I've now merged a major piece of development work into GitHub master.
This is a variant of 64-bit mode that uses 32-bit values. It was
triggered by the observation that Isabelle often peformed better with
the 32-bit version than the 64-bit despite the 64-bit version allowing a
much larger heap and having more registers. It appears that Isabelle,
at any rate, is limited more by the bandwidth between the processors and
main memory than by the processors themselves. This may well be the
result of the work on parallelising the application and the garbage
collector.
In effect there are now three versions on the X86: native 64-bit, native
32-bit and 32-bits on 64-bits. The new version can be built with the
--enable-compact32bit option to configure. It allows a heap size of up
to 16Gbytes by using 31-bit "object ids", essentially indexes into an
array of 8-byte cells. This heap contains only the data. Code, thread
stacks and other support structures are outside this.
It has been extensively tested with Isabelle (thanks to Makarius) and
seems to perform better than either of the native address versions. I
started the development about a year ago; development of the final
stages were supported by Data61 and CSIRO through Gerwin Klein.
I would be interested in feedback on this and also the native address
versions. Whether it well help other applications may depend on whether
the limitation is processor speed or memory bandwidth. It's likely that
many applications will continue to perform best with native addressing.
The update also includes changes to the X86 code-generator that affect
all the variants. These changes improve the way boolean values and
conditionals are handled. There also other changes to the IO system.
Garbage-collection of streams has been removed and overlapped IO is now
used on Windows which should improve performance and responsiveness.
There have been changes to the handling of floating point and Real32 has
been added.
Let me have any feedback.
Regards,
David
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