[m-rev.] for review: disable_warnings scope

[Zoltan Somogyi]

This is about the LLDS backend.

On Wed, 11 Jan 2017 15:06:17 +1100, Paul Bone <paul at bone.id.au> wrote:
> > > Is the decision of what to make a tail call made twice?  Or do the later
> > > passes simply follow the marked tailcalls?
> > 
> > The former.
> 
> Is that easy enough to change?  To remove the possability that these become
> out-of-sync.

The behavior of the LLDS code generator is dead easy to predict; there is
no realistic possibility of the HLDS-based mark_tail_calls.m making a different
decision than the LLDS code generator and optimizer, if it is run on the HLDS that
the code generator works from.

I now see that the reason for the existence of ml_tailcalls.m is the fact
this is *not* true for the MLDS backend. I will add documentation saying so
to both source files.

There *are* passes between mark_tail_calls.m and the LLDS code generator
that can affect what calls are tail calls. One example is the deep profiling
transformation. However, while this can transform calls that look like
tail calls (to mark_tail_calls.m) into non-tail calls, we still don't want
to generate non-tail-call warnings for these calls, since deep profiling
destroys tail-recursion *wholesale*, and the warning is intended to be
about *individual* places where a recursive call is prevented from being
tail recursive by circumstances *specific* to that call site. (I am ignoring
the deep-profiling transformation that aims to preserve tail recursion,
since it hasn't worked for a while.)

Zoltan.