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Re: Opcode handler dispatch in an interpreter: Implementing switch on OpCode.

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On 08/10/2017 19:36, George Neuner wrote:
> On Sat, 7 Oct 2017 19:05:10 +0100, bartc <bc@freeuk.com> wrote:

>> I tried an experiment a few years ago, where the byte-code for a test
>> program was expanded into a sequence of instructions in a statically
>> compiled language (or it might have been done at run-time; I can't
>> remember). Each byte-code was represented by some mix of function
>> call, or some inline code.
>>
>> This would have been expected to benefit by eliminating dispatch
>> overheads (it just steps into the next lot of code like an ordinary
>> program), and also by having dedicated code for some things that were
>> otherwise a parameter in a generic byte-code instruction.
>>
>> But in fact the results weren't that great at all; most normal
>> dispatchers were actually faster!

> Some of the things to come out of the discussions in c.l.a.x86 were
> that the fastest [simple] JIT sequence for an interpreter is a list of
> explicit call instructions: e.g.,
>
>     call <bytecode_function>
>     call <bytecode_function>
>       :
>
> rather than a list of function addresses with a dispatcher.   This is
> because the call leverages the CPU branch predictor rather than
> fighting with it.

I think that's the sort of thing I was doing. Of course when it comes to
function calls, conditional code and so on, all things that require
jumping about in the byte-code, then it starts to get mixed up with the
language used to express those calls.

But I thought I'd do one quick experiment on one specific program in the
interpreted language (a very poor benchmark but just trying to see if
the runtime can be improved):

    i:=0

    while i<100 million do
      ++i
    end

So a while-loop counting to 100 million. The normal byte-code that is
generated for that fragment:

      2: 00006  ----------push_ci                     0d
      3: 00008  ----------pop_f                       [while.start.i:-1]
      4: 00010  ----------jump                        L14
      5: 00012 L12:
                ----------incrto_f                    [while.start.i:-1]
      4: 00014 L14:
                ----------push_f                      [while.start.i:-1]
      4: 00016  ----------push_ci                     100000000d
      4: 00018  ----------jumplt                      L12

These are normally executed in some sort of loop (not the loop indicated
here). The experiment emulates five of those byte-codes, by implementing
them as functions callable from static code representing this program,
as shown below.

The normally interpreted byte-code took 2.2 seconds.
The static version took 1.9 seconds.

A small difference. But this code uses my own poor x64 compiler. If
converted to C and compiled with gcc -O3 -m64, then the figures are 1.7
seconds and 0.75 seconds. (1.8 and 1.1 using -m32.)

So a significant difference this time, up to twice the speed. However,
that's comparing against the slow function-pointer dispatcher; a
switch- or label-pointer-based dispatcher would be rather brisker. But
the static code would still be quite a bit faster I think.

(But note: the very small number of handlers in my test can be easily
inlined by gcc, especially as they are only called once or twice. It's
might be a different story when they are called 100s of times. And also
the code might be generated via JIT methods; inlining too much could
have its own problems.)

There's one more comparison to make however: normally I run my byte-code
using my ASM overlay dispatcher, coupled with my poor compiler. For this
test, it runs the normal byte-code in 0.55 seconds. Faster than the
static code compiled with gcc-O3.


-------------------------------------------------------------------

The test function 'testwhile()', and the five special byte-code
handlers, written in the same static language as the interpreter, and
hidden away in a corner of the interpreter.

The local 'i' variable of the original has been changes to a static 'ii'
variable here to simplify matters (no stack frame has been created):

varrec ii

proc testwhile=	# emulate the byte-code of the 'while' test

     do_push_ci(0)
     do_pop_m(&ii)

     goto L14
L12:
     do_incrto_m(&ii)
L14:
     do_push_m(&ii)
     do_push_ci(100 million)
     if do_lt() then goto L12 fi

     println "Done, ii = ",ii.value
end

proc do_push_ci(int a)=
     (--sptr)^.tagx:=tint
     sptr^.value:=a
end

proc do_pop_m(ref varrec a)=
     vx_ufree(a) when a^.hasref
     a^:=sptr++^
end

proc do_incrto_m(ref varrec a)=

     switch ttbasetype[a^.tag]
     when tint  then
         ++a^.value
     when trefvar then
         ++a^.varptr
     when trefpacked then
         a^.ptr+:=ttsize[a^.refelemtag]
     else
         pcustype("INCRTO_M",a)
     endswitch
end

proc do_push_m(ref varrec a)=
     (--sptr)^:= a^

     if sptr^.hasref then
         ++sptr^.objptr^.refcount
     fi
end

function do_lt:int=
     ref varrec x,y
     int xt,res

     y:=sptr++
     x:=sptr++
     xt:=x^.tag

     if xt<>y^.tag then
         if xt:=vx_mixed(x,y) then
             goto retry
         fi
         pcmxtypes("LT/MIXED",x,y)
     fi
     retry:

     switch xt
     when tint then
         if x^.value < y^.value then
             return 1
         fi
     ..... etc
     return 0
end

--
bartc

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Thread

Opcode handler dispatch in an interpreter: Implementing switch on OpCode. Robert Jacobson <rljacobson@gmail.com> - 2017-10-05 09:29 -0700
  Re: Opcode handler dispatch in an interpreter: Implementing switch on OpCode. bartc <bc@freeuk.com> - 2017-10-05 21:14 +0100
  Re: Opcode handler dispatch in an interpreter: Implementing switch on OpCode. Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2017-10-06 02:56 +0200
    Re: Opcode handler dispatch in an interpreter: Implementing switch on OpCode. George Neuner <gneuner2@comcast.net> - 2017-10-07 12:55 -0400
      Re: Opcode handler dispatch in an interpreter: Implementing switch on OpCode. bartc <bc@freeuk.com> - 2017-10-07 19:05 +0100
        Re: Opcode handler dispatch in an interpreter: Implementing switch on OpCode. George Neuner <gneuner2@comcast.net> - 2017-10-08 14:36 -0400
          Re: Opcode handler dispatch in an interpreter: Implementing switch on OpCode. bartc <bc@freeuk.com> - 2017-10-08 22:49 +0100
            Re: Opcode handler dispatch in an interpreter: Implementing switch on OpCode. Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2017-10-11 18:13 -0400
              Re: Opcode handler dispatch in an interpreter: Implementing switch on OpCode. bartc <bc@freeuk.com> - 2017-10-17 15:54 -0400
          Re: Opcode handler dispatch in an interpreter: Implementing switch on OpCode. bartc <bc@freeuk.com> - 2017-10-11 18:12 -0400
          Re: Opcode handler dispatch in an interpreter: Implementing switch on OpCode. Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2017-10-11 18:12 -0400
            Re: Opcode handler dispatch in an interpreter: Implementing switch on OpCode. George Neuner <gneuner2@comcast.net> - 2017-10-17 15:55 -0400
      Re: Opcode handler dispatch in an interpreter: Implementing switch on OpCode. anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2017-10-08 13:41 +0000
  Re: Opcode handler dispatch in an interpreter: Implementing switch on OpCode. anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2017-10-06 22:32 +0000

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