From a76aacd69fa3512c3c566c93248bc6c41bb00e97 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Reinhard=20Ni=C3=9Fl?= <rnissl@gmx.de>
Date: Thu, 12 Apr 2007 00:02:45 +0200
Subject: Speed up start code scanning. The current code implements hardware (a
 shift register) in software just to find the byte pattern 00 00 01 xx, which
 causes remarkable CPU load on less powerful machines. The new approach uses
 memchr() to find the 01 in the buffer, which most often hits a start code.
 memchr() seems to be even faster then implementing a real pattern search (i.
 e. by just looking at every third byte to find 01). The new implementation
 causes significantly fewer CPU load on less powerful machines.

---
 src/libmpeg2/decode.c | 99 +++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 72 insertions(+), 27 deletions(-)

(limited to 'src/libmpeg2/decode.c')

diff --git a/src/libmpeg2/decode.c b/src/libmpeg2/decode.c
index c3ba6ea4d..2b2426950 100644
--- a/src/libmpeg2/decode.c
+++ b/src/libmpeg2/decode.c
@@ -598,13 +598,63 @@ fprintf(stderr, "AFD changed from %d to %d\n", mpeg2dec->afd_value_reported, mpe
     return is_frame_done;
 }
 
+static inline int find_start_code (mpeg2dec_t * mpeg2dec,
+                                   uint8_t ** current, uint8_t * limit)
+{
+    uint8_t * p;
+
+    if (*current >= limit)
+	return 0;
+    if (mpeg2dec->shift == 0x00000100)
+	return 1;
+
+    mpeg2dec->shift = (mpeg2dec->shift | *(*current)++) << 8;
+
+    if (*current >= limit)
+	return 0;
+    if (mpeg2dec->shift == 0x00000100)
+	return 1;
+
+    mpeg2dec->shift = (mpeg2dec->shift | *(*current)++) << 8;
+
+    if (*current >= limit)
+	return 0;
+    if (mpeg2dec->shift == 0x00000100)
+	return 1;
+
+    limit--;
+
+    if (*current >= limit) {
+	mpeg2dec->shift = (mpeg2dec->shift | *(*current)++) << 8;
+	return 0;
+    }
+
+    p = *current;
+
+    while (p < limit && (p = (uint8_t *)memchr(p, 0x01, limit - p))) {
+	if (p[-2] || p[-1])
+	    p += 3;
+	else {
+	    *current = ++p;
+	    return 1;
+	}
+    }
+
+    *current = ++limit;
+    p = limit - 3;
+    mpeg2dec->shift = (mpeg2dec->shift | *p++) << 8;
+    mpeg2dec->shift = (mpeg2dec->shift | *p++) << 8;
+    mpeg2dec->shift = (mpeg2dec->shift | *p++) << 8;
+
+    return 0;
+}
+
 static inline uint8_t * copy_chunk (mpeg2dec_t * mpeg2dec,
 				    uint8_t * current, uint8_t * end)
 {
-    uint32_t shift;
-    uint8_t * chunk_ptr;
     uint8_t * limit;
-    uint8_t byte;
+    uint8_t * data = current;
+    int found, bite;
 
     /* sequence end code 0xb7 doesn't have any data and there might be the case
      * that no start code will follow this code for quite some time (e. g. in case
@@ -618,37 +668,32 @@ static inline uint8_t * copy_chunk (mpeg2dec_t * mpeg2dec,
 	return current;
     }
 
-    shift = mpeg2dec->shift;
-    chunk_ptr = mpeg2dec->chunk_ptr;
-    limit = current + (mpeg2dec->chunk_buffer + BUFFER_SIZE - chunk_ptr);
+    limit = current + (mpeg2dec->chunk_buffer + BUFFER_SIZE - mpeg2dec->chunk_ptr);
     if (limit > end)
 	limit = end;
 
-    while (1) {
-
-	byte = *current++;
-	if (shift != 0x00000100) {
-	    shift = (shift | byte) << 8;
-	    *chunk_ptr++ = byte;
-	    if (current < limit)
-		continue;
-	    if (current == end) {
-		mpeg2dec->chunk_ptr = chunk_ptr;
-		mpeg2dec->shift = shift;
-		return NULL;
-	    } else {
-		/* we filled the chunk buffer without finding a start code */
-		mpeg2dec->code = 0xb4;	/* sequence_error_code */
-		mpeg2dec->chunk_ptr = mpeg2dec->chunk_buffer;
-		return current;
-	    }
-	}
-	mpeg2dec->code = byte;
-	mpeg2dec->chunk_size = chunk_ptr - mpeg2dec->chunk_buffer - 3;
+    found = find_start_code(mpeg2dec, &current, limit);
+    bite = current - data;
+    if (bite) {
+	xine_fast_memcpy(mpeg2dec->chunk_ptr, data, bite);
+	mpeg2dec->chunk_ptr += bite;
+    }
+
+    if (found) {
+	mpeg2dec->code = *current++;
+	mpeg2dec->chunk_size = mpeg2dec->chunk_ptr - mpeg2dec->chunk_buffer - 3;
 	mpeg2dec->chunk_ptr = mpeg2dec->chunk_buffer;
 	mpeg2dec->shift = 0xffffff00;
 	return current;
     }
+ 
+    if (current == end)
+	return NULL;
+
+    /* we filled the chunk buffer without finding a start code */
+    mpeg2dec->code = 0xb4;	/* sequence_error_code */
+    mpeg2dec->chunk_ptr = mpeg2dec->chunk_buffer;
+    return current;
 }
 
 int mpeg2_decode_data (mpeg2dec_t * mpeg2dec, uint8_t * current, uint8_t * end,
-- 
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