[LLVMdev] Bitcode format

[Reid Spencer]

Hi Joshua,

On Mon, 2007-09-03 at 21:34 +0000, Joshua Haberman wrote:
> Greetings,
> 
> I am working on a project (unrelated to LLVM) that needed a 
> bytecode-like format.  I found Bitcode and it seems to fit the bill
> really nicely.
> 
> I am writing an independent implementation of Bitcode in C (I really 
> want to keep my runtime pure C).  In the process of doing this, I've
> discovered a few things I want to ask about.

Okay, cool :)

> 
> I notice the Bitcode format documentation [0] is somewhat incomplete --
> there have been a few questions I had to resolve by looking at LLVM
> source code.  This is totally understandable (I'm actually impressed
> that it's documented so well for something so new), but I wonder if you
> would accept a patch that clarified some things.  For example, the
> document doesn't mention endianness at all, but from the source code I
> discover that bytes are read in order and bits are read 
> least-significant first.

Absolutely, patches are always welcome.

> 
> I also have a few questions about the format:
> 
> - it appears that the only magic number in the file is 
>   application-specific.  This seems unfortunate, because it means that
>   application-neutral tools cannot be built that process bitcode files,
>   since they could not reliably detect that the file is a bitcode file.
>   It might seem like there is little room for application- neutral tools
>   since almost all the data in the file is application-specific, but off
>   the top of my head I can think of a few, like a tool to suggest
>   abbreviations that would give a file better compression.

Wouldn't those abbreviations imply some application-specific knowledge
of the format? I can't think of any situation where a generic tool could
do anything other than outline the contents of the file by looking at
the blocks, e.g. llvm-bcanalyzer.
> 
> - the LLVM code assumes that several VBR fields can be at most 32 bits 
>   (block ids, number of elements in an array, etc).  These assumptions
>   seem quite reasonable: can they be considered part of the format and
>   added to the document?

I don't see why not. The llvm bitcode documentation is specific to llvm
anyway so the limits should be defined. 

Thanks for your interest, Joshua.

Reid.

> 
> Cheers,
> Josh
> 
> [0] http://llvm.org/releases/2.0/docs/BitCodeFormat.html
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev