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Give a description of the Objective-C garbage collection module flags.

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The rule governing the flags is this:

  no-gc + no-gc   = no-gc
  no-gc +    gc   = no-gc
  no-gc + gc-only = error
     gc +    gc   = gc
     gc + gc-only = gc-only
gc-only + gc-only = gc-only


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@150646 91177308-0d34-0410-b5e6-96231b3b80d8
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Bill Wendling 2012-02-16 01:10:50 +00:00
parent 5e6162e756
commit f7b367ce45
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@ -110,6 +110,7 @@
</li>
<li><a href="#module_flags">Module Flags Metadata</a>
<ol>
<li><a href="#objc_gc_flags">Objective-C Garbage Collection Module Flags Metadata</a></li>
</ol>
</li>
<li><a href="#intrinsic_globals">Intrinsic Global Variables</a>
@ -3147,12 +3148,84 @@ call void @llvm.dbg.value(metadata !24, i64 0, metadata !25)
<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>
<!-- ======================================================================= -->
<h3>
<a name="objc_gc_flags">Objective-C Garbage Collection Module Flags Metadata</a>
</h3>
<div>
<p>On the Mach-O platform, Objective-C stores metadata about garbage collection
in a special section called "image info". The metadata consists of a version
number and a bitmask specifying what types of garbage collection are
supported (if any) by the file. If two or more modules are linked together
their garbage collection metadata needs to be merged rather than appended
together.</p>
<p>The Objective-C garbage collection module flags metadata consists of the
following key-value pairs:</p>
<table border="1" cellspacing="0" cellpadding="4">
<tbody>
<tr>
<th width="30%">Key</th>
<th>Value</th>
</tr>
<tr>
<td><tt>Objective-C&nbsp;Version</tt></td>
<td align="left"><b>[Required]</b> &mdash; The Objective-C ABI
version. Valid values are 1 and 2.</td>
</tr>
<tr>
<td><tt>Objective-C&nbsp;Image&nbsp;Info&nbsp;Version</tt></td>
<td align="left"><b>[Required]</b> &mdash; The version of the image info
section. Currently always 0.</td>
</tr>
<tr>
<td><tt>Objective-C&nbsp;Image&nbsp;Info&nbsp;Section</tt></td>
<td align="left"><b>[Required]</b> &mdash; The section to place the
metadata. Valid values are <tt>"__OBJC, __image_info, regular"</tt> for
Objective-C ABI version 1, and <tt>"__DATA,__objc_imageinfo, regular,
no_dead_strip"</tt> for Objective-C ABI version 2.</td>
</tr>
<tr>
<td><tt>Objective-C&nbsp;Garbage&nbsp;Collection</tt></td>
<td align="left"><b>[Required]</b> &mdash; Specifies whether garbage
collection is supported or not. Valid values are 0, for no garbage
collection, and 2, for garbage collection supported.</td>
</tr>
<tr>
<td><tt>Objective-C&nbsp;GC&nbsp;Only</tt></td>
<td align="left"><b>[Optional]</b> &mdash; Specifies that only garbage
collection is supported. If present, its value must be 6. This flag
requires that the <tt>Objective-C Garbage Collection</tt> flag have the
value 2.</td>
</tr>
</tbody>
</table>
<p>Some important flag interactions:</p>
<ul>
<li>If a module with <tt>Objective-C Garbage Collection</tt> set to 0 is
merged with a module with <tt>Objective-C Garbage Collection</tt> set to
2, then the resulting module has the <tt>Objective-C Garbage
Collection</tt> flag set to 0.</li>
<li>A module with <tt>Objective-C Garbage Collection</tt> set to 0 cannot be
merged with a module with <tt>Objective-C GC Only</tt> set to 6.</li>
</ul>
</div>
</div>
<!-- *********************************************************************** -->