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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [coroutines] errno address is reused after context switch"
href="https://bugs.llvm.org/show_bug.cgi?id=49913">49913</a>
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<th>Summary</th>
<td>[coroutines] errno address is reused after context switch
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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>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>C++2a
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>antoshkka@gmail.com
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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>Created <span class=""><a href="attachment.cgi?id=24742" name="attach_24742" title="Full example">attachment 24742</a> <a href="attachment.cgi?id=24742&action=edit" title="Full example">[details]</a></span>
Full example
Consider the example:
CoroTask CoroToDealWith() {
if (errno) first_call();
co_await writerQueue;
if (errno) second_call();
}
With `-O2 -std=c++20 -stdlib=libc++` flags clang generates code that stores the
`errno` address on first call:
call __errno_location
mov qword ptr [rbx + 24], rax
cmp dword ptr [rax], 0
je .LBB1_2
call first_call()
and reuses that address after wakeup:
CoroToDealWith() [clone .resume]: # @CoroToDealWith() [clone .resume]
push rbx
mov rbx, rdi
mov rax, qword ptr [rdi + 24]
cmp dword ptr [rax], 0
je .LBB2_2
call second_call()
The optimization is wrong: coroutine could be resumed on other thread and the
errno address changes.
Godbolt playground: <a href="https://godbolt.org/z/Wh9xd9oME">https://godbolt.org/z/Wh9xd9oME</a>
Looks like the common subexpression elimination pass must not be applied
between the context switches for the functions with __attribute__((const))</pre>
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