[llvm-bugs] [Bug 38871] New: std::atomic not always atomic when compiling with GCC

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38871

            Bug ID: 38871
           Summary: std::atomic not always atomic when compiling with GCC
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: richard-llvm at metafoo.co.uk
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

Compiling this with GCC:

#include <atomic>
struct X { char c[4]; };
std::atomic<X> x;

... results in a call to __atomic_load to load the X object. However, due to
gcc.gnu.org/PR87237, that builtin is not actually atomic in this case.

libstdc++ works around this by increasing the alignment of atomic<X> to
sizeof(X) if sizeof(X) is a power of 2 not greater than 16, so that's
presumably the set of cases where the GCC bug occurs. libc++ should probably do
the same.

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