__udivsi3: coldfire vs non-coldfire implementation

__udivsi3: coldfire vs non-coldfire implementation

[Luis Alves]

Hi all,

What's the reason for having two different algorithms for this function?
The coldfire version seems faster since it doesn't perform any div/mul
opcodes. Or am I wrong?

(arch/m68k/lib/udivsi3.S)

Regards,
Luis Alves

Re: __udivsi3: coldfire vs non-coldfire implementation

[Andreas Schwab]

Luis Alves <ljalvs@gmail.com> writes:

> What's the reason for having two different algorithms for this function?
> The coldfire version seems faster since it doesn't perform any div/mul
> opcodes. Or am I wrong?

The coldfire version would require a minimum of 1800 cycles on a 68000.
The 68000 version is dominated by either two divu (max 140 cycles each)
or an lsr loop of max 900 cycles plus divu and two mulu (max 70 cycles
each) plus some 100 cycles for the rest.  That should make it better for
all inputs.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: __udivsi3: coldfire vs non-coldfire implementation

[Luis Alves]

I wasn't aware of the differences in the divide and multiply
instructions (and that some coldfire's don't even have the division
opcodes).

Thanks for the cycle count, it makes sense.

Luis Alves


On Tue, Apr 24, 2012 at 4:39 PM, Andreas Schwab <schwab@linux-m68k.org> wrote:
> Luis Alves <ljalvs@gmail.com> writes:
>
>> What's the reason for having two different algorithms for this function?
>> The coldfire version seems faster since it doesn't perform any div/mul
>> opcodes. Or am I wrong?
>
> The coldfire version would require a minimum of 1800 cycles on a 68000.
> The 68000 version is dominated by either two divu (max 140 cycles each)
> or an lsr loop of max 900 cycles plus divu and two mulu (max 70 cycles
> each) plus some 100 cycles for the rest.  That should make it better for
> all inputs.
>
> Andreas.
>
> --
> Andreas Schwab, schwab@linux-m68k.org
> GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
> "And now for something completely different."