Re: Code generation for DOES> in Gforth

anton@mips.complang.tuwien.ac.at (Anton Ertl) writes:

I recently noticed that Gforth still used the following COMPILE,
implementation for words defined with CREATE...SET-DOES> (and
consequently also for words defined with CREATE...DOES>):
>
: does, ( xt -- ) does-check ['] does-xt peephole-compile, , ;
>
Ignore DOES-CHECK (it has to do with stack-depth checking, still
incomplete).  The rest means that it compiles the primitive DOES-XT
with the xt of the COMPILE,d word as immediate argument.  DOES-XT
pushes the body of the word and then EXECUTEs the xt that SET-DOES>
has registered for this word.  In most cases this is a colon
definition (always if DOES> is used), so the next thing that happens
is DOCOL, and then the code for the colon definition is run.
>
I have now replaced this with
>
: does, ( xt -- ) does-check dup >body lit, >extra @ compile, ;
>
What this does is to compile the body as a literal, and then it
COMPILE,s the xt that DOES-XT would EXECUTE.  In the common case of a
colon definition this compiles a call to the colon definition.  This
saves the overhead of accessing the doesfield and of dispatching on
its contents at run-time; all that is now done during compilation.

Another benefit: Gforth used to implement special COMPILE,
implementations for 2VALUE and FVALUE.  Here's the old implementation
of FVALUE:

: opt-fval ( xt -- )  >body postpone Literal postpone f@ ;

create dummy-fvalue
' f@ set-does>
' fvalue-to set-to
' opt-fval set-optimizer

: fvalue ( r "name" -- ) \ floating-ext f-value
    \g Define @i{name} @code{( -- r1 )} where @i{r1} initially is
    \g @i{r}; this value can be changed with @code{to @i{name}} or
    \g @code{->@i{name}}.
    ['] dummy-fvalue create-from reveal f, ;

The new DOES, generates exactly the same code for FVALUEs as OPT-FVAL
does, so we no longer need OPT-FVAL and the use of SET-OPTIMIZER here.
Likewise for 2VALUE.  This simplification reduces the image size by
927 bytes and the native-code size by 176 bytes.

The code for compiling an FVALUE looks as follows (before and after
the change):

5e fvalue x  ok
: bla x ;  ok
see-code bla
$7F1341F2D5A8 lit    1->2
$7F1341F2D5B0 x
7F1341A4EA63:   mov     r15,$08[rbx]
$7F1341F2D5B8 f@    2->1
7F1341A4EA67:   movsd   [r12],xmm15
7F1341A4EA6D:   movsd   xmm15,[r15]
7F1341A4EA72:   sub     r12,$08
$7F1341F2D5C0 ;s    1->1
7F1341A4EA76:   mov     rbx,[r14]
7F1341A4EA79:   add     r14,$08
7F1341A4EA7D:   mov     rax,[rbx]
7F1341A4EA80:   jmp     eax

- anton
--
M. Anton Ertl  http://www.complang.tuwien.ac.at/anton/home.html
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