[m-rev.] for review: support thread local trailing in .par grades

[Julien Fischer]

On Mon, 7 May 2007, Julien Fischer wrote:

>
> For preliminary review by Zoltan or PeterW.
>
> (This change requires more extensive bootchecking and I also need to
> make sure that it doesn't break G12.)
>
> Estimated hours taken: 8
> Branches: main
>
> Add support for thread local trailing in grades that support parallel
> execution.  Previously the trail state for each context was copied into the
> global variables defined in mercury_trail.[ch] when the context was loaded,
> i.e. we had thread-local trailing in grades that did not support parallel
> execution.
>
> This change extends the runtime to support thread-local trailing in the
> presence of parallel execution.  We do this by adding the relevant fields to
> the MR_engine structure and redefining the abstract registers related to
> trailing to make accesses to the trail state go via the relevant engine.
>
> This also works for parallel trailing grades with the high-level C backend
> because in those grades the trail state is store in a thread-local dummy
> engine.
>
> XXX the coupling between the high-level and low-level versions of the 
> runtimes
> w.r.t trailing is not good.  I intend to fix it as a separate change.
> (It also affects thread local mutables.)

Actually, ignore the hlc specific parts of this diff.  They don't
work properly because of dodgy depedencies on parts of the MR_engine
structure.  Using hlc.par.gc.tr will cause an abort in the runtime
at the moment.  This should be fixable once the XXX above is addressed.

Julien.
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