> When I implemented a similar change previously (in my old
> non-CVS speceval
> branch) results suggested that it made little difference to
> non-speculative execution.
>
> The problem was that most closures were either thunks, or, if
> they had already
> been
> evaluated, indirections. As a result, semitagging usually
> failed and so didn't on average win anything.
There's an important difference between this kind of semi-tagging and
the version you implemented before: this version doesn't need to
dereference the pointer to determine whether it is evaluated or not. I
think that should be a win.
We know from ticky results that around 50% of enters are to evaluated
objects (varies wildly from program to program, but 50% seems about
average), so I'd be surprised if avoiding these enters does not save
some memory accesses. I'm not surprised that the old kind of
semitagging didn't win: looking in the info table is just as bad as
entering the thing.
Cheers,
Simon
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