https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70490
Bug ID: 70490 Summary: __atomic_load_n(const __int128 *, ...) generates CMPXCHG16B with no warning Product: gcc Version: 5.2.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: mdpoole at troilus dot org Target Milestone: --- When compiling for x86-64 with the -mcx16 flag, there is no diagnostic for code like this: #include <sys/mman.h> __int128 test(void) { const void *ptr = mmap(0, 4096, PROT_READ, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0); const __int128 *p_v = (const __int128 *)ptr; return __atomic_load_n(p_v, __ATOMIC_SEQ_CST); } The CMPXCHG16B instruction that is generated unconditionally attempts to write to the address, which causes a fault at runtime. When __atomic_load_n() uses that instruction, it seems prudent to reject a const pointer as the first argument. Command line: gcc -Wall -Wextra -mcx16 -O2 -c atomic.c First line of "gcc --version": gcc (Ubuntu 5.2.1-22ubuntu2) 5.2.1 20151010