[ltt-dev] [PATCH 7/9] rculfhash: Simplify lookup_bucket()

[Mathieu Desnoyers]

* Lai Jiangshan (laijs at cn.fujitsu.com) wrote:
> They are the same, but I don't think the compiler
> can optimize it.
> 
> And it also helps for understanding the following code.
> 
> Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
> ---
>  rculfhash.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/rculfhash.c b/rculfhash.c
> index edbd24a..7b880d7 100644
> --- a/rculfhash.c
> +++ b/rculfhash.c
> @@ -719,7 +719,7 @@ struct _cds_lfht_node *lookup_bucket(struct cds_lfht *ht, unsigned long size,
>  
>  	assert(size > 0);
>  	index = hash & (size - 1);
> -	order = get_count_order_ulong(index + 1);
> +	order = fls_ulong(index);
>  
>  	dbg_printf("lookup hash %lu index %lu order %lu aridx %lu\n",
>  		   hash, index, order, index & (!order ? 0 : ((1UL << (order - 1)) - 1)));

merged as:

commit a4ea2223fa701bd34b09fb3eede037908dbdd0c8
Author: Lai Jiangshan <laijs at cn.fujitsu.com>
Date:   Mon Oct 17 10:19:10 2011 -0400

    rculfhash: Simplify lookup_bucket()
    
    They are the same, but I don't think the compiler can optimize it.
    
    And it also helps for understanding the following code.
    
    [ Edit by Mathieu Desnoyers:
      - Add a comment that describes the equivalence between get_count_order
        and fls for this lookup of index + 1. ]
    
    Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>

diff --git a/rculfhash.c b/rculfhash.c
index d733d6b..189f8c8 100644
--- a/rculfhash.c
+++ b/rculfhash.c
@@ -724,7 +724,12 @@ struct _cds_lfht_node *lookup_bucket(struct cds_lfht *ht, unsigned long size,
 
 	assert(size > 0);
 	index = hash & (size - 1);
-	order = get_count_order_ulong(index + 1);
+	/*
+	 * equivalent to get_count_order_ulong(index + 1), but optimizes
+	 * away the non-existing 0 special-case for
+	 * get_count_order_ulong.
+	 */
+	order = fls_ulong(index);
 
 	dbg_printf("lookup hash %lu index %lu order %lu aridx %lu\n",
 		   hash, index, order, index & (!order ? 0 : ((1UL << (order - 1)) - 1)));

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com