read-cache.c: allow unaligned mapping of the index file

Both the on-disk format v2 and v3 pads the "name" field to the multiple of
eight to make sure that various quantities in network long/short type can
be accessed with ntohl/ntohs without having to worry about alignment, but
this forces us to waste disk I/O bandwidth.

Introduce ntoh_s()/ntoh_l() macros that the callers can use as if they were
the regular ntohs()/ntohl() on a field that may not be aligned correctly.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -1285,6 +1285,26 @@ int read_index(struct index_state *istate)
 	return read_index_from(istate, get_index_file());
 }
 
+#ifndef NEEDS_ALIGNED_ACCESS
+#define ntoh_s(var) ntohs(var)
+#define ntoh_l(var) ntohl(var)
+#else
+static inline uint16_t ntoh_s_force_align(void *p)
+{
+	uint16_t x;
+	memcpy(&x, p, sizeof(x));
+	return ntohs(x);
+}
+static inline uint32_t ntoh_l_force_align(void *p)
+{
+	uint32_t x;
+	memcpy(&x, p, sizeof(x));
+	return ntohl(x);
+}
+#define ntoh_s(var) ntoh_s_force_align(&(var))
+#define ntoh_l(var) ntoh_l_force_align(&(var))
+#endif
+
 static struct cache_entry *create_from_disk(struct ondisk_cache_entry *ondisk)
 {
 	struct cache_entry *ce;
@@ -1293,14 +1313,14 @@ static struct cache_entry *create_from_disk(struct ondisk_cache_entry *ondisk)
 	unsigned int flags;
 
 	/* On-disk flags are just 16 bits */
-	flags = ntohs(ondisk->flags);
+	flags = ntoh_s(ondisk->flags);
 	len = flags & CE_NAMEMASK;
 
 	if (flags & CE_EXTENDED) {
 		struct ondisk_cache_entry_extended *ondisk2;
 		int extended_flags;
 		ondisk2 = (struct ondisk_cache_entry_extended *)ondisk;
-		extended_flags = ntohs(ondisk2->flags2) << 16;
+		extended_flags = ntoh_s(ondisk2->flags2) << 16;
 		/* We do not yet understand any bit out of CE_EXTENDED_FLAGS */
 		if (extended_flags & ~CE_EXTENDED_FLAGS)
 			die("Unknown index entry format %08x", extended_flags);
@@ -1315,16 +1335,16 @@ static struct cache_entry *create_from_disk(struct ondisk_cache_entry *ondisk)
 
 	ce = xmalloc(cache_entry_size(len));
 
-	ce->ce_ctime.sec = ntohl(ondisk->ctime.sec);
-	ce->ce_mtime.sec = ntohl(ondisk->mtime.sec);
-	ce->ce_ctime.nsec = ntohl(ondisk->ctime.nsec);
-	ce->ce_mtime.nsec = ntohl(ondisk->mtime.nsec);
-	ce->ce_dev   = ntohl(ondisk->dev);
-	ce->ce_ino   = ntohl(ondisk->ino);
-	ce->ce_mode  = ntohl(ondisk->mode);
-	ce->ce_uid   = ntohl(ondisk->uid);
-	ce->ce_gid   = ntohl(ondisk->gid);
-	ce->ce_size  = ntohl(ondisk->size);
+	ce->ce_ctime.sec = ntoh_l(ondisk->ctime.sec);
+	ce->ce_mtime.sec = ntoh_l(ondisk->mtime.sec);
+	ce->ce_ctime.nsec = ntoh_l(ondisk->ctime.nsec);
+	ce->ce_mtime.nsec = ntoh_l(ondisk->mtime.nsec);
+	ce->ce_dev   = ntoh_l(ondisk->dev);
+	ce->ce_ino   = ntoh_l(ondisk->ino);
+	ce->ce_mode  = ntoh_l(ondisk->mode);
+	ce->ce_uid   = ntoh_l(ondisk->uid);
+	ce->ce_gid   = ntoh_l(ondisk->gid);
+	ce->ce_size  = ntoh_l(ondisk->size);
 	ce->ce_flags = flags;
 
 	hashcpy(ce->sha1, ondisk->sha1);