Inline assembly and Profiling

[Matthew Dudley via Digitalmars-d-learn]

I'm working on a chess engine side-project, and I'm starting to 
get into profiling and optimization.

One of the optimizations I've made involves some inline assembly, 
and I ran across some apparently bizarre behavior today, and I 
just wanted to double-check that I'm not doing something wrong.

Here's the behavior boiled down:

import std.stdio;
ubyte LS1B(ulong board)
{
   asm
   {
     bsf RAX, board;
   }
}

void main()
{
   auto one = 0x939839FA;
   assert(one.LS1B == 1, "Wrong LS1B!");
}

If I run this through DMD without profiling on, it runs 
successfully, but with profiling on, the assertion fails. And in 
the actual code, it returns seeming random numbers.

Is the profiling code stomping on my toes here? Am I not allowed 
to just single instruction into RAX like this with profiling on? 
Or is this just a compiler bug?