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 3 of the 5 branches it contained.
Signed-off-by: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
---
Looks like I forgot to resend this with the new comment. I think
optimising the control frames too is fine even if we don't use them,
it's not /that/ hard to understand.
net/mac80211/ieee80211.c | 31 +++++++++++++++++++++----------
1 file changed, 21 insertions(+), 10 deletions(-)
--- wireless-dev.orig/net/mac80211/ieee80211.c 2007-04-27 12:03:38.334987898 +0200
+++ wireless-dev/net/mac80211/ieee80211.c 2007-04-27 12:03:43.034987898 +0200
@@ -255,21 +255,32 @@ int ieee80211_get_hdrlen(u16 fc)
switch (fc & IEEE80211_FCTL_FTYPE) {
case IEEE80211_FTYPE_DATA:
- if ((fc & IEEE80211_FCTL_FROMDS) && (fc & IEEE80211_FCTL_TODS))
+ if (unlikely((fc & IEEE80211_FCTL_FROMDS) && (fc & IEEE80211_FCTL_TODS)))
hdrlen = 30; /* Addr4 */
- if (fc & IEEE80211_STYPE_QOS_DATA)
- hdrlen += 2; /* QoS Control Field */
+ /*
+ * The QoS Control field is two bytes and its presence is
+ * indicated by the IEEE80211_STYPE_QOS_DATA bit. Add 2 to
+ * hdrlen if that bit is set.
+ * This works by masking out the bit and shifting it to
+ * bit position 1 so the result has the value 0 or 2.
+ */
+ 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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