4 ms·

What is Rust's policy for signed/unsigned int overflow? I assume that it's not modulo or else the complier should have generated ADC for you. I assume you've b

• masklinn 6y ago
  > What is Rust's policy for signed/unsigned int overflow? Defined, by default checked in debug mode and unchecked in release, unsigned wraps and signed wraps as two's complement. This can be overridden by explicitly setting overflow-checks in the relevant profile. It also, separately, has explicitly wrapping, saturating and checking versions of basic arithmetic operations.
• palmtree3000 6y ago
  overflowing_add, like I was using above, is explicitly wrapping. I've found it very difficult to provoke rustc into emitting an ADC: the only case where it does so AFAICT is when adding u128s, which are implemented using u64s. Not sure why, except that the shortest function I could think of to emulate ADC is kind of baroque, and it's possible the compiler can't figure it out. I've mostly been using cpuprofiler[1] and Vtune to simultaneously profile my code and show the assembly. In theory they both provide timing information per-instruction, but I don't really trust it. For the 6 adc instructions above, it shows the number of clock ticks as ranging from 22 million to 3 billion, which doesn't make sense to me. But at least it shows me the assembly! [1] https://docs.rs/cpuprofiler/0.0.4/cpuprofiler/index.html https://docs.rs/cpuprofiler/0.0.4/cpuprofiler/index.html
  • secondcoming 6y ago
    This will change your life! https://godbolt.org/z/fntvYa https://godbolt.org/z/fntvYa