This patch optimises the ieee80211_get_hdrlen function by exploiting the
bit masks directly.
On powerpc, this decreases the function by 4 instructions, but more
importantly it kills 4 of the 5 branches it contained.
Signed-off-by: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
---
And don't ask me how the compiler actually gets by with a single branch
now!
It was fun to do but in my userspace test program on powerpc doesn't
seem to change much, CPU time goes down by like 2%...
net/mac80211/ieee80211.c | 23 ++++++++++++++---------
1 file changed, 14 insertions(+), 9 deletions(-)
--- wireless-dev.orig/net/mac80211/ieee80211.c 2007-03-27 16:33:47.163155480 +0200
+++ wireless-dev/net/mac80211/ieee80211.c 2007-03-27 16:33:48.473155480 +0200
@@ -258,19 +258,24 @@ int ieee80211_get_hdrlen(u16 fc)
case IEEE80211_FTYPE_DATA:
if ((fc & IEEE80211_FCTL_FROMDS) && (fc & IEEE80211_FCTL_TODS))
hdrlen = 30; /* Addr4 */
- if (fc & IEEE80211_STYPE_QOS_DATA)
- hdrlen += 2; /* QoS Control Field */
+ /* QoS Control Field */
+ hdrlen += (fc & IEEE80211_STYPE_QOS_DATA)
+ >> (ilog2(IEEE80211_STYPE_QOS_DATA)-1);
break;
case IEEE80211_FTYPE_CTL:
- switch (fc & IEEE80211_FCTL_STYPE) {
- case IEEE80211_STYPE_CTS:
- case IEEE80211_STYPE_ACK:
+ /*
+ * ACK and CTS are 10 bytes, all others 16. To see how
+ * to get this condition consider
+ * subtype mask: 0b0000000011110000 (0x00F0)
+ * ACK subtype: 0b0000000011010000 (0x00D0)
+ * CTS subtype: 0b0000000011000000 (0x00C0)
+ * bits that matter: ^^^ (0x00E0)
+ * value of those: 0b0000000011000000 (0x00C0)
+ */
+ if ((fc & 0xE0) == 0xC0)
hdrlen = 10;
- break;
- default:
+ else
hdrlen = 16;
- break;
- }
break;
}
-
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