[MLton] Question on profile.fun

[Matthew Fluet]

> > You believe that the move of a constant integer to a known slot in
> > the gc state at transitions in the profile graph is too intrusive?
> 
> Yes.  The point is that it happens all the time, not just at (SSA)
> nontail calls, and not just at (SSA) basic block entries.
> Furthermore, to implement this portably within MLton, the right place
> to put it is at the Machine IL, which means it will interfere with
> codegen optimizations too.  I bet it'll hurt more than 50% on some
> benchmarks.  That's a lot of skew.  I'm already annoyed by the skew
> that we get with -profile time as it is, 20-30% on some benchmarks
> IIRC, although it would be worth rerunning to see where we are today.

Well, I added the -profile mark option which essentially adds the time
profiling labels, but doesn't install the sig handler (or produce a
mlmon.out file).  I ran that through the benchmarks, and I believe that
it's still 20-30% overhead.  So, I think the overhead is more from failing
to optimize in the presence of profiling statements than from gathering
the profile data itself.