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Document the new module flags.

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Bill Wendling 2012-02-11 11:59:36 +00:00
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@ -108,6 +108,10 @@
</li>
</ol>
</li>
<li><a href="#module_flags">Module Flags Metadata</a>
<ol>
</ol>
</li>
<li><a href="#intrinsic_globals">Intrinsic Global Variables</a>
<ol>
<li><a href="#intg_used">The '<tt>llvm.used</tt>' Global Variable</a></li>
@ -3029,6 +3033,128 @@ call void @llvm.dbg.value(metadata !24, i64 0, metadata !25)
</div>
<!-- *********************************************************************** -->
<h2>
<a name="module_flags">Module Flags Metadata</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>Information about the module as a whole is difficult to convey to LLVM's
subsystems. The LLVM IR isn't sufficient to transmit this
information. The <tt>llvm.module.flags</tt> named metadata exists in order to
facilitate this. These flags are in the form of key / value pairs &mdash;
much like a dictionary &mdash; making it easy for any subsystem who cares
about a flag to look it up.</p>
<p>The <tt>llvm.module.flags</tt> metadata contains a list of metadata
triplets. Each triplet has the following form:</p>
<ul>
<li>The first element is a <i>behavior</i> flag, which specifies the behavior
when two (or more) modules are merged together, and it encounters two (or
more) metadata with the same ID. The supported behaviors are described
below.</li>
<li>The second element is a metadata string that is a unique ID for the
metadata. How each ID is interpreted is documented below.</li>
<li>The third element is the value of the flag.</li>
</ul>
<p>When two (or more) modules are merged together, the resulting
<tt>llvm.module.flags</tt> metadata is the union of the
modules' <tt>llvm.module.flags</tt> metadata. The only exception being a flag
with the <i>Override</i> behavior, which may override another flag's value
(see below).</p>
<p>The following behaviors are supported:</p>
<table border="1" cellspacing="0" cellpadding="4">
<tbody>
<tr>
<th>Value</th>
<th>Behavior</th>
</tr>
<tr>
<td>1</td>
<td align="left">
<dt><b>Error</b></dt>
<dd>Emits an error if two values disagree. It is an error to have an ID
with both an Error and a Warning behavior.</dd>
</td>
</tr>
<tr>
<td>2</td>
<td align="left">
<dt><b>Warning</b></dt>
<dd>Emits a warning if two values disagree.</dd>
</td>
</tr>
<tr>
<td>3</td>
<td align="left">
<dt><b>Require</b></dt>
<dd>Emits an error when the specified value is not present or doesn't
have the specified value. It is an error for two (or more)
<tt>llvm.module.flags</tt> with the same ID to have the Require
behavior but different values. There may be multiple Require flags
per ID.</dd>
</td>
</tr>
<tr>
<td>4</td>
<td align="left">
<dt><b>Override</b></dt>
<dd>Uses the specified value if the two values disagree. It is an error
for two (or more) <tt>llvm.module.flags</tt> with the same ID to
have the Override behavior but different values.</dd>
</td>
</tr>
</tbody>
</table>
<p>An example of module flags:</p>
<pre class="doc_code">
!0 = metadata !{ i32 1, metadata !"foo", i32 1 }
!1 = metadata !{ i32 4, metadata !"bar", i32 37 }
!2 = metadata !{ i32 2, metadata !"qux", i32 42 }
!3 = metadata !{ i32 3, metadata !"qux",
metadata !{
metadata !"foo", i32 1
}
}
!llvm.module.flags = !{ !0, !1, !2, !3 }
</pre>
<ul>
<li><p>Metadata <tt>!0</tt> has the ID <tt>!"foo"</tt> and the value '1'. The
behavior if two or more <tt>!"foo"</tt> flags are seen is to emit an
error if their values are not equal.</p></li>
<li><p>Metadata <tt>!1</tt> has the ID <tt>!"bar"</tt> and the value '37'. The
behavior if two or more <tt>!"bar"</tt> flags are seen is to use the
value '37' if their values are not equal.</p></li>
<li><p>Metadata <tt>!2</tt> has the ID <tt>!"qux"</tt> and the value '42'. The
behavior if two or more <tt>!"qux"</tt> flags are seen is to emit a
warning if their values are not equal.</p></li>
<li><p>Metadata <tt>!3</tt> has the ID <tt>!"qux"</tt> and the value:</p>
<pre class="doc_code">
metadata !{ metadata !"foo", i32 1 }
</pre>
<p>The behavior is to emit an error if the <tt>llvm.module.flags</tt> does
not contain a flag with the ID <tt>!"foo"</tt> that has the value
'1'. If two or more <tt>!"qux"</tt> flags exist, then they must have
the same value or an error will be issued.</p></li>
</ul>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="intrinsic_globals">Intrinsic Global Variables</a>