Note: this instruction is new to ARMv8.

Simple example:

$ cat test.c
int main() {
    asm volatile("stlex r0, r1, [r2]");
}
$ gcc -marm -march=armv8-a test.c 
$ valgrind ./a.out
==28004== Memcheck, a memory error detector
==28004== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==28004== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==28004== Command: ./a.out
==28004== 
disInstr(arm): unhandled instruction: 0xE1820E91
                 cond=14(0xE) 27:20=24(0x18) 4:4=1 3:0=1(0x1)
...


Reproducible: Always