produced
the
>>> bitcode so that known incompatibilities with LTO compilation could
be
>>> detected. Roughly speaking, this version number would consist of
the major,
>>> minor and optionally the patch version of the compiler used to
produce the
>>> bitcode. The version information would be encoded in 4 bytes, with
the
>>> first byte representing the major version number, the second byte
the minor
>>> version number, and the third and fourth bytes optionally encoding
the
>>> patch version or other information. As to where to place this
information,
>>> we are considering two different possibilities for updating the
bitcode
>>> wrapper specification.
>>>
>>>
>>>
>>> The first is to simply add a single 32bit wide field at the end of
the
>>> existing bitcode wrapper format field. This would result in the new
>>> structure looking like this:
>>>
>>> [Magic_{32}, Version_{32}, Offset_{32}, Size_{32}, CPUType_{32},
>>> BitcodeVersion_{32}]
>>>
>>> All of the existing fields would keep their current meanings, and
the
>>> new field BitcodeVersion is simply appended with the format
described in
>>> the first paragraph.
>>>
>>> A second idea was to use the existing Version field in the bitcode
>>> wrapper format to store the bitcode version information. According
to the
>>> documentation (
>>> http://llvm.org/docs/BitCodeFormat.html#bitcode-wrapper-format)
this
>>> field is currently always set to 0. This would allow us to make use
of what
>>> is (presumably) an unused field.
>>>
>>>
>>>
>>> As this is a feature that we feel would be beneficial to the
community,
>>> we wanted to get feedback on the design for our upcoming patches.
Any
>>> thoughts or opinions on this would be greatly appreciated.
>>>
>>>
>>>
>>> Thanks!
>>>
>>>
>>>
>>> Douglas Yung
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>
>>
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>>
>>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>
>
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Hi –

I realize the thread has drifted a little, but I wanted to get back to my
original proposal. I would like to make a change to the bitcode file wrapper to
include the version of llvm that produced the bitcode. I would like to write
this version into the unused version field that currently exists. Would there be
any objections to this change?

Since the original wrapper is only emitted for Darwin targets, I ran an
experiment where I took bitcode files produced by the official Apple tools and
then modified the version field to be non-zero. From my simple tests, there
seemed to be no problems with existing tools when the version field is non-zero.

Douglas Yung

From: David Blaikie [mailto:dblaikie at gmail.com]
Sent: Sunday, September 28, 2014 1:02
To: Alex Rosenberg
Cc: Greg Bedwell; Yung, Douglas; cfe-dev at cs.uiuc.edu; llvmdev at cs.uiuc.edu
Subject: Re: [cfe-dev] [LLVMdev] Proposal to add Bitcode version field to
bitcode file wrapper



On Sat, Sep 27, 2014 at 11:35 PM, Alex Rosenberg <alexr at
leftfield.org<mailto:alexr at leftfield.org>> wrote:
How is this use case different from the LTO-supported toolchains shipped by
other vendors such as Apple? Do they have this theoretical problem too?

If the issue is solely constrained to debug info metadata, then why not use
metadata to describe the format/version of the debug info?

FWIW (I haven't followed the rest of this thread) - that's what we/Apple
have done. There's a module flag metadata that specifies the debug info
metadata schema version, then the verifier (or some other pass, I forget how
it's phrased) can check if the version matches the current LLVM's debug
info metadata version, and if it doesn't match, it strips out all the debug
info metadata.


Alex

On Sep 27, 2014, at 3:19 AM, Greg Bedwell <gregbedwell at
gmail.com<mailto:gregbedwell at gmail.com>> wrote:
As I understand it, the bitcode compatibility promise doesn't extend as far
as debug info metadata (happy to be wrong here!).  I think we have a usecase
where need to guarantee that debug information from any two arbitrary bitcode
files going into an LTO link will result in the expected/correct debug
information going into the resulting ELF file; unless we can be sure that this
will always work between bitcode files generated by different versions we'd
need some way of flagging up an incompatibility and providing useful information
on the reason to the user.

--Greg Bedwell
SN Systems - Sony Computer Entertainment Group

On 27 September 2014 02:24, Sean Silva <chisophugis at
gmail.com<mailto:chisophugis at gmail.com>> wrote:


On Fri, Sep 26, 2014 at 5:18 PM, Yung, Douglas <douglas_yung at
playstation.sony.com<mailto:douglas_yung at playstation.sony.com>>
wrote:
Sorry if I was unclear. There are currently no “known incompatibilities” that I
am aware of, although I fully admit to not being an expert on the topic. The
idea is that we add versioning information to the bitcode so that if an issue
were discovered, it could be easily detected and dealt with.

It sounds like time would be better invested in improving the testing of our
bitcode compatibility promise.

-- Sean Silva


Douglas Yung

From: Bob Wilson [mailto:bob.wilson at apple.com<mailto:bob.wilson at
apple.com>]
Sent: Friday, September 26, 2014 16:39
To: Yung, Douglas
Cc: llvmdev at cs.uiuc.edu<mailto:llvmdev at cs.uiuc.edu>; cfe-dev at
cs.uiuc.edu<mailto:cfe-dev at cs.uiuc.edu>
Subject: Re: [LLVMdev] Proposal to add Bitcode version field to bitcode file
wrapper

Bitcode backward compatibility, at least for the current major version, is
supposed to make this unnecessary. Can you provide more information about what
“known incompatibilities” you’re seeing?

On Sep 26, 2014, at 2:40 PM, Yung, Douglas <douglas_yung at
playstation.sony.com<mailto:douglas_yung at playstation.sony.com>>
wrote:

Hi,

We would like to add a version number to the bitcode wrapper. This feature would
allow easier identification of what compiler produced the bitcode so that known
incompatibilities with LTO compilation could be detected. Roughly speaking, this
version number would consist of the major, minor and optionally the patch
version of the compiler used to produce the bitcode. The version information
would be encoded in 4 bytes, with the first byte representing the major version
number, the second byte the minor version number, and the third and fourth bytes
optionally encoding the patch version or other information. As to where to place
this information, we are considering two different possibilities for updating
the bitcode wrapper specification.

The first is to simply add a single 32bit wide field at the end of the existing
bitcode wrapper format field. This would result in the new structure looking
like this:

[Magic_{32}, Version_{32}, Offset_{32}, Size_{32}, CPUType_{32},
BitcodeVersion_{32}]

All of the existing fields would keep their current meanings, and the new field
BitcodeVersion is simply appended with the format described in the first
paragraph.

A second idea was to use the existing Version field in the bitcode wrapper
format to store the bitcode version information. According to the documentation
(http://llvm.org/docs/BitCodeFormat.html#bitcode-wrapper-format) this field is
currently always set to 0. This would allow us to make use of what is
(presumably) an unused field.

As this is a feature that we feel would be beneficial to the community, we
wanted to get feedback on the design for our upcoming patches. Any thoughts or
opinions on this would be greatly appreciated.

Thanks!

Douglas Yung
_______________________________________________
LLVM Developers mailing list
LLVMdev at cs.uiuc.edu<mailto:LLVMdev at cs.uiuc.edu>        
http://llvm.cs.uiuc.edu<http://llvm.cs.uiuc.edu/>
http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev


_______________________________________________
LLVM Developers mailing list
LLVMdev at cs.uiuc.edu<mailto:LLVMdev at cs.uiuc.edu>        
http://llvm.cs.uiuc.edu
http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev


_______________________________________________
cfe-dev mailing list
cfe-dev at cs.uiuc.edu<mailto:cfe-dev at cs.uiuc.edu>
http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev

_______________________________________________
LLVM Developers mailing list
LLVMdev at cs.uiuc.edu<mailto:LLVMdev at cs.uiuc.edu>        
http://llvm.cs.uiuc.edu
http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

_______________________________________________
cfe-dev mailing list
cfe-dev at cs.uiuc.edu<mailto:cfe-dev at cs.uiuc.edu>
http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev

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On Sun, Oct 5, 2014 at 8:10 PM, Yung, Douglas <
douglas_yung at playstation.sony.com> wrote:
>  Hi –
>
>
>
> I realize the thread has drifted a little, but I wanted to get back to my
> original proposal. I would like to make a change to the bitcode file
> wrapper to include the version of llvm that produced the bitcode. I would
> like to write this version into the unused version field that currently
> exists. Would there be any objections to this change?
>
If the version field is currently unused, I don't see the harm in filling
it in. It probably would be fine to just use it to store the LLVM major
version, so that we can detect incompatibilities. I still would be opposed
to using a granularity finer than the "intended" breakage cycle (major
versions), since that would give the impression that it is "ok" to
break
compatibility since it can be detected through the version field.

-- Sean Silva

>
>
> Since the original wrapper is only emitted for Darwin targets, I ran an
> experiment where I took bitcode files produced by the official Apple tools
> and then modified the version field to be non-zero. From my simple tests,
> there seemed to be no problems with existing tools when the version field
> is non-zero.
>
>
>
> Douglas Yung
>
>
>
> *From:* David Blaikie [mailto:dblaikie at gmail.com]
> *Sent:* Sunday, September 28, 2014 1:02
> *To:* Alex Rosenberg
> *Cc:* Greg Bedwell; Yung, Douglas; cfe-dev at cs.uiuc.edu;
> llvmdev at cs.uiuc.edu
> *Subject:* Re: [cfe-dev] [LLVMdev] Proposal to add Bitcode version field
> to bitcode file wrapper
>
>
>
>
>
>
>
> On Sat, Sep 27, 2014 at 11:35 PM, Alex Rosenberg <alexr at
leftfield.org>
> wrote:
>
> How is this use case different from the LTO-supported toolchains shipped
> by other vendors such as Apple? Do they have this theoretical problem too?
>
>
>
> If the issue is solely constrained to debug info metadata, then why not
> use metadata to describe the format/version of the debug info?
>
>
>
> FWIW (I haven't followed the rest of this thread) - that's what
we/Apple
> have done. There's a module flag metadata that specifies the debug info
> metadata schema version, then the verifier (or some other pass, I forget
> how it's phrased) can check if the version matches the current
LLVM's debug
> info metadata version, and if it doesn't match, it strips out all the
debug
> info metadata.
>
>
>
>
> Alex
>
>
> On Sep 27, 2014, at 3:19 AM, Greg Bedwell <gregbedwell at gmail.com>
wrote:
>
>  As I understand it, the bitcode compatibility promise doesn't extend
as
> far as debug info metadata (happy to be wrong here!).  I think we have a
> usecase where need to guarantee that debug information from any two
> arbitrary bitcode files going into an LTO link will result in the
> expected/correct debug information going into the resulting ELF file;
> unless we can be sure that this will always work between bitcode files
> generated by different versions we'd need some way of flagging up an
> incompatibility and providing useful information on the reason to the user.
>
>
>
> --Greg Bedwell
>
> SN Systems - Sony Computer Entertainment Group
>
>
>
> On 27 September 2014 02:24, Sean Silva <chisophugis at gmail.com>
wrote:
>
>
>
>
>
> On Fri, Sep 26, 2014 at 5:18 PM, Yung, Douglas <
> douglas_yung at playstation.sony.com> wrote:
>
> Sorry if I was unclear. There are currently no “known incompatibilities”
> that I am aware of, although I fully admit to not being an expert on the
> topic. The idea is that we add versioning information to the bitcode so
> that if an issue were discovered, it could be easily detected and dealt
> with.
>
>
>
> It sounds like time would be better invested in improving the testing of
> our bitcode compatibility promise.
>
>
>
> -- Sean Silva
>
>
>
>
>
> Douglas Yung
>
>
>
> *From:* Bob Wilson [mailto:bob.wilson at apple.com]
> *Sent:* Friday, September 26, 2014 16:39
> *To:* Yung, Douglas
> *Cc:* llvmdev at cs.uiuc.edu; cfe-dev at cs.uiuc.edu
> *Subject:* Re: [LLVMdev] Proposal to add Bitcode version field to bitcode
> file wrapper
>
>
>
> Bitcode backward compatibility, at least for the current major version, is
> supposed to make this unnecessary. Can you provide more information about
> what “known incompatibilities” you’re seeing?
>
>
>
>  On Sep 26, 2014, at 2:40 PM, Yung, Douglas <
> douglas_yung at playstation.sony.com> wrote:
>
>
>
> Hi,
>
>
>
> We would like to add a version number to the bitcode wrapper. This feature
> would allow easier identification of what compiler produced the bitcode so
> that known incompatibilities with LTO compilation could be detected.
> Roughly speaking, this version number would consist of the major, minor and
> optionally the patch version of the compiler used to produce the bitcode.
> The version information would be encoded in 4 bytes, with the first byte
> representing the major version number, the second byte the minor version
> number, and the third and fourth bytes optionally encoding the patch
> version or other information. As to where to place this information, we are
> considering two different possibilities for updating the bitcode wrapper
> specification.
>
>
>
> The first is to simply add a single 32bit wide field at the end of the
> existing bitcode wrapper format field. This would result in the new
> structure looking like this:
>
> [Magic_{32}, Version_{32}, Offset_{32}, Size_{32}, CPUType_{32},
> BitcodeVersion_{32}]
>
> All of the existing fields would keep their current meanings, and the new
> field BitcodeVersion is simply appended with the format described in the
> first paragraph.
>
> A second idea was to use the existing Version field in the bitcode wrapper
> format to store the bitcode version information. According to the
> documentation (
> http://llvm.org/docs/BitCodeFormat.html#bitcode-wrapper-format) this
> field is currently always set to 0. This would allow us to make use of what
> is (presumably) an unused field.
>
>
>
> As this is a feature that we feel would be beneficial to the community, we
> wanted to get feedback on the design for our upcoming patches. Any thoughts
> or opinions on this would be greatly appreciated.
>
>
>
> Thanks!
>
>
>
> Douglas Yung
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>
>
>
>  _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>
>
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>
>
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