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[SYCL] Fix library destructor priority value #3054

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Jan 21, 2021
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[SYCL] Fix library destructor priority value #3054

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[SYCL] Fix library destructor priority value
Jan 19, 2021
313ea53
address feedback
Jan 19, 2021
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Update docs
Jan 20, 2021
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11 changes: 7 additions & 4 deletions sycl/doc/GlobalObjectsInRuntime.md
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Expand Up @@ -57,10 +57,13 @@ destruction of nested `std::unique_ptr`s.

### Linux

On Linux DPC++ runtime uses `__attribute__((destructor))` property with maximum
possible priority value 65535. This approach does not guarantee, that
`GlobalHandler` destructor is the last thing to run, as user code may contain
a similar function with the same priority value.
On Linux DPC++ runtime uses `__attribute__((destructor))` property with low
priority value 110. This approach does not guarantee, that `GlobalHandler`
destructor is the last thing to run, as user code may contain a similar function
with the same priority value. At the same time, users may specify priorities
within [101, 109] range in order to run destructor after SYCL runtime has been
de-initialized. A destructor without specific priority value is going to be
executed before runtime shutdown mechanisms.

Another approach would be to leak global objects. This would guarantee user,
that global objects live long enough. But some global objects allocate heap
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8 changes: 5 additions & 3 deletions sycl/source/detail/global_handler.cpp
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Expand Up @@ -142,9 +142,11 @@ BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpReserved) {
return TRUE; // Successful DLL_PROCESS_ATTACH.
}
#else
// Setting maximum priority on destructor ensures it runs after all other global
// destructors.
__attribute__((destructor(65535))) static void syclUnload() { shutdown(); }
// Setting low priority on destructor ensures it runs after all other global
// destructors. Priorities 0-100 are reserved by the compiler. The priority
// value 110 allows SYCL users to run their destructors after runtime library
// deinitialization.
__attribute__((destructor(110))) static void syclUnload() { shutdown(); }
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Should we document it somewhere what priority are for what? And how it interacts with no priority specified?

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I conducted a small experiment:

#include <iostream>

__attribute__((destructor(101))) static void syclUnload101() {
    std::cout << "101\n";
}

__attribute__((destructor(102))) static void syclUnload102() {
    std::cout << "102\n";
}

__attribute__((destructor(110))) static void syclUnload110() {
    std::cout << "110\n";
}

__attribute__((destructor(105))) static void syclUnload105() {
    std::cout << "105\n";
}

__attribute__((destructor)) static void syclUnload() {
    std::cout << "default\n";
}

int main() {
    std::cout << "Hi\n";
    return 0;
}

The result is the same for both GCC and Clang:

Hi
default
110
105
102
101

#endif
} // namespace detail
} // namespace sycl
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