Re: thoughts on native code

["Sjoerd van Leent Privé" <svanleent@gmail.com>]

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Hi Stefan,

Just my idea about an assembler in Scheme. Sounds interesting. If it's 
done properly, it can be very promising to use scheme itself to directly 
emit machine instructions. This would also be interesting for meta 
compilation in the future (think of aiding GCC).

So you are thinking about an assembler for x86? Perhaps I can help out 
on this one. I would like to do this part, as I haven't been able to aid 
on other parts besides voicing my ideas (anyways, I am on embedded 
development these days.)

The only discussion is the syntax I believe, I mean, should it be AT&T 
like, Intel like, or leave this domain and do something new. I would go 
for instructions like this (using macros):

(let ((target :x686))
   (assemble target
    ((mov long 100 EAX)
     (mov long 200 EBX)
     (add long EBX EAX))))

Giving back the native machine code instructions. Perhaps special 
constructions can be made to return partially complete instructions 
(such as missing labels or calls to guile procedures...)

Sjoerd


On 11/12/2012 10:50 PM, Stefan Israelsson Tampe wrote:
> Thanks for your mail Noah,
>
> Yea libjit is quite interesting. But playing around with an assembler 
> in scheme I do not want to go back to
> C or C++ land. The only problem is that we need a GNU scheme assembler 
> and right now I use sbcl's assembler
> ported to scheme. We could perhaps use weinholts assembler as well in 
> industria if he could sign papers to make it GNU. For the register 
> allocation part I would really like to play a little in scheme to 
> explore the idea you saw from my previous mail in this thread. Again I 
> think it's natural to have this features in scheme and do not want to 
> mess in C land too much.
>
> Am I wrong?
>
> Cheers
> Stefan
>
>
> On Sat, Nov 10, 2012 at 11:49 PM, Noah Lavine <noah.b.lavine@gmail.com 
> <mailto:noah.b.lavine@gmail.com>> wrote:
>
>     Hello,
>
>     I assume "compressed native" is the idea you wrote about in your
>     last email, where we generate native code which is a sequence of
>     function calls to VM operations.
>
>     I really like that idea. As you said, it uses the instruction
>     cache better. But it also fixes something I was worried about,
>     which is that it's a lot of work to port an assembler to a new
>     architecture, so we might end up not supporting many native
>     architectures. But it seems much easier to make an assembler that
>     only knows how to make call instructions and branches. So we could
>     support compressed native on lots of architectures, and maybe
>     uncompressed native only on some.
>
>     If you want a quick way to do compressed native with reasonable
>     register allocation, GNU libjit might work. I used it a couple
>     years ago for a JIT project that we never fully implemented. I
>     chose it over GNU Lightning specifically because it did register
>     allocation. It implements a full assembler, not just calls, which
>     could also be nice later.
>
>     Noah
>
>
>
>     On Sat, Nov 10, 2012 at 5:06 PM, Stefan Israelsson Tampe
>     <stefan.itampe@gmail.com <mailto:stefan.itampe@gmail.com>> wrote:
>
>         I would like to continue the discussion about native code.
>
>         Some facts are,
>         For example, consider this
>         (define (f x) (let loop ((s 0) (i 0)) (if (eq? i x) s (loop (+
>         s i) (+ i 1)))))
>
>         The timings for (f 100000000)  ~ (f 100M) is
>
>         1) current vm                 : 2.93s
>         2) rtl                              : 1.67s
>         3) compressed native     : 1.15s
>         4) uncompressed native : 0.54s
>
>         sbcl = compressed nativ + better register allocations (normal
>         optimization level) : 0.68s
>
>         To note is that for this example the call overhead is close to
>         5ns per iteration and meaning that
>         if we combined 4 with better register handling the potential
>         is to get this loop to run at 0.2s which means
>         that the loop has the potential of running 500M iterations in
>         one second without sacrifying safety and not
>         have a extraterestial code analyzer. Also to note is that the
>         native code for the compressed native is smaller then the
>         rtl code by some factor and if we could make use of registers
>         in a better way we would end up with even less overhead.
>
>         To note is that compressed native is a very simple mechanism
>         to gain some speed and also improve on memory
>         usage in the instruction flow, Also the assembler is very
>         simplistic and it would not be to much hassle to port a new
>         instruction format to that environment. Also it's probably
>         possible to handle the complexity of the code in pure C
>         for the stubs and by compiling them in a special way make sure
>         they output a format that can be combined
>         with the meta information in special registers needed to make
>         the execution of the compiled scheme effective.
>
>         This study also shows that there is a clear benefit to be able
>         to use the computers registers, and I think this is the way
>         you would like the system to behave in the end. sbcl does this
>         rather nicely and we could look at their way of doing it.
>
>         So, the main question now to you is how to implement the
>         register allocations? Basic principles of register allocation
>         can be gotten out from the internet, I'm assure of, but the
>         problem is how to handle the interaction with the helper
>         stubs. That is
>         something i'm not sure of yet.
>
>         A simple solution would be to assume that the native code have
>         a set of available registers r1,...,ri and then force the
>         compilation of the stubs to treat the just like the registers
>         bp, sp, and bx. I'm sure that this is possible to configure in
>         gcc.
>
>         So the task for me right now is to find out more how to do
>         this, if you have any pointers or ideas, please help out.
>
>         Cheers
>         Stefan
>
>
>
>
>
>
>         On Sat, Nov 10, 2012 at 3:41 PM, Stefan Israelsson Tampe
>         <stefan.itampe@gmail.com <mailto:stefan.itampe@gmail.com>> wrote:
>
>             Hi all,
>
>             After talking with Mark Weaver about his view on native
>             code, I have been pondering how to best model our needs.
>
>             I do have a framework now that translates almost all of
>             the rtl vm directly to native code and it do shows a speed
>             increase of say 4x compared to runing a rtl VM. I can also
>             generate rtl code all the way from guile scheme right now
>             so It's pretty easy to generate test cases. The problem
>             that Mark point out to is that we need to take care to not
>             blow the instructuction cache. This is not seen in these
>             simple examples but we need larger code bases to test out
>             what is actually true. What we can note though is that I
>             expect the size of the code to blow up with a factor of
>             around 10 compared to the instruction feed in the rtl code.
>
>             One interesting fact is that SBCL does fairly well by
>             basically using the native instruction as the instruction
>             flow to it's VM. For example if it can deduce that a +
>             operation works with fixnums it simply compiles that as a
>             function call to a general + routine e.g. it will do a
>             long jump to the + routine, do the plus, and longjump back
>             essentially dispatching general instructions like + * /
>             etc, directly e.g. sbcl do have a virtual machine, it just
>             don't to table lookup to do the dispatch, but function
>             call's in stead. If you count longjumps this means that
>             the number of jumps for these instructions are double that
>             of using the original table lookup methods. But for
>             calling functions and returning functions the number of
>             longjumps are the same and moving local variables in place
>             , jumping  is really fast.
>
>             Anyway, this method of dispatching would mean a fairly
>             small footprint with respect to the direct assembler.
>             Another big chunk of code that we can speedup without to
>             much bloat in the instruction cache is the lookup of
>             pairs, structs and arrays, the reason is that in many
>             cases we can deduce at compilation so much that we do not
>             need to check the type all the time but can safely lookup
>             the needed infromation.
>
>             Now is this method fast? well, looking a the sbcl code for
>             calculating 1+ 2 + 3 + 4 , (disassembling it) I see that
>             it do uses the mechanism above, it manages to sum 150M
>             terms in one second, that's quite a feat for a VM with no
>             JIT. The same with the rtl VM is 65M.
>
>             Now, sbcl's compiler is quite matured and uses native
>             registers quite well which explains one of the reasons why
>             the speed. My point is though that we can model
>             efficiently a VM by call's and using the native
>             instructions and a instructions flow.
>
>             Regards Stefan
>
>
>
>


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