So it seems that most the compilers optimise a 4 bit shift into a swapf/and
combo. Which is a good optimisation. I think I shall actually have a play
tonight

thanks for the insights.

Jon

> -----Original Message-----
> From: Sergio Masci [mailto:smpl@NTLWORLD.COM]
> Sent: 14 October 2003 18:02
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC:] Efficiency of C?
>
>
> From: Michael Rigby-Jones wrote:
>
> <snip>
>
> > Not bad, the first two instructions are for bank setting.
> I chose a
> > 16F877 so a smaller device may require less instructions.
> Note that
> > the compiler has optimised the last call to OutputNibble by
> converting
> > it to a jump. However also note that the nibble variable is not
> > actually required, a better implementation would be:
> >
> > void OutputByte( unsigned char w )
> > {
> >         OutputNibble((unsigned char)(w >> 4));
> >         OutputNibble((unsigned char)(w & 0xF));
> > }
> >
> > giving:
> >
> >     33  07EE                     _OutputByte
> >     34                           ;      _w assigned to
> ?a_OutputByte+0
> >     35  0000                     _OutputByte$w  set
> ?a_OutputByte
> >     36                           ;_w stored from w
> >     37  07EE  1283                      bcf     3,5
> >     38  07EF  1303                      bcf     3,6
> >     39  07F0  00A0                      movwf   ?a_OutputByte
> >     41  07F1  0E20                      swapf   ?a_OutputByte,w
> >     42  07F2  390F                      andlw   15
> >     43  07F3  27E9                      fcall   _OutputNibble
> >     45  07F4  0820                      movf    ?a_OutputByte,w
> >     46  07F5  390F                      andlw   15
> >     47  07F6  2FE9                      ljmp    _OutputNibble
> >
> > Now apart from the bank setting bits which weren't included in your
> > assembly example, this has exactly the same number of instructions,
> > but actually executes more quickly due to the jump
> optimisation.  The
> > ljmp macro degenerates to a single goto if the OutputNibble
> resides in
> > the same program memory page as the OutputByte function.
> >
> > Hope that calms your fears over the code generation quality
> of HiTech
> > :o)
> >
> > Mike
>
>
> The following shows equivalent code compiled using the XCSB
> compiler. Note the absence of call and return instructions.
>
> Admittedly the total of 12 instructions generated could be
> further reduced to 8 by a key hole optimiser but none the
> less still not bad going for a high level language generating
> code for a 16 series PIC
>
>         //----------------------
>         //----------------------
>
>         include "hwreg-p16f628.h"
>
>         proc inline OutputNibble(ubyte val)
>
>                 PORTA = val
>         endproc
>
>         proc main()
>
>                 ubyte   x
>
>                 OutputNibble(x >> 4)
>                 OutputNibble(x & 0x0f)
>
>         endproc
>
>         //----------------------
>         //----------------------
>
>         0069                    main
>
>                                 ;;;  OutputNibble (
> x_main_func_local >> 4 )
>         0069    08 22   f:22            movf    x_main_func_local,w
>         006A    00 A7   f:27            movwf
> xacc_2_main_func_eval_stk
>
>         006B    0E A7   f:27            swapf
> xacc_2_main_func_eval_stk
>         006C    30 0F                   movlw   15
>         006D    05 A7   f:27            andwf
> xacc_2_main_func_eval_stk
>
>                                 ;;;  PORTA = xacc_2_main_func_eval_stk
>         006E    08 27   f:27            movf
> xacc_2_main_func_eval_stk,w
>         006F    00 85   f:05            movwf   PORTA
>
>                                 ;;;  OutputNibble (
> x_main_func_local & 15 )
>         0070    08 22   f:22            movf    x_main_func_local,w
>         0071    39 0F                   andlw   15
>         0072    00 A7   f:27            movwf
> xacc_2_main_func_eval_stk
>
>                                 ;;;  PORTA = xacc_2_main_func_eval_stk
>         0073    08 27   f:27            movf
> xacc_2_main_func_eval_stk,w
>         0074    00 85   f:05            movwf   PORTA
>
>         0075                    main_func_exit_point
>
>
> Regards
> Sergio Masci
>
http://www.xcprod.com/titan/XCSB - optimising structured PIC BASIC compiler

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads