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The pre-ARMv7 CPUs are missing atomic compare and exchange and/or barrier instructions. Therefore those are implemented using kernel assistance, calling a kernel function at a specific address, and passing the arguments in the r0 to r4 registers. This is done by specifying registers for local variables. The a_ptr variable is placed in the r2 register and declared with __typeof (mem). According to the GCC documentation on local register variables, if mem is a constant pointer, the compiler may substitute the variable with its initializer in asm statements, which may cause the corresponding operand to appear in a different register. This happens in __libc_start_main with the pointer to the thread counter for static binaries (but not the shared ones): # ifdef SHARED unsigned int *ptr = __libc_pthread_functions.ptr_nthreads; # ifdef PTR_DEMANGLE PTR_DEMANGLE (ptr); # endif # else extern unsigned int __nptl_nthreads __attribute ((weak)); unsigned int *const ptr = &__nptl_nthreads; # endif This causes static binaries using threads to crash when the GNU libc is built with GCC 8 and most notably tst-cancel21-static. To fix that, use the same trick than for the volatile qualifier, defining a_ptr as a union. Changelog: [BZ #24034] * sysdeps/unix/sysv/linux/arm/atomic-machine.h (__arm_assisted_compare_and_exchange_val_32_acq): Use uint32_t rather than __typeof (...) for the a_ptr variable. |
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