mlprof

[Stephen Weeks]

> I've started in on fixing mlprof to work with the native backend.  I don't
> forsee any problems with that, but I am getting strange behavior with
> executables compiled with mlton -p or mlton -p -g.

Here's what I've learned so far.  I compiled logic.sml with -p and observed the
same errors as Matthew.  Running with strace, I saw the following two system
calls made before the mlton generated code started running.

rt_sigaction(SIGPROF, {0x400e8790, ~[], SA_RESTART|0x4000000}, {SIG_DFL}, 8) = 0
setitimer(ITIMER_PROF, {it_interval={0, 1}, it_value={0, 1}}, {it_interval={0, 0}, it_value={0, 0}}) = 0

During execution, I saw the following.

--- SIGPROF (Profiling timer expired) ---
sigreturn()                             = ? (mask now [])

The errors always happen after at least one SIGPROF.

To confirm that it is the SIGPROF stuff that is causing the problem, I prefixed
the following onto logic.sml and compiled -p.

local
   open MLton
   val t = Time.zeroTime
   val _ = Itimer.set {which = Itimer.Prof,
		       value = t,
		       interval = t}
in
end

In this case, the following system call happens as soon as the code starts
running. 

setitimer(ITIMER_PROF, {it_interval={0, 0}, it_value={0, 0}}, NULL) = 0

This eliminates the segfault. 

Finally, to test whether it MLton's interaction with signals or the specific
signal handling that goes on with profiling, I prefixed logic.sml with the
following code and compiled without -p.

local
   fun handler () = TextIO.output (TextIO.stdErr, "got a sigprof\n")
   open MLton
   structure Handler = Signal.Handler
   val _ = Handler.set (Signal.prof, Handler.simple handler)
   val t = Time.fromMicroseconds 1
   val _ = Itimer.set {which = Itimer.Prof,
		       value = t,
		       interval = t}
in
end

With this prefix, the code works fine.

So, I think there is some bad interaction with the specific SIGPROF handling
stuff that is used by -p and code generated by the native backend (i.e. I agree
with Matthew).  Signals by themselves seem to work fine.