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

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Message-ID: <87twz7ho25.fsf@rasmusvillemoes.dk>
Date:	Sat, 31 Jan 2015 00:00:02 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Roland Dreier <roland@...nel.org>
Cc:	Sean Hefty <sean.hefty@...el.com>,
	Hal Rosenstock <hal.rosenstock@...il.com>,
	linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] infiniband: Help gcc generate better code for ocrdma_srq_toggle_bit

ping

On Fri, Jan 16 2015, Rasmus Villemoes <linux@...musvillemoes.dk> wrote:

> 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)
--
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