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Date: Thu, 20 Nov 2014 07:32:09 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: question about malloc's bin_index
* Rich Felker <dalias@...c.org> [2014-11-19 23:06:33 -0500]:
> On Wed, Nov 19, 2014 at 06:43:23PM -0800, Weiming Zhao wrote:
> > I'm wondering what's the purpose of the union? How should I rewrite it to
> > use integer only?
>
> The value we want, for categorizing sizes into bins, is exactly the
> floating point exponent and first 2 bits of the mantissa. It's
> basically a sort of base-2 log with 4 linear steps between successive
> logarithmic-scale points. I think nsz has an integer-only version of
> this code that might be useful; it performs mildly better on archs
> without fpu and mildly worse on ones with fpu and it's somewhat larger
> (but of course not as large as soft-float code). I'll see if I can
> find a copy or if he can post it.
i have this in src/malloc/malloc.c with #ifdef NOFPU:
/* non-float bin index */
static const unsigned char bintab[32]={
0, 0, 0, 0,32,33,34,35,36,36,37,37,38,38,39,39,
40,40,40,40,41,41,41,41,42,42,42,42,43,43,43,43
};
static int bin_index(size_t x)
{
x = x / SIZE_ALIGN - 1;
if (x <= 32) return x;
x /= 8;
if (x < 32)
return bintab[x];
x /= 8;
if (x < 32)
return bintab[x]+12;
x /= 8;
if (x < 16)
return bintab[x]+24;
return 63;
}
static int bin_index_up(size_t x)
{
x = x / SIZE_ALIGN - 1;
if (x <= 32) return x;
x--;
x /= 8;
if (x < 32)
return bintab[x]+1;
x /= 8;
if (x < 32)
return bintab[x]+13;
x /= 8;
return bintab[x]+25;
}
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