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Date: Wed, 24 Jun 2015 07:32:56 +0300 (MSK)
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
Subject: Re: [PATCH 2/5] dynlink.c: compute modulus via magic
multiplication
On Wed, 24 Jun 2015, Rich Felker wrote:
> On Wed, Jun 24, 2015 at 02:24:52AM +0300, Alexander Monakov wrote:
> > +static uint32_t umod(uint32_t x, uint32_t div, struct udiv *p)
> > +{
> > + if (!(div&(div-1)))
> > + return x&(div-1);
> > + uint32_t v = x;
> > + if (p->s1) v >>= p->s1;
> > + else if (v != ~0u) v += p->inc;
> > + int s32=32, s2=p->s2;
> > + if (sizeof(long) == 8)
> > + s32+=s2, s2=0;
> > + v = (1ull * v * p->mul) >> s32;
> > + v >>= s2;
> > + return x-v*div;
> > +}
>
> I think having the div argument here is a pessimization.
How can I do the last step, 'x-v*div' without it?
> Power-of-two sizes are unlikely to be used,
probably so, but...
> and it's possible for precompute_udiv
> to setup the struct udiv to work perfectly fine for powers of two,
I gave it a thought, and I didn't find how to do that either.
> so that there is a single code path with no branches. The p->s1 branch
> should not be needed either.
p->s1 check is for this reason: shift are relatively costly, and s1 is rarely
non-zero, so try to skip the shift if possible; in the rare case the pre-shift
is non-zero, the check allows to skip the saturating increment operation.
Alexander
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