I posted this on the learn section of the forum earlier in the week, but I'm reasonably certain this is a compiler bug. 

Essentially, when I run some code which has inline assembly through the profiler, I get unexpected results.

Here's a reproduction:

// bsf is Bitscan Forward. It should return the index of the least significant bit which is on.
ubyte LS1B(ulong board)
{
  asm
  {
    bsf RAX, board;
  }
}

void main()
{
    import std.conv;

    for (int i = 0; i < 63; ++i)
    {
        auto slide = (1UL << i);
        auto ls1b = slide.LS1B;
        assert(ls1b == i, "Incorrect LS1B ~ expected: " ~ i.to!string ~ " got: " ~ ls1b.to!string);
    }
}



If I run the above like so:

dmd -debug app.d; ./app

It doesn't assert. But if I run it like so:

dmd -profile app.d; ./app

It asserts, and comes back with a different answer every time.