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SubjectRe: [PATCH 1/2] infiniband: Help gcc generate better code for ocrdma_srq_toggle_bit
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On Fri, Jan 16 2015, Rasmus Villemoes <linux@rasmusvillemoes.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@rasmusvillemoes.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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