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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Add function __attribute__((idempotent))"
href="https://bugs.llvm.org/show_bug.cgi?id=34600">34600</a>
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<th>Summary</th>
<td>Add function __attribute__((idempotent))
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
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</tr>
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<th>Component</th>
<td>-New Bugs
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</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>bugzilla@tecnocode.co.uk
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</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
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<div>
<pre>(Copied from a similar gcc bug here:
<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32911">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32911</a>)
tl;dr: It would be useful for GObject-based C code if Clang supported a new
__attribute__((idempotent)) to mark functions as doing something (and probably
changing global state) the first time they are called, but being effectively
__attribute__((const)) for every subsequent call.
In GNOME's gobject object-oriented code, each type has a "get_type" function.
Something like, eg:
GType
pango_layout_line_get_type(void)
{
static GType our_type = 0;
if (our_type == 0)
our_type = g_boxed_type_register_static (I_("PangoLayoutLine"),
(GBoxedCopyFunc) pango_layout_line_ref,
(GBoxedFreeFunc) pango_layout_line_unref);
return our_type;
}
Since these functions are called quite frequently (as part of casts among other
things), people have started marking them with __attribute__((const)) so the
compiler can optimize away multiple calls to them. However, there is a down
side to it. Sometimes one needs to make sure the type is registered, so they
call the _get_type() function to ensure that, but the compiler, seeing the
((const)) attribute, optimizes away the call.
This is of course a bug to mark functions with side-effects as ((const)).
That's why a new function attribute like "idempotent" is useful. What it means
is that subsequent calls to this function return the same value as the first
call, have no side-effect, and can be optimized away, but not the first call...
More information about GObject requirements here:
<a href="https://bugzilla.gnome.org/show_bug.cgi?id=64994">https://bugzilla.gnome.org/show_bug.cgi?id=64994</a>
This would presumably also be useful for various implementations of singletons
in other idiomatic C code.</pre>
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