3 ms·

The fence instruction that Intel recommends (lfence) is way slower than the techniques described here. We measured a 5x slowdown on Web Assembly trying to use i

• br1 9y ago
  Have you considered arr[max(i, arr.len)] instead of AND?
  • tetromino_ 9y ago
    Max() is not a CPU instruction. It's an abstraction, a function that could be implemented either using a branch (which defeats the whole purpose) or, on some architectures, with something like cmovXX. Perhaps they wanted a solution that works on Arm, which I think doesn't have cmovXX. Or maybe Intel does speculation with cmovXX used on array index.
    • olliej 9y ago
      Cmov can speculate as well (it makes sense from a performance standpoint but that isn’t the same as secure ;) )
      • om2 9y ago
        On some CPUs, cmov is known not to speculate, but in those cases it is apparently super slow.