[m-dev.] for review: --optimize-mostly-det-tailcalls

[Fergus Henderson]

On 06-Jun-1999, Zoltan Somogyi <zs at cs.mu.OZ.AU> wrote:
> 
> Add a new option, --optimize-mostly-det-tailcalls, that enables the use of
> Prolog style nondet tail calls (which check whether the current frame is
> on top of the nondet stack and only do a tail call if it is).
...
> --- jumpopt.m	1999/05/28 05:26:18	1.46
> @@ -65,8 +82,16 @@
>  		Procmap0, Procmap, Sdprocmap0, Sdprocmap, Succmap0, Succmap),
>  	map__init(Forkmap0),
>  	jumpopt__build_forkmap(Instrs0, Sdprocmap, Forkmap0, Forkmap),
> +	( MostlyDetTailCall = yes ->
> +		opt_util__get_prologue(Instrs0, ProcLabel, _, _, _),
> +		opt_util__new_label_no(Instrs0, 500, LabelNum),
> +		MostlyDetTailCallInfo = yes(ProcLabel - LabelNum)
> +	;
> +		MostlyDetTailCallInfo = no
> +	),

What's the magic number 500 for?

I think it would be a good idea to centralize the code which allocates
label numbers so that all such magic numbers occur in a single part
of the code.

options.m:
> @@ -2077,6 +2081,12 @@
> +		"\tConvert nondet calls into tail calls whenever possible,",
> +		"\teven if the code before the call is not guaranteed to be det.",
> +		"\tThis increases speed only if the code before the call turns",
> +		"\tout to be det most of the time, but can reduce speed if this",
> +		"\tis not the case.",

> +++ user_guide.texi	1999/06/06 04:38:39
> @@ -3483,6 +3483,14 @@
>  Disable elimination of jumps to ordinary code.
>  
>  @sp 1
> + at item --optimize-mostly-det-tailcalls
> +Convert nondet calls into tail calls whenever possible,
> +even if the code before the call is not guaranteed to be det.
> +This increases speed only if the code before the call turns
> +out to be det most of the time, but can reduce speed if this
> +is not the case.

I think this documentation will be very hard for most users to
understand.  I think it would be better to explain things in more detail,
e.g. mentioning the run-time test.

Regarding "Convert nondet calls" -- a "nondet call" is a call to a nondet
procedure, but here I think you mean a call _from_ a nondet (or multi)
procedure.

Also, "is not guaranteed to be det" is not the right condition.
The right condition is "might return more than one solution" or
something like that.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
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