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Document the new !tbaa.struct metadata.

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llvm-svn: 164398
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Dan Gohman 2012-09-21 18:21:48 +00:00
parent 022673f281
commit cacdd7ae8e
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@ -103,6 +103,7 @@
<li><a href="#metadata">Metadata Nodes and Metadata Strings</a>
<ol>
<li><a href="#tbaa">'<tt>tbaa</tt>' Metadata</a></li>
<li><a href="#tbaa.struct">'<tt>tbaa.struct</tt>' Metadata</a></li>
<li><a href="#fpmath">'<tt>fpmath</tt>' Metadata</a></li>
<li><a href="#range">'<tt>range</tt>' Metadata</a></li>
</ol>
@ -3050,6 +3051,44 @@ call void @llvm.dbg.value(metadata !24, i64 0, metadata !25)
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="tbaa.struct">'<tt>tbaa.struct</tt>' Metadata</a>
</h4>
<div>
<p>The <a href="#int_memcpy"><tt>llvm.memcpy</tt></a> is often used to implement
aggregate assignment operations in C and similar languages, however it is
defined to copy a contiguous region of memory, which is more than strictly
necessary for aggregate types which contain holes due to padding. Also, it
doesn't contain any TBAA information about the fields of the aggregate.</p>
<p><tt>!tbaa.struct</tt> metadata can describe which memory subregions in a memcpy
are padding and what the TBAA tags of the struct are.</p>
<p>The current metadata format is very simple. <tt>!tbaa.struct</tt> metadata nodes
are a list of operands which are in conceptual groups of three. For each
group of three, the first operand gives the byte offset of a field in address
units, the second gives its size in address units, and the third gives its
tbaa tag. e.g.:</p>
<div class="doc_code">
<pre>
!4 = metadata !{ i64 0, i64 4, metadata !1, i64 8, i64 4, metadata !2 }
</pre>
</div>
<p>This describes a struct with two fields. The first is at offset 0 address units
with size 4 address units, and has tbaa tag !1. The second is at offset 8 address
units and has size 4 address units and has tbaa tag !2.</p>
<p>Note that the fields need not be contiguous. In this example, there is a
4 byte gap between the two fields. This gap represents padding which
does not carry useful data and need not be preserved.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="fpmath">'<tt>fpmath</tt>' Metadata</a>