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

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Message-Id: <1421419196-4659-1-git-send-email-linux@rasmusvillemoes.dk>
Date:	Fri, 16 Jan 2015 15:39:55 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Roland Dreier <roland@...nel.org>,
	Sean Hefty <sean.hefty@...el.com>,
	Hal Rosenstock <hal.rosenstock@...il.com>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	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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