[m-rev.] for review: trail resets

[Julien Fischer]

On Wed, 11 Jun 2008, Julien Fischer wrote:

> Estimated hours taken: 3
> Branches: main
>
> Provide a mechanism for resetting the trail.  This allows the reuse of
> memory that is occupied by any entries on the trail.  This can only be done
> safely after it is certain that any calls that added the trail entries
> cannot be backtracked over.  For some programs doing this can prevent trail
> exhaustation.
>
> This diff does not provide a way of doing this at the Mercury level, nor
> can the compiler currently automatically add calls to reclaim memory used
> by trail.  Both these things are future work.
>
> XXX the interaction between this and the debugger is a bit unclear since
> trailing and debugging don't currently work together properly.
>
> runtime/mercury_trail.h:
> runtime/mercury_trail.c:
> 	Add a new function MR_reset_trail().  When called this
> 	function walks back along the trail calling function trail entries
> 	with the MR_gc untrail reason.  It then zeros out the trail zone
> 	and resets the ticket counter and ticket high water mark to their
> 	initial values.
>
> 	Define two macros, MR_TRAIL_ZONE and MR_TRAIL_BASE that expand
> 	to the addresses of the trail zone and the base of the trail
> 	respectively in a grade independent manner.
>
> 	Redefine MR_num_trail_entries() using the MR_TRAIL_BASE macro.
>
> 	Document that MR_gc is now used for this.
>
> 	Fix a typo.
>
> tests/trailing/Mmakefile:
> tests/trailing/test_trail_reset.m:
> tests/trailing/test_trail_reset.exp:
> 	A test of the trail reset functionality.

I'm going to go ahead and commit this since any potential reviewers
are either busy marking exams or on holidays.  I fixed the typos
in the original diff that prevented it working when MR_THREAD_SAFE
was defined.

Julien.
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