setBit at runtime and compile-time

main.d

auto setBit(ubyte idx)(ulong bitfield)
{
    enum mask = 1UL << idx;
    return bitfield |= mask;
}

ulong setBit(ulong bitfield, ulong idx)
{
    ulong mask = 1UL << idx;
    return bitfield |= mask;
}

private void doNotOptimizeAway(T)(auto ref T t)
{
    import core.thread : getpid;
    import std.stdio : writeln;
    if(getpid() == 1) {
        writeln(*cast(char*)&t);
    }
}

__gshared ulong r;

void main()
{
    import std.datetime: benchmark, Duration;
    import std.stdio : writefln;
    import std.conv : to;

    ulong bitfield = 42_000;
    ubyte idx = 10;

    auto bench = benchmark!(
        { r += setBit(bitfield, idx); },
        { r += setBit!10(bitfield); },
    )(2_000_000_000);

    string[] names = ["setbit.runtime", "setbit.ctfe"];

    foreach(j,r;bench)
        writefln("%-14s = %s", names[j], r.to!Duration);
}

output

> dmd -release -O -boundscheck=off foo.d main.d && ./main

setbit.runtime = 6 secs, 176 ms, 765 μs, and 7 hnsecs
setbit.ctfe    = 5 secs, 427 ms, 177 μs, and 1 hnsec

> ldmd -inline -release -O3 -boundscheck=off main.d && ./main

setbit.runtime = 1 sec, 374 ms, 360 μs, and 5 hnsecs
setbit.ctfe    = 676 ms, 810 μs, and 4 hnsecs