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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 - Bad inlining decision when caller puts on stack bunch of unused in callee data (boost::variant visitation)"
href="https://bugs.llvm.org/show_bug.cgi?id=41491">41491</a>
</td>
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<th>Summary</th>
<td>Bad inlining decision when caller puts on stack bunch of unused in callee data (boost::variant visitation)
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Keywords</th>
<td>code-quality
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Interprocedural Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>nok.raven@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Blocks</th>
<td>4931
</td>
</tr></table>
<p>
<div>
<pre>boost::variant visitation recursively wraps visitor reference about 3 times.
LLVM decides to not inline even relatively-small switches, leaving switch in
one function and all visitor wrapping in another. Because of that it cannot
optimize-out those wrappers and produces bunch of store/loads.
I don't know whether GCC is more clever or aggressive, but it will inline the
switch even a huge one. <a href="https://godbolt.org/z/R6NpPa">https://godbolt.org/z/R6NpPa</a>
// Removes switch bloat due to std::abort, will be part of Boost 1.71
#define BOOST_VARIANT_DETAIL_FORCED_RETURN_HPP
namespace boost { namespace detail { namespace variant {
template <typename T> T forced_return() { __builtin_unreachable(); }
}}}
#include <boost/variant.hpp>
struct A {}; struct B {}; struct C {};
struct D {}; struct E {}; struct F {};
struct G {}; struct H {}; struct J {};
template <typename T> void f(T);
struct Visitor
{
template <typename T> void operator()(T v) const { f(v); }
};
void foo(boost::variant<A, B, C, D, E, F, G, H, J> v)
{
boost::apply_visitor(Visitor{}, v);
}</pre>
</div>
</p>
<div id="referenced">
<hr style="border: 1px dashed #969696">
<b>Referenced Bugs:</b>
<ul>
<li>
[<a class="bz_bug_link
bz_status_NEW "
title="NEW - [meta] Improve inlining effectiveness"
href="https://bugs.llvm.org/show_bug.cgi?id=4931">Bug 4931</a>] [meta] Improve inlining effectiveness
</li>
</ul>
</div>
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