[llvm-commits] [llvm] r164398 - /llvm/trunk/docs/LangRef.html

[Hal Finkel]

On Fri, 21 Sep 2012 18:21:48 -0000
Dan Gohman <gohman at apple.com> wrote:

> Author: djg
> Date: Fri Sep 21 13:21:48 2012
> New Revision: 164398
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=164398&view=rev
> Log:
> Document the new !tbaa.struct metadata.
> 
> Modified:
>     llvm/trunk/docs/LangRef.html
> 
> Modified: llvm/trunk/docs/LangRef.html
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/LangRef.html?rev=164398&r1=164397&r2=164398&view=diff
> ==============================================================================
> --- llvm/trunk/docs/LangRef.html (original) +++
> llvm/trunk/docs/LangRef.html Fri Sep 21 13:21:48 2012 @@ -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>
> @@ -3052,6 +3053,44 @@
>  
>  <!--
> _______________________________________________________________________
> --> <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>

So an 'address unit' is always a byte, or just is in this case? I can't
find this term elsewhere in the language reference, so please define it
first.

Thanks again,
Hal

> +
> +</div>
> +
> +<!--
> _______________________________________________________________________
> --> +<h4> <a name="fpmath">'<tt>fpmath</tt>' Metadata</a>
>  </h4>
>   
> 
> 
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-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory