Spurious clang-cl warnings compiling atomics for ARM64EC

[skoulik]

The problem is only reproducible when compiling ARM64EC targets. Using std::atomic_bool::load, std::atomic_signal_fence and others produce warnings: '+cx16' is not a recognized feature for this target (ignoring feature)

Compiling for ARM64, AMD64 does not produce the warnings.

clang-cl --version
clang version 21.1.0
Target: x86_64-pc-windows-msvc
Thread model: posix

test.cpp:
#include <atomic>
int main()
{
std::atomic_signal_fence(std::memory_order_seq_cst);
return 0;
}

test.cpp (another example):
#include <atomic>
int main()
{
std::atomic_bool test {false};
auto t = test.load(std::memory_order_seq_cst);
return 0;
}

clang-cl --target=arm64ec-pc-windows-msvc test.cpp
'+cx16' is not a recognized feature for this target (ignoring feature)
'+cx16' is not a recognized feature for this target (ignoring feature)

clang-cl --target=arm64-pc-windows-msvc test.cpp
(empty output)

clang-cl --target=amd64-pc-windows-msvc test.cpp
(empty output)

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: Sergei Kulik (skoulik)

The problem is only reproducible when compiling ARM64EC targets. Using std::atomic_bool::load, std::atomic_signal_fence and others produce warnings: '+cx16' is not a recognized feature for this target (ignoring feature)

Compiling for ARM64, AMD64 does not produce the warnings.

clang-cl --version
clang version 21.1.0
Target: x86_64-pc-windows-msvc
Thread model: posix

test.cpp:
#include &lt;atomic&gt;
int main()
{
std::atomic_signal_fence(std::memory_order_seq_cst);
return 0;
}

test.cpp (another example):
#include &lt;atomic&gt;
int main()
{
std::atomic_bool test {false};
auto t = test.load(std::memory_order_seq_cst);
return 0;
}

clang-cl --target=arm64ec-pc-windows-msvc test.cpp
'+cx16' is not a recognized feature for this target (ignoring feature)
'+cx16' is not a recognized feature for this target (ignoring feature)

clang-cl --target=arm64-pc-windows-msvc test.cpp
(empty output)

clang-cl --target=amd64-pc-windows-msvc test.cpp
(empty output)

[efriedma-quic]

Reduced:

#pragma clang attribute _STD_ATOMIC_HEADER.push([[gnu::target("cx16")]], apply_to = function)
extern "C" inline void atomic_signal_fence() noexcept {}
#pragma clang attribute _STD_ATOMIC_HEADER.pop

int main()
{
  atomic_signal_fence();
  return 0;
}

There is a clang bug here: the frontend should print a proper error/warning instead of just passing the raw feature flag to the backend. But the STL headers shouldn't do this.

It looks like there's a fix in the latest STL (microsoft/STL#5717); CC @StephanTLavavej .

[StephanTLavavej]

Yes, until VS shipped Clang 20, we weren't able to properly test and support it, and we had zero test coverage for Clang ARM64EC. After microsoft/STL#5717, Clang ARM64EC is tested and supported.