Precompute the fanout decoding and the oid offset in a pack-*.idx

The fanout table is fairly commonly accessed, we need to read it
twice for each object we lookup in any given pack file.  Most of
the processors running Git are running in little-endian mode, as
they are variants of the x86 platform, so reading the fanout is
a costly operation as we need to convert from network byte order
to local byte order.  By decoding the fanout table into a malloc
obtained buffer we can save these 2 decode operations per lookup
and make search go more quickly.

This also cleans up the initialization of the search functions
by cutting out a few instructions, saving a small amount of time.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

src/odb.c

@@ -46,6 +46,8 @@ struct git_pack {
 	/** The .idx file, mapped into memory. */
 	git_file idx_fd;
 	gitfo_map idx_map;
+	uint32_t *im_fanout;
+	unsigned char *im_oid;
 	uint32_t *im_crc;
 	uint32_t *im_offset32;
 	uint32_t *im_offset64;
@@ -520,11 +522,10 @@ static int pack_openidx_map(git_pack *p)
 
 static int idxv1_search(off_t *out, git_pack *p, const git_oid *id)
 {
-	unsigned char *data = p->idx_map.data;
-	size_t lo = id->id[0] ? decode32(data + ((id->id[0]-1) << 2)) : 0;
-	size_t hi = decode32(data + (id->id[0] << 2));
+	unsigned char *data = p->im_oid;
+	size_t lo = id->id[0] ? p->im_fanout[id->id[0] - 1] : 0;
+	size_t hi = p->im_fanout[id->id[0]];
 
-	data += 1024;
 	do {
 		size_t mid = (lo + hi) >> 1;
 		size_t pos = 24 * mid;
@@ -542,30 +543,42 @@ static int idxv1_search(off_t *out, git_pack *p, const git_oid *id)
 
 static int pack_openidx_v1(git_pack *p)
 {
-	uint32_t *fanout = p->idx_map.data;
+	uint32_t *src_fanout = p->idx_map.data;
+	uint32_t *im_fanout;
 	size_t expsz;
 	int j;
 
+	if (!(im_fanout = git__malloc(sizeof(*im_fanout) * 256)))
+		return GIT_ERROR;
+
+	im_fanout[0] = decode32(&src_fanout[0]);
 	for (j = 1; j < 256; j++) {
-		if (decode32(&fanout[j]) < decode32(&fanout[j - 1]))
+		im_fanout[j] = decode32(&src_fanout[j]);
+		if (im_fanout[j] < im_fanout[j - 1]) {
+			free(im_fanout);
 			return GIT_ERROR;
 		}
-	p->obj_cnt = decode32(&fanout[255]);
+	}
+	p->obj_cnt = im_fanout[255];
+
 	expsz = 4 * 256 + 24 * p->obj_cnt + 2 * 20;
-	if (expsz != p->idx_map.len)
+	if (expsz != p->idx_map.len) {
+		free(im_fanout);
 		return GIT_ERROR;
+	}
 
 	p->idx_search = idxv1_search;
+	p->im_fanout = im_fanout;
+	p->im_oid = (unsigned char*)(src_fanout + 256);
 	return GIT_SUCCESS;
 }
 
 static int idxv2_search(off_t *out, git_pack *p, const git_oid *id)
 {
-	unsigned char *data = ((unsigned char*)p->idx_map.data) + 8;
-	size_t lo = id->id[0] ? decode32(data + ((id->id[0]-1) << 2)) : 0;
-	size_t hi = decode32(data + (id->id[0] << 2));
+	unsigned char *data = p->im_oid;
+	size_t lo = id->id[0] ? p->im_fanout[id->id[0] - 1] : 0;
+	size_t hi = p->im_fanout[id->id[0]];
 
-	data += 1024;
 	do {
 		size_t mid = (lo + hi) >> 1;
 		size_t pos = 20 * mid;
@@ -587,17 +600,28 @@ static int idxv2_search(off_t *out, git_pack *p, const git_oid *id)
 
 static int pack_openidx_v2(git_pack *p)
 {
-	void *data = ((unsigned char*)p->idx_map.data) + 8;
-	uint32_t *fanout = data;
+	unsigned char *data = p->idx_map.data;
+	uint32_t *src_fanout = (uint32_t*)(data + 8);
+	uint32_t *im_fanout;
 	int j;
 
+	if (!(im_fanout = git__malloc(sizeof(*im_fanout) * 256)))
+		return GIT_ERROR;
+
+	im_fanout[0] = decode32(&src_fanout[0]);
 	for (j = 1; j < 256; j++) {
-		if (decode32(&fanout[j]) < decode32(&fanout[j - 1]))
+		im_fanout[j] = decode32(&src_fanout[j]);
+		if (im_fanout[j] < im_fanout[j - 1]) {
+			free(im_fanout);
 			return GIT_ERROR;
 		}
-	p->obj_cnt = decode32(&fanout[255]);
+	}
+	p->obj_cnt = im_fanout[255];
+
 	p->idx_search = idxv2_search;
-	p->im_crc = (uint32_t*)(data + 1024 + (20 * p->obj_cnt));
+	p->im_fanout = im_fanout;
+	p->im_oid = (unsigned char*)(src_fanout + 256);
+	p->im_crc = (uint32_t*)(p->im_oid + 20 * p->obj_cnt);
 	p->im_offset32 = p->im_crc + p->obj_cnt;
 	p->im_offset64 = p->im_offset32 + p->obj_cnt;
 	return GIT_SUCCESS;
@@ -661,6 +685,7 @@ static void pack_dec(git_pack *p)
 		if (p->idx_search) {
 			gitfo_free_map(&p->idx_map);
 			gitfo_close(p->idx_fd);
+			free(p->im_fanout);
 		}
 
 		gitlck_free(&p->lock);