[llvm-bugs] [Bug 30675] New: atomic::compare_exchange returns wrong value

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=30675

            Bug ID: 30675
           Summary: atomic::compare_exchange returns wrong value
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: release blocker
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: dvyukov at google.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com
    Classification: Unclassified

llvm/clang/libc++ are on rev 271514.

Test is:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <atomic>

template<int kSize>
struct MyStruct {
  char data[kSize];

  explicit MyStruct(char v = 0) noexcept {
    memset(&data[0], v, sizeof(data));
  }

  bool operator == (const MyStruct &other) const {
    return memcmp(&data[0], &other.data[0], sizeof(data)) == 0;
  }

  bool operator != (const MyStruct &other) const {
    return !(*this == other);
  }

  operator int() const {
    return data[0];
  }
};

int main() {
  typedef MyStruct<6> T;
  std::atomic<T> a(T(0));
  T cmp(17);
  if (a.compare_exchange_strong(cmp, T(18), std::memory_order_acquire)) {
    fprintf(stderr, "%d: bad atomic value %d\n", __LINE__, (int)cmp);
    exit(1);
  }
  if (cmp != T(0)) {
    fprintf(stderr, "%d: bad atomic value %d\n", __LINE__, (int)cmp);
    exit(1);
  }
}

The test works with libstdc++:

$ bin/clang++ /tmp/atomic.cc -std=c++11 -latomic && ./a.out

But fails with libc++:

$ bin/clang++ /tmp/atomic.cc -std=c++11 -stdlib=libc++ -latomic &&
LD_LIBRARY_PATH=./lib/ ./a.out
36: bad atomic value 17

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