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Add a little wrapper header that is put around bc files when emitting

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bc files for modules with a target triple that indicates they are for
darwin.  The reader unconditionally handles this, and the writer could
turn this on for more targets if we care.

This change has two benefits for darwin:

1) it allows us to encode the cpu type of the file in an easy to read
   place that doesn't require decoding the bc file.
2) it works around a bug (IMO) in darwin's AR where it is incapable of
   handling files that are not a multiple of 8 bytes long.  BC files
   are only guaranteed to be multiples of 4 bytes long.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@53275 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner
2008-07-09 05:14:23 +00:00
parent b02b8af378
commit 6fa6a32e4e
5 changed files with 179 additions and 9 deletions
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40
docs/BitCodeFormat.html
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@@ -22,6 +22,8 @@
<li><a href="#stdblocks">Standard Blocks</a></li>
</ol>
</li>
<li><a href="#wrapper">Bitcode Wrapper Format</a>
</li>
<li><a href="#llvmir">LLVM IR Encoding</a>
<ol>
<li><a href="#basics">Basics</a></li>
@@ -65,8 +67,12 @@ Unlike XML, the bitstream format is a binary encoding, and unlike XML it
provides a mechanism for the file to self-describe "abbreviations", which are
effectively size optimizations for the content.</p>
<p>This document first describes the LLVM bitstream format, then describes the
record structure used by LLVM IR files.
<p>LLVM IR files may be optionally embedded into a <a
href="#wrapper">wrapper</a> structure that makes it easy to embed extra data
along with LLVM IR files.</p>
<p>This document first describes the LLVM bitstream format, describes the
wrapper format, then describes the record structure used by LLVM IR files.
</p>
</div>
@@ -544,6 +550,36 @@ corresponding blocks. It is not safe to skip them.
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="wrapper">Bitcode Wrapper Format</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>Bitcode files for LLVM IR may optionally be wrapped in a simple wrapper
structure. This structure contains a simple header that indicates the offset
and size of the embedded BC file. This allows additional information to be
stored alongside the BC file. The structure of this file header is:
</p>
<p>
<pre>
[Magic<sub>32</sub>,
Version<sub>32</sub>,
Offset<sub>32</sub>,
Size<sub>32</sub>,
CPUType<sub>32</sub>]
</pre></p>
<p>Each of the fields are 32-bit fields stored in little endian form (as with
the rest of the bitcode file fields). The Magic number is always
<tt>0x0B17C0DE</tt> and the version is currently always <tt>0</tt>. The Offset
field is the offset in bytes to the start of the bitcode stream in the file, and
the Size field is a size in bytes of the stream. CPUType is a target-specific
value that can be used to encode the CPU of the target.
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="llvmir">LLVM IR Encoding</a></div>
<!-- *********************************************************************** -->