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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Coroutine suspend/resume in loop not always optimized away"
href="https://bugs.llvm.org/show_bug.cgi?id=35405">35405</a>
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</tr>
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<th>Summary</th>
<td>Coroutine suspend/resume in loop not always optimized away
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>strager@fb.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
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<pre>It seems like Clang+LLVM has some trouble optimizing my code. In a loop, I am
suspending a coroutine then calling
std::experimental::coroutine_handle::resume. See main_v0_loop in the program
below.
I expect Clang+LLVM to optimize the loop away completely, but the generated
code repeatedly suspends and resumes the coroutine.
Clang+LLVM *is* able to properly optimize similar loops. It also is able to
properly optimize the loop if I unroll the loop, but not always. See the other
main_* functions in the program below.
Let me know if (and how) I should reduce the program further.
Program:
<a href="https://godbolt.org/g/m2YSsW">https://godbolt.org/g/m2YSsW</a>
main_v0_loop: gen_pop-push 1000 times, then gen_pop once (suspending the
coroutine forever). Conceptually, the gen_pop-s are done asynchronously on a
different "thread".
main_v1_loop: push-gen_pop 1000 times. Conceptually, the gen_pop-s are done
synchronously on the main "thread".
main_v2_loop: push-pop 1000 times.
main_v3_loop: push-gen_pop 1000 times, then gen_pop once (suspending the
coroutine forever). Conceptually, the gen_pop-s are done asynchronously on a
different "thread".
main_v0_single: Like main_v0_loop, but iterate only once and manually unroll
the loop.
main_v1_single: Like main_v1_loop, but iterate only once and manually unroll
the loop.
main_v2_single: Like main_v2_loop, but iterate only once and manually unroll
the loop.
main_v3_single: Like main_v3_loop, but iterate only once and manually unroll
the loop.
main_v0_triple: Like main_v0_loop, but iterate only 3 times and manually unroll
the loop.
main_v1_triple: Like main_v1_loop, but iterate only 3 times and manually unroll
the loop.
main_v2_triple: Like main_v2_loop, but iterate only 3 times and manually unroll
the loop.
main_v3_triple: Like main_v3_loop, but iterate only 3 times and manually unroll
the loop.
Clang+LLVM version: 6.0.0 trunk 318899 (according to Godbolt's Compiler
Explorer <<a href="https://godbolt.org/">https://godbolt.org/</a>>)
Clang driver flags: -std=c++17 -fcoroutines-ts -stdlib=libc++ -O2 -DNDEBUG=1
-fno-exceptions
Platform: Linux x86_64 (I think; I used Godbolt's Compiler Explorer
<<a href="https://godbolt.org/">https://godbolt.org/</a>>)
Expected results:
X: The generated code for main_v0_*, main_v1_*, and main_v3_* has no calls to
operator new or operator delete (due to the coroutine created by pop_once or
keep_popping).
Y: The generated code for all functions has no indirect jumps (due to the call
to coroutine_handle::resume).
Z: The generated code for all functions is just 'ret', since the function has
no side effects.
Observed results:
X: As expected, the generated code for all functions has no calls to operator
new or operator delete.
Y: The generated code for main_v0_loop, main_v0_triple, main_v3_loop, and
main_v3_triple has indirect jumps. The generated code for all other functions
has no indirect jumps as expected.
Z: The generated code for main_v0_loop, main_v0_triple, main_v3_loop, and
main_v3_triple is non-trivial. The generated code for all other functions is
just 'ret' as expected.</pre>
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