linux-kernel - Re: [PATCH 1/4] lib: vsprintf: Optimize division by 10 for small integers.

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Date:	24 Sep 2012 07:27:12 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	linux@...izon.com, vda.linux@...glemail.com
Cc:	hughd@...gle.com, linux-kernel@...r.kernel.org, mina86@...a86.com
Subject: Re: [PATCH 1/4] lib: vsprintf: Optimize division by 10 for small integers.

>> +/* See comment in put_dec_full9 for choice of constants */
>>  static noinline_for_stack
>>  char *put_dec_full4(char *buf, unsigned q)
>>  {
>>         unsigned r;
>> -       r      = (q * 0xcccd) >> 19;
>> +       r      = (q * 0xccd) >> 15;
>>         *buf++ = (q - 10 * r) + '0';
>> -       q      = (r * 0x199a) >> 16;
>> +       q      = (r * 0xcd) >> 11;

> I would use 16-bit shifts instead of smaller ones.
> There may be CPUs on which "get upper half of 32-bit reg"
> operation is cheaper or smaller than a shift.

Good point, but wouldn't those CPUs *also* have multi-cycle multiply,
or have to synthesize it out of shift-and-add, in which case smaller
constants would save even more cycles?

I'm thinking original MC68010 here, which I'm not sure is even
meaningful any more.  ColdFire has single-cycle shifts.

Can you think of a processor where that would actually be
an improvement?
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