Re: [PATCH v2] src/port/snprintf.c: Optimize the common base=10 case in fmtint

On Wed, 27 Oct 2021 at 04:58, Arjan van de Ven <arjan(at)linux(dot)intel(dot)com> wrote:
> [PATCH v2] src/port/snprintf.c: Optimize the common base=10 case in fmtint
>
> fmtint() turns an integer into a string for a given base, and to do this
> it does a divide/modulo operation iteratively.
> The only possible base values are 8, 10 and 16
>
> On just about any CPU, divides are a pretty expensive operation, generally
> 10x to 20x or more expensive than adds or multiplies.
>
> By special casing the base values, the compiler (gcc or other) can (and will)
> replace the divide by a multiply with 0xcccccccccccccccd (for base 10) or bitops
> for base 8 and 16, yielding a lot faster code.
>
> I considered a switch statement, but since base 10 is the most common by far,
> I implemented it as a series of if/else statements with a likely() marking the 10 case.
>
> Even though this only shows up in the database creation phase of pgbench and not so much
> during the normal run time, the optimization is simple and high value enough that
> in my opinion it's worth doing
>
>

+ if (likely(base == 10)) {
+ do
+ {
+ convert[sizeof(convert) - (++vallen)] = cvt[uvalue % 10];
+ uvalue = uvalue / 10;
+ } while (uvalue);
+ } else if (base == 16) {

Why do we need likely() for base=10, however, base=16 and base=8 don't need?

--
Regrads,
Japin Li.
ChengDu WenWu Information Technology Co.,Ltd.