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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [coroutines] Bad codegen that causes a coroutine to resume at the wrong suspend-point under -O2 and above"
href="https://bugs.llvm.org/show_bug.cgi?id=45321">45321</a>
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<th>Summary</th>
<td>[coroutines] Bad codegen that causes a coroutine to resume at the wrong suspend-point under -O2 and above
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>10.0
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>LLVM Codegen
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>lewissbaker@gmail.com
</td>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>The following godbolt contains a sample that shows correct output under clang
10 with -O1 but incorrect output under -O2 (also observed under -O3).
<a href="https://godbolt.org/z/dUn2-p">https://godbolt.org/z/dUn2-p</a>
Note that the issue does not seem to occur under current clang trunk so may
have already been fixed. Reporting here in case a fix needs to be backported to
clang 10 branch. Feel free to close if a backport to clang 10 is not required.
The code generated for the coroutine:
immediate_task task() {
awaitable a(0);
awaitable b(1);
std::printf("Expected 0: %i\n", co_await a);
std::printf("Expected 1: %i\n", co_await b);
}
When compiled with: -std=c++2a -stdlib=libc++ -O2
Suspends first at `co_await a', writing its coroutine_handle to a global var
which is then used to resume it.
However, when it suspends at `co_await b` and is subsequently resumed it
resumes at the `co_await a` expression instead of the `co_await b` expression.
Note that minor changes to the code can make it generate the correct output.
eg. changing immediate_task::promise_type::final_suspend() to return
suspend_never instead of returning an awaitable that calls .destroy() inside
the await_suspend() method.</pre>
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