linux-kernel - RE: [PATCH 1/2] infiniband: Help gcc generate better code for ocrdma_srq_toggle

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Date:	Mon, 2 Feb 2015 12:04:28 +0000
From:	Selvin Xavier <Selvin.Xavier@...lex.Com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Roland Dreier <roland@...nel.org>,
	Sean Hefty <sean.hefty@...el.com>,
	Hal Rosenstock <hal.rosenstock@...il.com>
CC:	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/2] infiniband: Help gcc generate better code for
 ocrdma_srq_toggle_bit

Acked-by: Selvin Xavier <selvin.xavier@...lex.com>

Thanks!

> -----Original Message-----
> From: linux-rdma-owner@...r.kernel.org [mailto:linux-rdma-
> owner@...r.kernel.org] On Behalf Of Rasmus Villemoes
> Sent: Friday, January 16, 2015 8:10 PM
> To: Roland Dreier; Sean Hefty; Hal Rosenstock
> Cc: Rasmus Villemoes; linux-rdma@...r.kernel.org; linux-
> kernel@...r.kernel.org
> Subject: [PATCH 1/2] infiniband: Help gcc generate better code for
> ocrdma_srq_toggle_bit
> 
> gcc emits a surprising amount of code in order to flip a bit. One would think
> that a single instruction is enough.
> 
> $ scripts/bloat-o-meter /tmp/ocrdma_verbs.o
> drivers/infiniband/hw/ocrdma/ocrdma_verbs.o
> add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-142 (-142)
> function                                     old     new   delta
> ocrdma_post_srq_recv                         498     460     -38
> ocrdma_poll_cq                              2010    1962     -48
> ocrdma_discard_cqes                          495     439     -56
> 
> All three calls of ocrdma_srq_toggle_bit happen within spinlocks, so saving a
> few useless instructions might be worthwhile.
> 
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
>  drivers/infiniband/hw/ocrdma/ocrdma_verbs.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/drivers/infiniband/hw/ocrdma/ocrdma_verbs.c
> b/drivers/infiniband/hw/ocrdma/ocrdma_verbs.c
> index fb8d8c4dfbb9..eff11e6c6183 100644
> --- a/drivers/infiniband/hw/ocrdma/ocrdma_verbs.c
> +++ b/drivers/infiniband/hw/ocrdma/ocrdma_verbs.c
> @@ -1484,10 +1484,7 @@ static void ocrdma_srq_toggle_bit(struct
> ocrdma_srq *srq, int idx)
>  	int i = idx / 32;
>  	unsigned int mask = (1 << (idx % 32));
> 
> -	if (srq->idx_bit_fields[i] & mask)
> -		srq->idx_bit_fields[i] &= ~mask;
> -	else
> -		srq->idx_bit_fields[i] |= mask;
> +	srq->idx_bit_fields[i] ^= mask;
>  }
> 
>  static int ocrdma_hwq_free_cnt(struct ocrdma_qp_hwq_info *q)
> --
> 2.1.3
> 
> --
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