6502/llvm-6502
1
0
Fork 0
mirror of https://github.com/c64scene-ar/llvm-6502.git synced 2024-07-20 10:30:03 +00:00
Code Issues Projects Releases Wiki Activity

Document tbaa metadata in LangRef (documentation largely based on

Browse Source 
comments at top of TypeBasedAliasAnalysis.cpp).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143134 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Peter Collingbourne 2011-10-27 19:19:07 +00:00
parent 32a43cc0fc
commit 249d953ed7
2 changed files with 56 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
docs/GetElementPtr.html
View File

@ -594,10 +594,10 @@ idx3 = (char*) &MyVar + 8
because LLVM has no restrictions on mixing types in addressing, loads or
stores.</p>
<p>It would be possible to add special annotations to the IR, probably using
metadata, to describe a different type system (such as the C type system),
and do type-based aliasing on top of that. This is a much bigger
undertaking though.</p>
<p>LLVM's type-based alias analysis pass uses metadata to describe a different
type system (such as the C type system), and performs type-based aliasing
on top of that. Further details are in the
<a href="LangRef.html#tbaa">language reference</a>.</p>
</div>

53
docs/LangRef.html
View File

@ -100,7 +100,11 @@
<li><a href="#othervalues">Other Values</a>
<ol>
<li><a href="#inlineasm">Inline Assembler Expressions</a></li>
<li><a href="#metadata">Metadata Nodes and Metadata Strings</a></li>
<li><a href="#metadata">Metadata Nodes and Metadata Strings</a>
<ol>
<li><a href="#tbaa">'<tt>tbaa</tt>' Metadata</a></li>
</ol>
</li>
</ol>
</li>
<li><a href="#intrinsic_globals">Intrinsic Global Variables</a>
@ -2915,6 +2919,53 @@ call void @llvm.dbg.value(metadata !24, i64 0, metadata !25)
</pre>
</div>
<p>More information about specific metadata nodes recognized by the optimizers
and code generator is found below.</p>
<h4>
<a name="tbaa">'<tt>tbaa</tt>' Metadata</a>
</h4>
<div>
<p>In LLVM IR, memory does not have types, so LLVM's own type system is not
suitable for doing TBAA. Instead, metadata is added to the IR to describe
a type system of a higher level language. This can be used to implement
typical C/C++ TBAA, but it can also be used to implement custom alias
analysis behavior for other languages.</p>
<p>The current metadata format is very simple. TBAA metadata nodes have up to
three fields, e.g.:</p>
<div class="doc_code">
<pre>
!0 = metadata !{ metadata !"an example type tree" }
!1 = metadata !{ metadata !"int", metadata !0 }
!2 = metadata !{ metadata !"float", metadata !0 }
!3 = metadata !{ metadata !"const float", metadata !2, i64 1 }
</pre>
</div>
<p>The first field is an identity field. It can be any value, usually
a metadata string, which uniquely identifies the type. The most important
name in the tree is the name of the root node. Two trees with
different root node names are entirely disjoint, even if they
have leaves with common names.</p>
<p>The second field identifies the type's parent node in the tree, or
is null or omitted for a root node. A type is considered to alias
all of its descendants and all of its ancestors in the tree. Also,
a type is considered to alias all types in other trees, so that
bitcode produced from multiple front-ends is handled conservatively.</p>
<p>If the third field is present, it's an integer which if equal to 1
indicates that the type is "constant" (meaning
<tt>pointsToConstantMemory</tt> should return true; see
<a href="AliasAnalysis.html#OtherItfs">other useful
<tt>AliasAnalysis</tt> methods</a>).</p>
</div>
</div>
</div>