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

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
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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
<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
<http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>
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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.

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<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

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