Re: Subtraction + comparison in one asm instruction?

On 10 Aug 2002 02:12:44 -0400, "Vincent Lefevre"
<vincent+news@vinc17.org> wrote:


[...]
>
> while (--c > 0)
> { ... }
>
>(where c is a signed integer).
>
>Are there compilers that do the subtraction and the comparison in
>the same asm instruction (for the processors for which there is a
>difference)? For the other compilers, have there been discussions,
>plans to add such an optimization, etc?


I have one example.


The CCS PIC C compiler (Not Quite C for a tiny 8-bit microcontroller)
generated the following code for a project I worked on some time ago:


.................... do
.................... {
                                                          [... loop stuff ...]
....................
.................... } while (--bit_ctr);
00D8: DECFSZ 72,F
00D9: GOTO 0D0


The DECFSZ decrements a memory location (72 in this case, the ",F"
indicates the result should be stored back in memory instead of into
the W or "Working" register), and skips the next instruction if the
result is zero. The "GOTO" instruction is a jump back to the top of
the loop. The variable (bit_ctr) is an 8-bit unsigned value.


See www.ccsinfo.com for more info on the compiler, and
www.microchip.com for more info on the processor.


Regards,


                                                              -=Dave
--
Change is inevitable, progress is not.

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