Rust Optimizations That C++ Can't Do (Version 2)

Note that if you're very explicit about v not changing in the C++ code, like so:

    const long w = v;
    for (int i = 0; i < 100; ++i) {
      sum += w;
      callback();
    }

...then clang++ generates the following:

    .LBB0_1:
        call    rbx
        dec     ebp
        jne     .LBB0_1
        imul    rax, r14, 100

It optimised away all of the additions, calling callback in a loop and then multiplying w with 100. This is something the rust compiler doesn't currently do.

What this boils down to is that there isn't a good way of telling the compiler that the content of a reference will never change in C++. As noted in the other thread (https://news.ycombinator.com/item?id=14040021), const says that the current context can't change it, but someone else might still be able to.