[PATCH v2] ext4: improve str2hashbuf by processing 4-byte chunks and removing function pointers

[Guan-Chun Wu <409411716@gms.tku.edu.tw>]

The original byte-by-byte implementation with modulo checks is less
efficient. Refactor str2hashbuf_unsigned() and str2hashbuf_signed()
to process input in explicit 4-byte chunks instead of using a
modulus-based loop to emit words byte by byte.

Additionally, the use of function pointers for selecting the appropriate
str2hashbuf implementation has been removed. Instead, the functions are
directly invoked based on the hash type, eliminating the overhead of
dynamic function calls.

Performance test (x86_64, Intel Core i7-10700 @ 2.90GHz, average over 10000
runs, using kernel module for testing):

    len | orig_s | new_s | orig_u | new_u
    ----+--------+-------+--------+-------
      1 |   70   |   71  |   63   |   63
      8 |   68   |   64  |   64   |   62
     32 |   75   |   70  |   75   |   63
     64 |   96   |   71  |  100   |   68
    255 |  192   |  108  |  187   |   84

This change improves performance, especially for larger input sizes.

Signed-off-by: Guan-Chun Wu <409411716@gms.tku.edu.tw>
---
v1 -> v2:
- Drop redundant (int) casts.
- Replace indirect calls with simple conditionals.
- Use get_unaligned_be32() instead of manual byte extraction.
- Link to v1: https://lore.kernel.org/lkml/20251116130105.1988020-1-409411716@gms.tku.edu.tw/
---
 fs/ext4/hash.c | 64 +++++++++++++++++++++++++++++++++-----------------
 1 file changed, 42 insertions(+), 22 deletions(-)

diff --git a/fs/ext4/hash.c b/fs/ext4/hash.c
index 33cd5b6b02d5..97b7a3b0603e 100644
--- a/fs/ext4/hash.c
+++ b/fs/ext4/hash.c
@@ -9,6 +9,7 @@
 #include <linux/unicode.h>
 #include <linux/compiler.h>
 #include <linux/bitops.h>
+#include <linux/unaligned.h>
 #include "ext4.h"
 
 #define DELTA 0x9E3779B9
@@ -141,21 +142,28 @@ static void str2hashbuf_signed(const char *msg, int len, __u32 *buf, int num)
 	pad = (__u32)len | ((__u32)len << 8);
 	pad |= pad << 16;
 
-	val = pad;
 	if (len > num*4)
 		len = num * 4;
-	for (i = 0; i < len; i++) {
-		val = ((int) scp[i]) + (val << 8);
-		if ((i % 4) == 3) {
-			*buf++ = val;
-			val = pad;
-			num--;
-		}
+
+	while (len >= 4) {
+		val = (scp[0] << 24) + (scp[1] << 16) + (scp[2] << 8) + scp[3];
+		*buf++ = val;
+		scp += 4;
+		len -= 4;
+		num--;
 	}
+
+	val = pad;
+
+	for (i = 0; i < len; i++)
+		val = scp[i] + (val << 8);
+
 	if (--num >= 0)
 		*buf++ = val;
+
 	while (--num >= 0)
 		*buf++ = pad;
+
 }
 
 static void str2hashbuf_unsigned(const char *msg, int len, __u32 *buf, int num)
@@ -167,21 +175,28 @@ static void str2hashbuf_unsigned(const char *msg, int len, __u32 *buf, int num)
 	pad = (__u32)len | ((__u32)len << 8);
 	pad |= pad << 16;
 
-	val = pad;
 	if (len > num*4)
 		len = num * 4;
-	for (i = 0; i < len; i++) {
-		val = ((int) ucp[i]) + (val << 8);
-		if ((i % 4) == 3) {
-			*buf++ = val;
-			val = pad;
-			num--;
-		}
+
+	while (len >= 4) {
+		val = get_unaligned_be32(ucp);
+		*buf++ = val;
+		ucp += 4;
+		len -= 4;
+		num--;
 	}
+
+	val = pad;
+
+	for (i = 0; i < len; i++)
+		val = ucp[i] + (val << 8);
+
 	if (--num >= 0)
 		*buf++ = val;
+
 	while (--num >= 0)
 		*buf++ = pad;
+
 }
 
 /*
@@ -205,8 +220,7 @@ static int __ext4fs_dirhash(const struct inode *dir, const char *name, int len,
 	const char	*p;
 	int		i;
 	__u32		in[8], buf[4];
-	void		(*str2hashbuf)(const char *, int, __u32 *, int) =
-				str2hashbuf_signed;
+	bool use_unsigned = false;
 
 	/* Initialize the default seed for the hash checksum functions */
 	buf[0] = 0x67452301;
@@ -232,12 +246,15 @@ static int __ext4fs_dirhash(const struct inode *dir, const char *name, int len,
 		hash = dx_hack_hash_signed(name, len);
 		break;
 	case DX_HASH_HALF_MD4_UNSIGNED:
-		str2hashbuf = str2hashbuf_unsigned;
+		use_unsigned = true;
 		fallthrough;
 	case DX_HASH_HALF_MD4:
 		p = name;
 		while (len > 0) {
-			(*str2hashbuf)(p, len, in, 8);
+			if (use_unsigned)
+				str2hashbuf_unsigned(p, len, in, 8);
+			else
+				str2hashbuf_signed(p, len, in, 8);
 			half_md4_transform(buf, in);
 			len -= 32;
 			p += 32;
@@ -246,12 +263,15 @@ static int __ext4fs_dirhash(const struct inode *dir, const char *name, int len,
 		hash = buf[1];
 		break;
 	case DX_HASH_TEA_UNSIGNED:
-		str2hashbuf = str2hashbuf_unsigned;
+		use_unsigned = true;
 		fallthrough;
 	case DX_HASH_TEA:
 		p = name;
 		while (len > 0) {
-			(*str2hashbuf)(p, len, in, 4);
+			if (use_unsigned)
+				str2hashbuf_unsigned(p, len, in, 4);
+			else
+				str2hashbuf_signed(p, len, in, 4);
 			TEA_transform(buf, in);
 			len -= 16;
 			p += 16;
-- 
2.34.1