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Date: Sat, 17 Dec 2016 08:36:00 +0100
From: u-uy74@...ey.se
To: musl@...ts.openwall.com
Subject: Re: [PATCH] use lookup table for malloc bin index instead of
 float conversion

On Sat, Dec 17, 2016 at 12:50:58AM -0500, Rich Felker wrote:
> On Sun, Nov 27, 2016 at 03:15:41PM +0100, Szabolcs Nagy wrote:
> > +static const unsigned char bin_tab[64] = {
> > +	 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,
> > +	44,44,44,44,44,44,44,44,45,45,45,45,45,45,45,45,
> > +	46,46,46,46,46,46,46,46,47,47,47,47,47,47,47,47,
> > +};
> > +
> >  static int bin_index(size_t x)
> >  {
> >  	x = x / SIZE_ALIGN - 1;
> >  	if (x <= 32) return x;
> > +	if (x < 512) return bin_tab[x/8];
> >  	if (x > 0x1c00) return 63;
> > -	return ((union { float v; uint32_t r; }){(int)x}.r>>21) - 496;
> > +	return bin_tab[x/128] + 16;
> >  }
> >  
> >  static int bin_index_up(size_t x)
> >  {
> >  	x = x / SIZE_ALIGN - 1;
> >  	if (x <= 32) return x;
> > -	return ((union { float v; uint32_t r; }){(int)x}.r+0x1fffff>>21) - 496;
> > +	x--;
> > +	if (x < 512) return bin_tab[x/8] + 1;
> > +	return bin_tab[x/128] + 17;
> >  }
> >  
> >  #if 0
> > -- 
> > 2.10.2

> Looks good mostly, but wouldn't it be better to drop the 4 unused
> entries from the table and add -4's to the indices?

Wouldn't this enlarge the code more than reduce the data?

Rune

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