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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 - Coroutine symmetric transfer tail call optimization not working on AArch64"
href="https://bugs.llvm.org/show_bug.cgi?id=43508">43508</a>
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<th>Summary</th>
<td>Coroutine symmetric transfer tail call optimization not working on AArch64
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>9.0
</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>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++2a
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>bartde@microsoft.com
</td>
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<th>CC</th>
<td>blitzrakete@gmail.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>The following code:
task<void> sync_async() { co_return; }
task<void> do_async()
{
for (int i = 0; i < 1024 * 1024; i++)
{
co_await sync_async();
}
}
Causes a stack overflow
sync_async
do_async
sync_async
do_async
…
using clang9 on AArch64, without optimization (-O0). The task<T> implementation
uses symmetric transfer for the final awaiter:
template <typename Promise>
coroutine_handle_t await_suspend(std::experimental::coroutine_handle<Promise>
h) const noexcept
{
return h.promise().m_waiter;
}
and for its operator co_await implementation:
coroutine_handle_t await_suspend(coroutine_handle_t h) const
{
m_coro.promise().m_waiter = h;
return m_coro;
}
I think the task<T> provided by cppcoro should behave completely similar, so it
can be used for the repro.
The stack overflow doesn't repro on x86/x64, or for higher levels of
optimization on AArch64. Both for -O0 builds, the x86 version emits a tail call
by means of a jmp:
b68d2: e8 49 bc f7 ff callq 32520
<_ZNKSt12experimental16coroutine_handleIvE7addressEv>
b68d7: 48 89 c1 mov %rax,%rcx
b68da: 48 8b 00 mov (%rax),%rax
b68dd: 48 89 cf mov %rcx,%rdi
b68e0: 48 81 c4 a0 00 00 00 add $0xa0,%rsp
b68e7: 5d pop %rbp
b68e8: ff e0 jmpq *%rax
while the AArch64 version emits:
a8be0: 97fe1fbf bl 30adc
<_ZNKSt12experimental16coroutine_handleIvE7addressEv>
a8be4: f9400008 ldr x8, [x0]
a8be8: d63f0100 blr x8
a8bec: a9497bfd ldp x29, x30, [sp, #144]
a8bf0: 910283ff add sp, sp, #0xa0
a8bf4: d65f03c0 ret
which seems to perform a regular call using blr.</pre>
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