Re: Avoid treating the stack as an array [Re: "Back & Forth" is back!]

On 14 Sep 2024 at 08:19:52 CEST, "Anton Ertl" <Anton Ertl> wrote:

locals    stack
401       336   gforth-fast (AMD64)
179       132   lxf 1.6-982-823 (IA-32)
182       119   VFX FX Forth for Linux IA32 Version: 4.72 (IA-32)
241       159   VFX Forth 64 5.43 (AMD64)
163       175   iforth-5.1 mini (AMD64)

There are design decisions within locals that can impact optimisation.
The design of locals in VFX was influenced by Don Colburn's Forth's
and by a desire to use locals to simplify source code when interfacing
to a host operating system. Many operating systems return data
to the caller by passing the address of a variable/buffer as an input
parameter. Locals that can have an accessible address make such
code much easier to read and write. The example below comes from
early system access code in VFX (see kernel/386Lin/syspatch.fth).
The locals design dates from long before ANS.

$541B equ FIONREAD

: (OS_key?)    { | nread[ cell ] -- flag }
  ?PrepTerm  nread[ off
  nread[ FIONREAD stdin @ dll_ioctl @ 3 nxcall -1 = if
    0                    \ Error return from ioctl
  else
    nread[ @ 0<>
  then
;

: (OS_Key)    \ -- key ; SFP003
  { | iobuff[ cell ] -- char }
  ?PrepTerm
  1 iobuff[ stdin @ dll_ReadFile @ 3 nxcall drop
  iobuff[ c@
;

Code such as this has been around for a very long time and the use
of addresses of locals, and of local buffers, has proven itself over
time. Yes, we could put in a great effort to improve the performance
of locals, but this is Forth and there are other optimisations that may
produce bigger changes to application performance. In the last
decade or so there has been very little customer demand for
faster code. However, higher level source code has been much
in demand. An example is Nick Nelson's value flavoured structures,
which are of particular merit when converting code from 32 bit to
64 bit host Forths.

Just because many of the Forth applications visible to the Forth
community now run on CPUs with 16 or 32 address registers
does not mean that all systems can implement the compiler
techniques required for high-performance locals.

I can buy a lot of CPU cycles for the cost of one day of programmer
time. I am reminded when looking at locals that a client's Forth
engine is currently at 4GHz on a 12nm process. The performance
was detuned to 4GHz becuase the machine was more than fast
enough.

Stephen
--
Stephen Pelc, stephen@vfxforth.com
MicroProcessor Engineering, Ltd. - More Real, Less Time
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