[llvm-bugs] [Bug 50372] New: [Coroutine] Coroutine collect unneeded variables to coroutine frame.

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50372

            Bug ID: 50372
           Summary: [Coroutine] Coroutine collect unneeded variables to
                    coroutine frame.
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: yedeng.yd at linux.alibaba.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

I am not sure if it is OK to mark this with a bug. It is like a feature more to
me. I mark this with bugzilla since I found this:
https://bugs.llvm.org/show_bug.cgi?id=45566, which tells an optimization
instead of a bug.

The motivation example comes from: https://godbolt.org/z/c4zx9fE88.

The key codes is:
```
cppcoro::task<int> simple() noexcept {
    // 552 byte coroutine frame
    char dummy[500];
    for (int i = 0;i < 500; i++)
        dummy[i] = rand()%8;
    consume(dummy[rand()%500]);
    co_await std::experimental::suspend_always{};
    co_return 10;
}
```

Here we can find that the `dummy` isn't used across suspend point. So the dummy
could show as a stack variable. If compilers could optimize this, we could get
a smaller coroutine frame.

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