read-cache.c: optimize reading index format v4

Index format v4 requires some more computation to assemble a path
based on a previous one. The current code is not very efficient
because

 - it doubles memory copy, we assemble the final path in a temporary
   first before putting it back to a cache_entry

 - strbuf_remove() in expand_name_field() is not exactly a good fit
   for stripping a part at the end, _setlen() would do the same job
   and is much cheaper.

 - the open-coded loop to find the end of the string in
   expand_name_field() can't beat an optimized strlen()

This patch avoids the temporary buffer and writes directly to the new
cache_entry, which addresses the first two points. The last point
could also be avoided if the total string length fits in the first 12
bits of ce_flags, if not we fall back to strlen().

Running "test-tool read-cache 100" on webkit.git (275k files), reading
v2 only takes 4.226 seconds, while v4 takes 5.711 seconds, 35% more
time. The patch reduces read time on v4 to 4.319 seconds.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -1713,63 +1713,24 @@ int read_index(struct index_state *istate)
 	return read_index_from(istate, get_index_file(), get_git_dir());
 }
 
-static struct cache_entry *cache_entry_from_ondisk(struct mem_pool *mem_pool,
-						   struct ondisk_cache_entry *ondisk,
-						   unsigned int flags,
-						   const char *name,
-						   size_t len)
-{
-	struct cache_entry *ce = mem_pool__ce_alloc(mem_pool, len);
-
-	ce->ce_stat_data.sd_ctime.sec = get_be32(&ondisk->ctime.sec);
-	ce->ce_stat_data.sd_mtime.sec = get_be32(&ondisk->mtime.sec);
-	ce->ce_stat_data.sd_ctime.nsec = get_be32(&ondisk->ctime.nsec);
-	ce->ce_stat_data.sd_mtime.nsec = get_be32(&ondisk->mtime.nsec);
-	ce->ce_stat_data.sd_dev   = get_be32(&ondisk->dev);
-	ce->ce_stat_data.sd_ino   = get_be32(&ondisk->ino);
-	ce->ce_mode  = get_be32(&ondisk->mode);
-	ce->ce_stat_data.sd_uid   = get_be32(&ondisk->uid);
-	ce->ce_stat_data.sd_gid   = get_be32(&ondisk->gid);
-	ce->ce_stat_data.sd_size  = get_be32(&ondisk->size);
-	ce->ce_flags = flags & ~CE_NAMEMASK;
-	ce->ce_namelen = len;
-	ce->index = 0;
-	hashcpy(ce->oid.hash, ondisk->sha1);
-	memcpy(ce->name, name, len);
-	ce->name[len] = '\0';
-	return ce;
-}
-
-/*
- * Adjacent cache entries tend to share the leading paths, so it makes
- * sense to only store the differences in later entries.  In the v4
- * on-disk format of the index, each on-disk cache entry stores the
- * number of bytes to be stripped from the end of the previous name,
- * and the bytes to append to the result, to come up with its name.
- */
-static unsigned long expand_name_field(struct strbuf *name, const char *cp_)
-{
-	const unsigned char *ep, *cp = (const unsigned char *)cp_;
-	size_t len = decode_varint(&cp);
-
-	if (name->len < len)
-		die("malformed name field in the index");
-	strbuf_remove(name, name->len - len, len);
-	for (ep = cp; *ep; ep++)
-		; /* find the end */
-	strbuf_add(name, cp, ep - cp);
-	return (const char *)ep + 1 - cp_;
-}
-
-static struct cache_entry *create_from_disk(struct mem_pool *mem_pool,
+static struct cache_entry *create_from_disk(struct index_state *istate,
 					    struct ondisk_cache_entry *ondisk,
 					    unsigned long *ent_size,
-					    struct strbuf *previous_name)
+					    const struct cache_entry *previous_ce)
 {
 	struct cache_entry *ce;
 	size_t len;
 	const char *name;
 	unsigned int flags;
+	size_t copy_len;
+	/*
+	 * Adjacent cache entries tend to share the leading paths, so it makes
+	 * sense to only store the differences in later entries.  In the v4
+	 * on-disk format of the index, each on-disk cache entry stores the
+	 * number of bytes to be stripped from the end of the previous name,
+	 * and the bytes to append to the result, to come up with its name.
+	 */
+	int expand_name_field = istate->version == 4;
 
 	/* On-disk flags are just 16 bits */
 	flags = get_be16(&ondisk->flags);
@@ -1789,21 +1750,54 @@ static struct cache_entry *create_from_disk(struct mem_pool *mem_pool,
 	else
 		name = ondisk->name;
 
-	if (!previous_name) {
-		/* v3 and earlier */
-		if (len == CE_NAMEMASK)
-			len = strlen(name);
-		ce = cache_entry_from_ondisk(mem_pool, ondisk, flags, name, len);
+	if (expand_name_field) {
+		const unsigned char *cp = (const unsigned char *)name;
+		size_t strip_len, previous_len;
 
-		*ent_size = ondisk_ce_size(ce);
+		previous_len = previous_ce ? previous_ce->ce_namelen : 0;
+		strip_len = decode_varint(&cp);
+		if (previous_len < strip_len) {
+			if (previous_ce)
+				die(_("malformed name field in the index, near path '%s'"),
+				    previous_ce->name);
+			else
+				die(_("malformed name field in the index in the first path"));
+		}
+		copy_len = previous_len - strip_len;
+		name = (const char *)cp;
+	}
+
+	if (len == CE_NAMEMASK) {
+		len = strlen(name);
+		if (expand_name_field)
+			len += copy_len;
+	}
+
+	ce = mem_pool__ce_alloc(istate->ce_mem_pool, len);
+
+	ce->ce_stat_data.sd_ctime.sec = get_be32(&ondisk->ctime.sec);
+	ce->ce_stat_data.sd_mtime.sec = get_be32(&ondisk->mtime.sec);
+	ce->ce_stat_data.sd_ctime.nsec = get_be32(&ondisk->ctime.nsec);
+	ce->ce_stat_data.sd_mtime.nsec = get_be32(&ondisk->mtime.nsec);
+	ce->ce_stat_data.sd_dev   = get_be32(&ondisk->dev);
+	ce->ce_stat_data.sd_ino   = get_be32(&ondisk->ino);
+	ce->ce_mode  = get_be32(&ondisk->mode);
+	ce->ce_stat_data.sd_uid   = get_be32(&ondisk->uid);
+	ce->ce_stat_data.sd_gid   = get_be32(&ondisk->gid);
+	ce->ce_stat_data.sd_size  = get_be32(&ondisk->size);
+	ce->ce_flags = flags & ~CE_NAMEMASK;
+	ce->ce_namelen = len;
+	ce->index = 0;
+	hashcpy(ce->oid.hash, ondisk->sha1);
+
+	if (expand_name_field) {
+		if (copy_len)
+			memcpy(ce->name, previous_ce->name, copy_len);
+		memcpy(ce->name + copy_len, name, len + 1 - copy_len);
+		*ent_size = (name - ((char *)ondisk)) + len + 1 - copy_len;
 	} else {
-		unsigned long consumed;
-		consumed = expand_name_field(previous_name, name);
-		ce = cache_entry_from_ondisk(mem_pool, ondisk, flags,
-					     previous_name->buf,
-					     previous_name->len);
-
-		*ent_size = (name - ((char *)ondisk)) + consumed;
+		memcpy(ce->name, name, len + 1);
+		*ent_size = ondisk_ce_size(ce);
 	}
 	return ce;
 }
@@ -1898,7 +1892,7 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	struct cache_header *hdr;
 	void *mmap;
 	size_t mmap_size;
-	struct strbuf previous_name_buf = STRBUF_INIT, *previous_name;
+	const struct cache_entry *previous_ce = NULL;
 
 	if (istate->initialized)
 		return istate->cache_nr;
@@ -1936,11 +1930,9 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	istate->initialized = 1;
 
 	if (istate->version == 4) {
-		previous_name = &previous_name_buf;
 		mem_pool_init(&istate->ce_mem_pool,
 			      estimate_cache_size_from_compressed(istate->cache_nr));
 	} else {
-		previous_name = NULL;
 		mem_pool_init(&istate->ce_mem_pool,
 			      estimate_cache_size(mmap_size, istate->cache_nr));
 	}
@@ -1952,12 +1944,12 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 		unsigned long consumed;
 
 		disk_ce = (struct ondisk_cache_entry *)((char *)mmap + src_offset);
-		ce = create_from_disk(istate->ce_mem_pool, disk_ce, &consumed, previous_name);
+		ce = create_from_disk(istate, disk_ce, &consumed, previous_ce);
 		set_index_entry(istate, i, ce);
 
 		src_offset += consumed;
+		previous_ce = ce;
 	}
-	strbuf_release(&previous_name_buf);
 	istate->timestamp.sec = st.st_mtime;
 	istate->timestamp.nsec = ST_MTIME_NSEC(st);