[m-rev.] for review: fix `mmc --make' signal bugs

[Fergus Henderson]

On 21-Nov-2002, Simon Taylor <stayl at cs.mu.OZ.AU> wrote:
> On 21-Nov-2002, Fergus Henderson <fjh at cs.mu.OZ.AU> wrote:
> > That diff looks fine, but while reviewing it I spotted a bug
> > in the existing code that you were modifying:
> > 
> > On 21-Nov-2002, Simon Taylor <stayl at cs.mu.OZ.AU> wrote:
> > > 
> > > +++ compiler/process_util.m	21 Nov 2002 00:46:43 -0000
> > > @@ -193,7 +193,7 @@
> > >  
> > >  	MR_incr_hp_msg(SigintHandler,
> > >  		MR_bytes_to_words(sizeof(MR_signal_action)),
> > > -		MR_PROC_LABEL, ""make.util.signal_action/0"");
> > > +		MR_PROC_LABEL, ""libs.process_util.signal_action/0"");
> > 
> > There is no guarantee that the memory allocated by MR_incr_hp_msg()
> > will be sufficiently aligned to hold a value of type MR_signal_action.
> 
> How aligned does it need to be?

That depends on the C implementation.

In practice I suspect that word alignment would be sufficient for most
implementations.  But it's not guaranteed to work.

> runtime/mercury_heap.h contains the following code in
> MR_MAYBE_BOX_FOREIGN_TYPE.
> 
> 	/* XXX this assumes that nothing requires */
> 	/* stricter alignment than MR_Float */
> 	MR_make_hp_float_aligned();
> 	MR_incr_hp(MR_LVALUE_CAST(MR_Word, (box)), size_in_words);              
> 
> Is putting a call to MR_make_hp_float_aligned() before the call
> to MR_incr_hp_msg() acceptable, or should I be using some other
> method to allocate the memory.

I'd be reluctant to duplicate that hack; it would be 
be better to encapsulate it in a single routine

 	/* XXX this assumes that nothing requires */
 	/* stricter alignment than MR_Float */
	#define MR_make_hp_fully_aligned() MR_make_hp_float_aligned()

so that the XXX can be fixed in one place later.

But an even better solution would be to use `pragma foreign_type'
for the signal action type.  Then the Mercury compiler will
handle the necessary memory allocation automatically
(using MR_MAYBE_BOX_FOREIGN_TYPE()).

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  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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