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Date: Wed, 24 Jun 2015 01:39:06 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH 1/5] dynlink.c: use bloom filter in gnu hash lookup

On Wed, Jun 24, 2015 at 02:24:51AM +0300, Alexander Monakov wrote:
> Introduce gnu_lookup_filtered and use it to speed up symbol lookups in
> find_sym (do_dlsym is left as is, based on an expectation that frequently
> dlsym queries will use a dlopen handle rather than RTLD_NEXT or RTLD_DEFAULT,
> and will not need to look at more than one DSO).  The size of the bloom filter
> table minus 1, ghashmask, is stored in the dso struct together with the
> hashtable pointer to reduce pointer chasing on the hot path.
> ---
>  src/ldso/dynlink.c | 30 ++++++++++++++++++++++++++----
>  1 file changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/src/ldso/dynlink.c b/src/ldso/dynlink.c
> index b77c6f6..fa91b39 100644
> --- a/src/ldso/dynlink.c
> +++ b/src/ldso/dynlink.c
> @@ -54,6 +54,7 @@ struct dso {
>  	Sym *syms;
>  	uint32_t *hashtab;
>  	uint32_t *ghashtab;
> +	uint32_t ghashmask;
>  	int16_t *versym;
>  	char *strings;
>  	unsigned char *map;
> @@ -200,6 +201,19 @@ static Sym *gnu_lookup(const char *s, uint32_t h1, struct dso *dso)
>  	return 0;
>  }
>  
> +static Sym *gnu_lookup_filtered(const char *s, uint32_t h1, struct dso *dso, uint32_t fofs, size_t fmask)
> +{
> +	uint32_t *hashtab = dso->ghashtab;
> +	size_t *bloomwords = hashtab+4;
> +	size_t f = bloomwords[fofs & dso->ghashmask];

Is this measurably faster than fofs & hashtab[2]-1 ?

If suspect not, in which case it seems desirable not to increase the
size of struct dso. If it is worthwhile, at least don't sandwich a
uint32_t between two pointers where it might incur another 32 bits of
padding.

Rich

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