String copy idiom.

[Regis Crinon]

> ><
> Posted from  ark at alice.UUCP (Andrew Koenig)
> > If s and t are char pointers in registers,
> > 
> > 	while (*s++ = *t++) ;
> > 
> > generates the best code I could possibly imagine.
> > 
> > 	while ((*s = *t) != '\0') {s++; t++;}
> > 
> > is considerably worse.  Try it with register variables on your compiler.
> 
> Ok, here's the results.  This was done on a Pyramid 90x ("-O" on the cc command;
> disassembled the results).
> 
> ---First Method---
> Code:
>     while (*s++ = *t++)
> 	;
> 
> Assembly:
>  00000058: 01000870                movw     lr1,tr0
>  0000005c: 14000061                addw     $1,lr1
>  00000060: 01000831                movw     lr0,tr1
>  00000064: 14000060                addw     $1,lr0
>  00000068: 81100c31                movb     (tr0),(tr1)
>  0000006c: 32200c70                cvtbw    (tr1),tr0
>  00000070: f024fffa                bfc      z,0x58
> 
> ---Second Method---
> Code:
>     while ((*s = *t) != '\0') {
> 	s++;
> 	t++;
>     }
> 
> Assembly:
>  00000054: f0200003                br       0x60
>  00000058: 14000060                addw     $1,lr0
>  0000005c: 14000061                addw     $1,lr1
>  00000060: 81100860                movb     (lr1),(lr0)
>  00000064: 32200830                cvtbw    (lr0),tr0
>  00000068: f024fffc                bfc      z,0x58
> 
> ------
>  Seems that the second method (the longer C version) actually takes one less
>  instruction and also uses one less register (the first one used tr0 & tr1; the
>  second only needed tr0). [For the unfamilier, at any given time, the Pyramid
>  has available 16 global registers (gr0-gr15), 16 parameter registers
>  (pr0-pr15), 16 local registers (lr0-lr15) and 16 temporary registers
>  (tr0-tr15).]
> 
>  I think what matters here is whether your machine has an auto-increment/
>  decrement type of instruction.  I'm not sure if the Pyramid does or not,
>  but obviously, the C compiler doesn't use it, so I think I can safely assume
>  that it does not.  Anyone want to try this on a 68000?
> 
> 				robert
> -- 
> Robert Viduya
> Georgia Institute of Technology
> 
> ...!{akgua,allegra,amd,hplabs,ihnp4,masscomp,ut-ngp}!gatech!gitpyr!robert
> ...!{rlgvax,sb1,uf-cgrl,unmvax,ut-sally}!gatech!gitpyr!robert

*** REPLACE THIS LINE WITH YOUR MESSAGE ***

Ok. Following are the 68000 compiler results:

	i) Using while(*s++ = *t++);

07530	12D8		MOVE.B	(A0)+,(A1)+
07532	6600FFFC	BNE.L	$007530
07536

	ii) Using while((*s = *t) != '\0'){s++;t++}

07530	1290		MOVE.B	(A0),(A1)
07532	6706		BEQ.S	$00753A
07534	5286		ADDQ.L	#1,A1
07536	5288		ADDQ.L	#1,A0
07538	60F6		BRA.S 	$007530
0753A

	It seems to me that version i) is faster and requires less memory.

-- 
crinon