Re: Is This a Dumb Idea? paralellizing byte codes

gah4 <gah4@u.washington.edu> writes:
>Seems to me it is not that parallelizing byte codes that is
>a dumb idea, but byte codes themselves are.
>
>This was known when Alpha replaced VAX. Work on making faster VAX
>systems was stuck with the byte oriented instruction stream which was
>impossible to pipeline.


Pipelined VAX implementation exist, e.g. the VAX 8800 and NVAX. AMD64
is also a byte-granularity instruction set. While byte granularity
makes wide decoders more expensive (by roughly a factor of 4 compared
to an instruction set with 32-bit granularity for the same number of
decoded bytes). RISC-V's compressed (C) instructions have 16-bit
granularity, so at least the RISC-V designers think that the benefits
of instruction compression outweigh the costs in decoding.


Anyway, this has little to do with the question of the original
poster. Virtual machines (often called byte codes, even if they use a
different instruction granularity) use instruction sets that are quite
different from that of VAX or AMD64; in particular, they have no
"instruction formats" with "addressing modes", where every base
instruction can be combined with a set of addressing modes in an
orthogonal way. That's because they are not decoded like hardware
instructions.


There are, however, costs to byte granularity in VM interpreters:


* Byte granularity makes it harder to use direct-threaded code and
    optimizations that start with that, in particular dynamic
    superinstructions: You cannot rewrite the VM code into
    direct-threaded code in-place, but have to translate it to another
    place, which also means that you cannot reuse the branch targets
    as-is.


* Also, if the VM instruction has an inline argument wider than one
    byte, the access to that argument can be significantly more
    expensive on architectures with alignment restrictions (e.g.,
    ironically, SPARC). But alignment restrictions have died out in
    general-purpose computers, and VM interpreters are not that popular
    on embedded systems.


- anton
--
M. Anton Ertl
anton@mips.complang.tuwien.ac.at
http://www.complang.tuwien.ac.at/anton/

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