zcommon: add specialized versions of cityhash4

Specializing cityhash4 on 32-bit architectures can reduce the size
of stack frames as well as instruction count. This is a tiny but
useful optimization, since some callers invoke it frequently.

When specializing into 1/2/3/4-arg versions, the stack usage
(in bytes) on some 32-bit arches are listed as follows:

- x86: 32, 32, 32, 40
- arm-v7a: 20, 20, 28, 36
- riscv: 0, 0, 0, 16
- power: 16, 16, 16, 32
- mipsel: 8, 8, 8, 24

And each actual argument (even if passing 0) contributes evenly
to the number of multiplication instructions generated:

- x86: 9, 12, 15 ,18
- arm-v7a: 6, 8, 10, 12
- riscv / power: 12, 18, 20, 24
- mipsel: 9, 12, 15, 19

On 64-bit architectures, the tendencies are similar. But both stack
sizes and instruction counts are significantly smaller thus negligible.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Shengqi Chen <harry-chen@outlook.com>
Closes #16131
Closes #16483

module/zcommon/cityhash.c

@@ -49,8 +49,8 @@ cityhash_helper(uint64_t u, uint64_t v, uint64_t mul)
 	return (b);
 }
 
-uint64_t
-cityhash4(uint64_t w1, uint64_t w2, uint64_t w3, uint64_t w4)
+static inline uint64_t
+cityhash_impl(uint64_t w1, uint64_t w2, uint64_t w3, uint64_t w4)
 {
 	uint64_t mul = HASH_K2 + 64;
 	uint64_t a = w1 * HASH_K1;
@@ -59,9 +59,38 @@ cityhash4(uint64_t w1, uint64_t w2, uint64_t w3, uint64_t w4)
 	uint64_t d = w3 * HASH_K2;
 	return (cityhash_helper(rotate(a + b, 43) + rotate(c, 30) + d,
 	    a + rotate(b + HASH_K2, 18) + c, mul));
+}
 
+/*
+ * Passing w as the 2nd argument could save one 64-bit multiplication.
+ */
+uint64_t
+cityhash1(uint64_t w)
+{
+	return (cityhash_impl(0, w, 0, 0));
+}
+
+uint64_t
+cityhash2(uint64_t w1, uint64_t w2)
+{
+	return (cityhash_impl(w1, w2, 0, 0));
+}
+
+uint64_t
+cityhash3(uint64_t w1, uint64_t w2, uint64_t w3)
+{
+	return (cityhash_impl(w1, w2, w3, 0));
+}
+
+uint64_t
+cityhash4(uint64_t w1, uint64_t w2, uint64_t w3, uint64_t w4)
+{
+	return (cityhash_impl(w1, w2, w3, w4));
 }
 
 #if defined(_KERNEL)
+EXPORT_SYMBOL(cityhash1);
+EXPORT_SYMBOL(cityhash2);
+EXPORT_SYMBOL(cityhash3);
 EXPORT_SYMBOL(cityhash4);
 #endif