Re: [PATCH] use lookup table for malloc bin index instead of float conversion

[Rich Felker <dalias@libc.org>]

On Sun, Nov 27, 2016 at 03:15:41PM +0100, Szabolcs Nagy wrote:
> float conversion is slow and big on soft-float targets.
> 
> The lookup table increases code size a bit on most hard float targets
> (and adds 64byte rodata), performance can be a bit slower because of
> position independent data access and cpu internal state dependence
> (cache, extra branches), but the overall effect should be minimal
> (common, small size allocations should be unaffected).
> ---
>  src/malloc/malloc.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/src/malloc/malloc.c b/src/malloc/malloc.c
> index b90636c..ce2e97d 100644
> --- a/src/malloc/malloc.c
> +++ b/src/malloc/malloc.c
> @@ -111,19 +111,29 @@ static int first_set(uint64_t x)
>  #endif
>  }
>  
> +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?

Rich