From 5ae4c7816aeaf1e4b832e4f389b379570bccfcc4 Mon Sep 17 00:00:00 2001
From: Shengqi Chen <harry-chen@outlook.com>
Date: Sat, 7 Sep 2024 21:55:03 +0800
Subject: [PATCH] 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 callsites 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

Same tendency applies to the count of instructions.
Therefore 1-arg version is defined as a macro to the 2-arg one.

On all 64-bit arches, the differences are negligible.

See more discussion at https://github.com/openzfs/zfs/pull/16483.

Acked-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Shengqi Chen <harry-chen@outlook.com>
---
 include/cityhash.h        |  8 ++++++++
 lib/libzfs/libzfs.abi     | 13 +++++++++++++
 module/zcommon/cityhash.c | 23 +++++++++++++++++++++--
 3 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/include/cityhash.h b/include/cityhash.h
index 3b2d1e84b5b3..b78b5b174ad0 100644
--- a/include/cityhash.h
+++ b/include/cityhash.h
@@ -32,6 +32,14 @@
 extern "C" {
 #endif
 
+/*
+ * We have 2/3-argument specialized versions of cityhash4,
+ * which can reduce instruction count and stack usage on some 32-bit arches.
+ * For 1-arg version, using cityhash2 is enough.
+ */
+#define	cityhash1(w) (cityhash2(w, 0))
+_SYS_CITYHASH_H uint64_t cityhash2(uint64_t, uint64_t);
+_SYS_CITYHASH_H uint64_t cityhash3(uint64_t, uint64_t, uint64_t);
 _SYS_CITYHASH_H uint64_t cityhash4(uint64_t, uint64_t, uint64_t, uint64_t);
 
 #ifdef	__cplusplus
diff --git a/lib/libzfs/libzfs.abi b/lib/libzfs/libzfs.abi
index 51b29643ee0c..c3f8a7dc5569 100644
--- a/lib/libzfs/libzfs.abi
+++ b/lib/libzfs/libzfs.abi
@@ -153,6 +153,8 @@
     <elf-symbol name='avl_update_lt' type='func-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
     <elf-symbol name='avl_walk' type='func-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
     <elf-symbol name='bookmark_namecheck' type='func-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
+    <elf-symbol name='cityhash2' type='func-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
+    <elf-symbol name='cityhash3' type='func-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
     <elf-symbol name='cityhash4' type='func-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
     <elf-symbol name='color_end' type='func-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
     <elf-symbol name='color_start' type='func-type' binding='global-binding' visibility='default-visibility' is-defined='yes'/>
@@ -9179,6 +9181,17 @@
     </function-decl>
   </abi-instr>
   <abi-instr address-size='64' path='module/zcommon/cityhash.c' language='LANG_C99'>
+    <function-decl name='cityhash2' mangled-name='cityhash2' visibility='default' binding='global' size-in-bits='64' elf-symbol-id='cityhash2'>
+      <parameter type-id='9c313c2d' name='w1'/>
+      <parameter type-id='9c313c2d' name='w2'/>
+      <return type-id='9c313c2d'/>
+    </function-decl>
+    <function-decl name='cityhash3' mangled-name='cityhash3' visibility='default' binding='global' size-in-bits='64' elf-symbol-id='cityhash3'>
+      <parameter type-id='9c313c2d' name='w1'/>
+      <parameter type-id='9c313c2d' name='w2'/>
+      <parameter type-id='9c313c2d' name='w3'/>
+      <return type-id='9c313c2d'/>
+    </function-decl>
     <function-decl name='cityhash4' mangled-name='cityhash4' visibility='default' binding='global' size-in-bits='64' elf-symbol-id='cityhash4'>
       <parameter type-id='9c313c2d' name='w1'/>
       <parameter type-id='9c313c2d' name='w2'/>
diff --git a/module/zcommon/cityhash.c b/module/zcommon/cityhash.c
index 413a96df2cda..303d6855a376 100644
--- a/module/zcommon/cityhash.c
+++ b/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 +59,28 @@ 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));
+}
+
+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(cityhash2);
+EXPORT_SYMBOL(cityhash3);
 EXPORT_SYMBOL(cityhash4);
 #endif