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Message-ID: <20161217055058.GG1555@brightrain.aerifal.cx>
Date: Sat, 17 Dec 2016 00:50:58 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] use lookup table for malloc bin index instead of
float conversion
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
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