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NNTP-Posting-Date: Sat, 27 Oct 2012 11:44:47 -0500
From: Chris Hinsley <chris.hinsley@gmail.com>
Newsgroups: comp.lang.forth
Date: Sat, 27 Oct 2012 17:44:47 +0100
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Subject: Re: well formed flag ?
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On 2012-10-27 16:06:07 +0000, Alex McDonald said:

> On Oct 27, 4:33 pm, Alex McDonald <b...@rivadpm.com> wrote:
>> On Oct 27, 4:03 pm, Chris Hinsley <chris.hins...@gmail.com> wrote:
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>>> On 2012-10-27 14:59:48 +0000, Chris Hinsley said:
>> 
>>>> Folks, I'm doing some tweaking to my hobby Forth compiler and just been
>>>> doing some inlineing work.
>> 
>>>> I'm not very happy about the size of the x86 code for comparision words
>>>> (I know I could reduce the code size if I had a full optimizing code
>>>> generator, but I don't yet…) this is my code for "<"
>> 
>>>>    defword ">", 0, WORD_GT, WORD_INLINE_COMMA
>>>>    POPDSP eax
>>>>    cmp eax, ebx
>>>>    setg bl
>>>>    movzx ebx, bl
>>>>    neg ebx
>>>>    ret
>>>>    defword_end
>> 
>>>> I followed the 'well formed flag' comments in Ms Rathers book for this,
>>>> but the IF and TRUE, FALSE words only talk about 0 for false or not
>>>> zero for true. Is it definately the case that Forth needs -1 to come
>>>> from the comparason words ?
>> 
>>>> Are you allowed to write code that _needs_ the -1 from the ">" ? ie if
>>>> it produce just a 1, would that be incorrect ?
>> 
>>>> Chris
>> 
>>> I might also add that my 0BRANCH word is:
>> 
>>>         defword "0BRANCH", 0, WORD_ZBRANCH, WORD_INLINE_COMMA
>>>         mov eax, ebx
>>>         POPDSP ebx
>>>         test eax, eax
>>>         jz strict near i_jmp
>>>         ret
>>>         defword_end
>> 
>>> Regards
>> 
>>> Chris
>> 
>> : < ( 2 -- 1 )
>> \ ' nseopt compiles; code=$4014DF len=16 type=1
>> \ defined in src\kernel\gkernel32.f at line 564
>> ( $0 )    mov     edx eax                           \ 8BD0
>> ( $2 )    or      ecx $-1                           \ 83C9FF
>> ( $5 )    xor     eax eax                           \ 33C0
>> ( $7 )    cmp     dword { $0 ebp } edx              \ 395500
>> ( $A )    cmovl   eax ecx                           \ 0F4CC1
>> ( $D )    add     ebp $4                            \ 83C504
>> ( $10 )   ret                                       \ C3 ( end ) ok
>> 
>> EAX is top of stack, EBP is the stack pointer and { EBP } is next to
>> top.
>> 
>> I'd definitely stick to well formed flags since this;
>> 
>> variable var 0 var !
>> : foo 10 < negate var +! ;
>> 
>> is well-formed Forth.
> 
> There's also
> 
> : >= ( 2 -- 1 )
> ( $0 )    sub     eax dword { $0 ebp }              \ 2B4500
> ( $3 )    cdq                                       \ 99
> ( $4 )    mov     eax edx                           \ 8BC2
> ( $6 )    add     ebp $4                            \ 83C504
> ( $9 )    ret                                       \ C3 ( end ) ok
> 
> and correspondingly < is   neg eax

One thing I just noticed is that NASM isn't encodeing small constant 
adds as a single byte, it's useing the full 4 byte encodeing ! I don't 
recall there being an option to tell NASM to optimise constants, I 
thought there was only -O flags to say optimize branches ?

Need to double check the NASM manual...