[m-rev.] for review: fix purity in tabling and optimization passes

[Fergus Henderson]

On 24-Mar-2001, Simon Taylor <stayl at cs.mu.OZ.AU> wrote:
> 
> compiler/deforest.m:
> 	Use the same method as inlining.m to work out whether
> 	a procedure can be inlined. Don't inline predicates which
> 	are promised pure because the extra impurity which
> 	will be propagated through the goal will stop deforestation
> 	working on the goal.

What about predicates which are inferred semipure and promised pure?
For those, the extra impurity (or perhaps I should say the extra
semi-impurity) shouldn't prevent deforestation, should it?
It seems to me that the right condition to check here
is not whether the predicate was promised pure, but whether
the predicate body is impure.

> -warn_unnecessary_promise_pure(ModuleInfo, PredInfo, PredId) -->
> +warn_unnecessary_promise_pure(ModuleInfo, PredInfo, PredId, PromisedPurity) -->
>  	{ pred_info_context(PredInfo, Context) },
>  	write_context_and_pred_id(ModuleInfo, PredInfo, PredId),
>  	prog_out__write_context(Context),
> -	report_warning("  warning: unnecessary `promise_pure' pragma.\n"),
> +	{
> +		PromisedPurity = pure,
> +		Pragma = "promise_pure",
> +		CodeStr = "impure or semipure"
> +	;
> +		PromisedPurity = (semipure),
> +		Pragma = "promise_semipure",
> +		CodeStr = "impure"
> +	;
> +		PromisedPurity = (impure),
> +		error("purity__warn_unnecessary_promise_pure: promise_impure?")
> +	},
> +
> +	report_warning("  warning: unnecessary `"),
> +	io__write_string(Pragma),
> +	io__write_string("' pragma.\n"),
>  	globals__io_lookup_bool_option(verbose_errors, VerboseErrors),
>  	( { VerboseErrors = yes } ->
>  		prog_out__write_context(Context),
>  		{ pred_info_get_is_pred_or_func(PredInfo, PredOrFunc) },
>  		io__write_string("  This "),
>  		hlds_out__write_pred_or_func(PredOrFunc),
> -		io__write_string(
> -		    " does not invoke any impure or semipure code,\n"
> -		),
> +		io__write_string(" does not invoke any "),
> +		io__write_string(CodeStr),  
> +		io__write_string(",\n"),

I think the word "code" got dropped there.

> +:- type post_typecheck_message
> +	--->	error(post_typecheck_error)
> +	;	warning(post_typecheck_warning)
> +	.
> +
> +:- type post_typecheck_messages == list(post_typecheck_message).
> +	
> +:- type post_typecheck_error
> +	--->	missing_body_impurity_error(prog_context, pred_id)
> +	;	impure_closure(prog_context, purity)
> +	;	impure_unification_expr_error(prog_context, purity)
> +	;	aditi_builtin_error(aditi_builtin_error)
> +	.
> +
> +:- type post_typecheck_warning
> +	--->	unnecessary_body_impurity_decl(prog_context, pred_id, purity).

Hmm, should all go in post_typecheck.m rather than purity.m?
Or should those categories be renamed purity_error, purity_warning, etc.?

> --- doc/reference_manual.texi	2001/03/15 06:32:06	1.203
> +++ doc/reference_manual.texi	2001/03/24 02:10:09
> @@ -5846,20 +5846,24 @@
>  declared purity of the calls it executes,  the lowest purity bound is
>  propagated up from callee to caller through the program.
>  
> -However, some predicates which call impure or semipure predicates are
> -themselves pure. 
> -The only way for the programmer to stop the propagation of impurity is
> -to explicitly promise that a predicate or function is pure.
> +In some cases the impurity of a predicate's body is an implementation
> +detail which should not be exposed to callers. These predicates should
> +be considered to be pure or semipure even though they call impure or
> +semipure predicates. The only way for the programmer to stop the
> +propagation of impurity is to explicitly promise that the predicate
> +or function is pure or semipure.

s/should be considered/are/

> +		% XXX Storing the purity in the pred_info is the
> +		% wrong thing to do, because optimizations can
> +		% make some procedures more pure than others.

Hmm, yes.  I need to think about that one after I've had some sleep.

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