Black box function

[kazimuth]

It would be nice to have a function to disable dead-code elimination / constant propagation. Rust has this; it's implemented as the identity function, but with magic inside:

assert_eq!(100, std::hints::black_box(100));

Rust implementation:

/// A function that is opaque to the optimizer, to allow benchmarks to
/// pretend to use outputs to assist in avoiding dead-code
/// elimination.
///
/// This function is a no-op, and does not even read from `dummy`.
#[inline]
#[unstable(feature = "test", issue = "27812")]
#[allow(unreachable_code)] // this makes #[cfg] a bit easier below.
pub fn black_box<T>(dummy: T) -> T {
    // We need to "use" the argument in some way LLVM can't introspect, and on
    // targets that support it we can typically leverage inline assembly to do
    // this. LLVM's intepretation of inline assembly is that it's, well, a black
    // box. This isn't the greatest implementation since it probably deoptimizes
    // more than we want, but it's so far good enough.
    #[cfg(not(any(
        target_arch = "asmjs",
        all(
            target_arch = "wasm32",
            target_os = "emscripten"
        )
    )))]
    unsafe {
        asm!("" : : "r"(&dummy));
        return dummy;
    }

    // Not all platforms support inline assembly so try to do something without
    // inline assembly which in theory still hinders at least some optimizations
    // on those targets. This is the "best effort" scenario.
    unsafe {
        let ret = crate::ptr::read_volatile(&dummy);
        crate::mem::forget(dummy);
        ret
    }
}

I don't know how hard this would be to translate to Julia.

[gdalle]

See #92

[Zentrik]

I think Base.donotdelete should be similar to Rust's version. It doesn't affect constant folding but I don't think Rust's does either.