Messages in this thread |  | | | From | Segher Boessenkool <> | | Subject | Re: [PATCH] common implementation of iterative div/mod | | Date | Thu, 8 May 2008 22:52:12 +0200 | |
> We have a few instances of the open-coded iterative div/mod loop, used
> when we don't expcet the dividend to be much bigger than the divisor.
> Unfortunately modern gcc's have the tendency to strength "reduce" this
> into a full mod operation, which isn't necessarily any faster, and
> even if it were, doesn't exist if gcc implements it in libgcc.
>
> The workaround is to put a dummy asm statement in the loop to prevent
> gcc from performing the transformation.
It's not a "dummy" asm, it actually does something: it tells the
compiler that it has to iterate the loop exactly as written, and
not do something else. I.e., exactly the behaviour we want here.
> + ticks = iter_div_u64_rem(blocked, NS_PER_TICK, &blocked);
What a terrible function name.
> static inline void timespec_add_ns(struct timespec *a, u64 ns)
> {> - ns += a->tv_nsec;> - while(unlikely(ns >= NSEC_PER_SEC)) {> - /* The following asm() prevents the compiler from
> - * optimising this loop into a modulo operation. */
> - asm("" : "+r"(ns));
> -
> - ns -= NSEC_PER_SEC;
> - a->tv_sec++;
> - }
> + a->tv_sec += iter_div_u64_rem(a->tv_nsec + ns, NSEC_PER_SEC, &ns);
> a->tv_nsec = ns;
> }
...and now the "meat" of this function isn't inline anymore. If we
cared about not doing a divide here, you'll have to explain why
taking this trivial loop out of line is a good idea.
> +unsigned iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
> +{> + unsigned ret = 0;> +> + while(dividend >= divisor) {
You removed the unlikely() here. Why?
> + /* The following asm() prevents the compiler from
> + optimising this loop into a modulo operation. */
> + asm("" : "+rm"(dividend));
You changed "+r" to "+rm" here. Why? Also, "rm" is an x86-ism,
and this is generic code (it does work here, but why is it better
than "r"?)
> +EXPORT_SYMBOL(iter_div_u64_rem);
Does this need to be exported?
Can I suggest an alternative approach? Define a macro (with a
good, descriptive name!) for just the asm("" : "+r"(x)), and use
that. Much smaller patch, much clearer code, and doesn't result
in different (and worse) code generation, so it's a much safer
change.
Segher
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