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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed optimization: static guard variable generated for empty virtual destructor"
href="https://bugs.llvm.org/show_bug.cgi?id=36591">36591</a>
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<th>Summary</th>
<td>Missed optimization: static guard variable generated for empty virtual destructor
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>arthur.j.odwyer@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>Consider the following program:
struct Base {
constexpr Base() = default;
virtual int foo();
// virtual ~Base() = default;
};
struct Derived : public Base {
constexpr Derived() = default;
virtual int foo() override;
};
Base& get_base() {
static Derived d;
return d;
}
This is a simplified version of the current idioms around `std::error_category`
and `std::pmr::memory_resource`, except that for now I have commented-out the
virtual destructor.
Notice that because `Derived d` is trivially destructible, no atomic guard
variable is generated to guard its construction with
__cxa_guard_acquire/__cxa_guard_release.
Now uncomment the defaulted virtual destructor of `Base`. The semantics of
`Derived d` have not changed: we are still expecting to call ~Derived by
non-virtual dispatch, and ~Derived is still known statically to be a no-op.
(And Derived has a vtable either way, too.) But now suddenly Clang decides to
generate an atomic guard variable!
I think Clang could do better here, and if it did, it would eliminate a lot of
atomic instructions in the critical path for things like `std::error_code{}`
and `std::pmr::new_delete_resource()`.
(Slightly different symptom, same bug, filed as
<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84411">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84411</a> .)</pre>
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