Change in osmo-tetra[master]: phy/tetra_burst.c: use bitwise operations to speed up synchronization

[Jan Hrach]

Jan Hrach has uploaded this change for review. ( https://gerrit.osmocom.org/10263


Change subject: phy/tetra_burst.c: use bitwise operations to speed up synchronization
......................................................................

phy/tetra_burst.c: use bitwise operations to speed up synchronization

Finding synchronization sequence eats several times more CPU time than the
actual decoding. This is especially pronounced on channels with lots of errors
(where synchronization is lost frequently) and channels that are most of the
time empty (such as uplink channels, support for which is coming in following
patches).

Profiling shows that all the time is spent in memcmp calls.

A complicated and efficient algorithm, e.g. Aho-Corasick, turned out to be
not necessary. Compilers can optimize even a simple bit filter into fast code.

This provides only a modest (~25 %) performance gain, more fixes are coming.

Fixes: OS#1897
Change-Id: I3b90cc70c2ec67253a0fd2f00c6957a80971c38b
---
M src/phy/tetra_burst.c
1 file changed, 30 insertions(+), 0 deletions(-)



  git pull ssh://gerrit.osmocom.org:29418/osmo-tetra refs/changes/63/10263/1

diff --git a/src/phy/tetra_burst.c b/src/phy/tetra_burst.c
index 62efb72..5bede06 100644
--- a/src/phy/tetra_burst.c
+++ b/src/phy/tetra_burst.c
@@ -269,9 +269,39 @@
 int tetra_find_train_seq(const uint8_t *in, unsigned int end_of_in,
 			 uint32_t mask_of_train_seq, unsigned int *offset)
 {
+	static uint32_t tsq_bytes[5];
+
+	if (tsq_bytes[0] == 0) {
+#define FILTER_LOOKAHEAD_LEN 22
+#define FILTER_LOOKAHEAD_MASK ((1<<FILTER_LOOKAHEAD_LEN)-1)
+		for (int i = 0; i < FILTER_LOOKAHEAD_LEN; i++) {
+			tsq_bytes[0] = (tsq_bytes[0] << 1) | y_bits[i];
+			tsq_bytes[1] = (tsq_bytes[1] << 1) | n_bits[i];
+			tsq_bytes[2] = (tsq_bytes[2] << 1) | p_bits[i];
+			tsq_bytes[3] = (tsq_bytes[3] << 1) | q_bits[i];
+			tsq_bytes[4] = (tsq_bytes[4] << 1) | x_bits[i];
+		}
+	}
+
+	uint32_t filter = 0;
+
+	for (int i = 0; i < FILTER_LOOKAHEAD_LEN-2; i++)
+		filter = (filter << 1) | in[i];
+
 	const uint8_t *cur;
 
 	for (cur = in; cur < in + end_of_in; cur++) {
+		filter = ((filter << 1) | cur[FILTER_LOOKAHEAD_LEN-1]) & FILTER_LOOKAHEAD_MASK;
+
+		int match = 0;
+
+		for (int i = 0; i < 5; i++)
+			if (filter == tsq_bytes[i])
+				match = 1;
+
+		if (!match)
+			continue;
+
 		int remain_len = (in + end_of_in) - cur;
 
 		if (mask_of_train_seq & (1 << TETRA_TRAIN_SYNC) &&

-- 
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Gerrit-Project: osmo-tetra
Gerrit-Branch: master
Gerrit-MessageType: newchange
Gerrit-Change-Id: I3b90cc70c2ec67253a0fd2f00c6957a80971c38b
Gerrit-Change-Number: 10263
Gerrit-PatchSet: 1
Gerrit-Owner: Jan Hrach <jenda.2vf9h at hrach.eu>
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