x86: Optimize memchr() for x86-64
Subsystems (2)
cover x86: Optimize memchr() for x86-64
*** BLURB HERE ***
These patch series add an optimized "memchr()" for x86-64 and USER-MODE LINUX (UML).
There exists an assemebly implementation for x86-32. However, for x86-64, there isn't any optimized version. We implement word-wise comparison so that 8 characters can be compared at the same time on x86-64 CPU. The optimized “memchr()” is nearly 4x faster than the orginal implementation for long strings.
We test the optimized “memchr()” in UML and also recompile the 5.18 Kernel with the optimized “memchr()”. They run correctly.
In this patch we add a new file "string_64.c", which only contains "memchr()". We can add more optimized string functions in it in the future.
Yu-Jen Chang (2):
x86/lib: Optimize memchr()
x86/um: Use x86_64-optimized memchr
arch/x86/include/asm/string_64.h | 3 ++ arch/x86/lib/Makefile | 1 + arch/x86/lib/string_64.c | 78 ++++++++++++++++++++++++++++++++ arch/x86/um/Makefile | 2 +- 4 files changed, 83 insertions(+), 1 deletion(-) create mode 100644 arch/x86/lib/string_64.c
Signature · 1 line
2.25.1
PATCH 1/2 x86/lib: Optimize memchr()
The original assembly version of memchr() is implemented with the byte-wise comparing technique, which does not fully use 64-bits registers in x86_64 CPU. We use word-wide comparing so that 8 characters can be compared at the same time on x86_64 CPU. First we align the input and then use word-wise comparing to find the first 64-bit word that contain the target. Secondly, we compare every byte in the word and get the output.
We create two files to measure the performance. The first file contains on average 10 characters ahead the target character. The second file contains at least 1000 characters ahead the target character. Our implementation of “memchr()” is slightly better in the first test and nearly 4x faster than the orginal implementation in the second test.
--- arch/x86/include/asm/string_64.h | 3 ++ arch/x86/lib/Makefile | 1 + arch/x86/lib/string_64.c | 78 ++++++++++++++++++++++++++++++++ 3 files changed, 82 insertions(+) create mode 100644 arch/x86/lib/string_64.c
arch/x86/include/asm/string_64.h +3 -0
@@ -14,6 +14,9 @@extern void *memcpy (void *to,const void *from,size_t len);extern void *__memcpy (void *to,const void *from,size_t len); +#define __HAVE_ARCH_MEMCHR +extern void *memchr (const void *cs,int c,size_t length); +#define __HAVE_ARCH_MEMSETvoid *memset (void *s,int c,size_t n);void *__memset (void *s,int c,size_t n);
arch/x86/lib/Makefile +1 -0
@@ -69,5 +69,6 @@ else lib-y += clear_page_64.o copy_page_64.o lib-y += memmove_64.o memset_64.o lib-y += copy_user_64.o + lib-y += string_64.o lib-y += cmpxchg16b_emu.o endif
arch/x86/lib/string_64.c new file +78 -0
78 lines not on this page · raw message
Signature · 7 lines
2.25.1 _______________________________________________ linux-um mailing list [email protected] http://lists.infradead.org/mailman/listinfo/linux-um
Re: [PATCH 1/2] x86/lib: Optimize memchr()
Quoted 58 lines · Yu-Jen Chang
The original assembly version of
memchr()is implemented with the byte-wise comparing technique, which does not fully use 64-bits registers inx86_64CPU. We use word-wide comparing so that 8 characters can be compared at the same time onx86_64CPU. First we align the input and then use word-wise comparing to find the first 64-bit word that contain the target. Secondly, we compare every byte in the word and get the output.We create two files to measure the performance. The first file contains on average 10 characters ahead the target character. The second file contains at least 1000 characters ahead the target character. Our implementation of “memchr()” is slightly better in the first test and nearly 4x faster than the orginal implementation in the second test.
Signed-off-by Yu-Jen Chang <[email protected]>Signed-off-by Ching-Chun (Jim) Huang <[email protected]>--- arch/x86/include/asm/string_64.h | 3 ++ arch/x86/lib/Makefile | 1 + arch/x86/lib/string_64.c | 78 ++++++++++++++++++++++++++++++++ 3 files changed, 82 insertions(+) create mode 100644 arch/x86/lib/string_64.carch/x86/include/asm/string_64.h +3
-0@@ -14,6 +14,9 @@extern void *memcpy (void *to,const void *from,size_t len);extern void *__memcpy (void *to,const void *from,size_t len); +#define __HAVE_ARCH_MEMCHR +extern void *memchr (const void *cs,int c,size_t length); +#define __HAVE_ARCH_MEMSETvoid *memset (void *s,int c,size_t n);void *__memset (void *s,int c,size_t n);arch/x86/lib/Makefile +1
-0@@ -69,5 +69,6 @@ else lib-y += clear_page_64.o copy_page_64.o lib-y += memmove_64.o memset_64.o lib-y += copy_user_64.o + lib-y += string_64.o lib-y += cmpxchg16b_emu.o endifarch/x86/lib/string_64.c new file +3
-0@@ -0,0 +1,78 @@ +// SPDX-License-Identifier: GPL-2.0 +#include <linux/string.h> +#include <linux/export.h>
arch/x86/lib/string_64.c line 13
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+#include <linux/align.h> + +/* How many bytes are loaded each iteration of the word copy loop */ +#define LBLOCKSIZE (sizeof(long)) + +#ifdef __HAVE_ARCH_MEMCHR + +void *memchr (const void *cs,int c,size_t length) +{ +const unsigned char *src = (const unsigned char *)cs, d = c;
I don't know why this 'd = c' is not error.
d is a char pointer and c is int. At least this is not safe me do not know.
arch/x86/lib/string_64.c line 18
Quoted 4 lines
arch/x86/lib/string_64.c new file +4
-0+while (!IS_ALIGNED ((long )src,sizeof (long ))) { +if (!length--) +return NULL ; +if (*src == d)
Compare a character value to a pointer value and this value is c. May be right do not know.
Or:
char d = c;
...
arch/x86/lib/string_64.c line 27
Quoted 9 lines
arch/x86/lib/string_64.c new file +9
-0+return (void *)src; + src++; + } +if (length >=LBLOCKSIZE ) { +unsigned long mask = d <<8 | d; +unsigned int i =32 ; +long xor, data; +const long consta =0xFEFEFEFEFEFEFEFF , + constb =0x8080808080808080 ;
Two magic number..
Quoted 20 lines
arch/x86/lib/string_64.c new file +20
-0+/* +* Create a 8-bytes mask for word-wise comparing. +* For example, a mask for 'a' is 0x6161616161616161. +*/ + + mask |= mask <<16 ; +for (i =32 ; i <LBLOCKSIZE *8 ; i <<=1 ) + mask |= mask << i; +/* +* We perform word-wise comparing with following operation: +* 1. Perform xor on the long word @src and @mask +* and put into @xor. +* 2. Add @xor with @consta. +* 3. ~@xor & @constb. +* 4. Perform & with the result of step 2 and 3. +* +* Step 1 creates a byte which is 0 in the long word if +* there is at least one target byte in it. +* +* Step 2 to Step 4 find if there is a byte with 0 in
arch/x86/lib/string_64.c line 58
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+* the long word. +*/ + asmvolatile ("1:\n\t" +"movq (%0),%1\n\t" +"xorq %6,%1\n\t" +"lea (%1,%4), %2\n\t" +"notq %1\n\t" +"andq %5,%1\n\t" +"testq %1,%2\n\t" + "jne 2f\n\t"
s/jne/jnz/
Lack much here from me. But I give example that should check the CF flag is zero.
1) contain matching byte.
1111111011111111(consta)
+add
0000000001101100(xor)
------------------------
1111111101101011 (%1)
1111111110010100(~xor)
&and
1000000010000000(constb)
------------------------
1000000010000000 (%2)
the logical and of %1 and %2 is
1000000000000000 that is not zero.
2) not contain matching byte
1111111011111111
+
0110111011011100
----------------
0110110111011011(%1)
1001000100100011
&
1000000010000000
----------------
1000000000000000(%2)
%1 and %2 is
0000000000000000 that is zero.
I guess that here should use jump instruction jnz instead. Even though, I do not know why that two magic number is so magical..
Thanks,
Tao
Quoted 23 lines
arch/x86/lib/string_64.c new file +20
-0+ "add $8,%0\n\t" + "sub $8,%3\n\t" + "cmp $7,%3\n\t" + "ja 1b\n\t" + "2:\n\t" + : "=D"(src), "=r"(xor), "=r"(data), "=r"(length) + : "r"(consta), "r"(constb), "r"(mask), "0"(src), + "1"(xor), "2"(data), "3"(length) + : "memory", "cc"); + } + +while (length--) { +if (*src == d) +return (void *)src; + src++; + } +return NULL ; +} +EXPORT_SYMBOL (memchr); +#endif Signature · 1 line
2.25.1
Re: [PATCH 1/2] x86/lib: Optimize memchr()
Quoted 66 lines · Tao Zhou
Quoted 63 lines · Yu-Jen Chang
The original assembly version of
memchr()is implemented with the byte-wise comparing technique, which does not fully use 64-bits registers inx86_64CPU. We use word-wide comparing so that 8 characters can be compared at the same time onx86_64CPU. First we align the input and then use word-wise comparing to find the first 64-bit word that contain the target. Secondly, we compare every byte in the word and get the output.We create two files to measure the performance. The first file contains on average 10 characters ahead the target character. The second file contains at least 1000 characters ahead the target character. Our implementation of “memchr()” is slightly better in the first test and nearly 4x faster than the orginal implementation in the second test.
Signed-off-by Yu-Jen Chang <[email protected]>Signed-off-by Ching-Chun (Jim) Huang <[email protected]>--- arch/x86/include/asm/string_64.h | 3 ++ arch/x86/lib/Makefile | 1 + arch/x86/lib/string_64.c | 78 ++++++++++++++++++++++++++++++++ 3 files changed, 82 insertions(+) create mode 100644 arch/x86/lib/string_64.carch/x86/include/asm/string_64.h +3
-0@@ -14,6 +14,9 @@extern void *memcpy (void *to,const void *from,size_t len);extern void *__memcpy (void *to,const void *from,size_t len); +#define __HAVE_ARCH_MEMCHR +extern void *memchr (const void *cs,int c,size_t length); +#define __HAVE_ARCH_MEMSETvoid *memset (void *s,int c,size_t n);void *__memset (void *s,int c,size_t n);arch/x86/lib/Makefile +1
-0@@ -69,5 +69,6 @@ else lib-y += clear_page_64.o copy_page_64.o lib-y += memmove_64.o memset_64.o lib-y += copy_user_64.o + lib-y += string_64.o lib-y += cmpxchg16b_emu.o endifarch/x86/lib/string_64.c new file +8
-0@@ -0,0 +1,78 @@ +// SPDX-License-Identifier: GPL-2.0 +#include <linux/string.h> +#include <linux/export.h> +#include <linux/align.h> + +/* How many bytes are loaded each iteration of the word copy loop */ +#define LBLOCKSIZE (sizeof(long)) +
Quoted 9 lines
arch/x86/lib/string_64.c line 13
Quoted 5 lines
arch/x86/lib/string_64.c new file +5
-0+#ifdef __HAVE_ARCH_MEMCHR + +void *memchr (const void *cs,int c,size_t length) +{ +const unsigned char *src = (const unsigned char *)cs, d = c;I don't know why this 'd = c' is not error.
d is a char pointer and c is int. At least this is not safe me do not know.
The parameter d is a character value, not a pointer. Therefore 'd = c' is not an error.
Quoted 94 lines
arch/x86/lib/string_64.c line 18
Quoted 4 lines
arch/x86/lib/string_64.c new file +4
-0+while (!IS_ALIGNED ((long )src,sizeof (long ))) { +if (!length--) +return NULL ; +if (*src == d)Compare a character value to a pointer value and this value is c. May be right do not know.
Or:
char d = c;
...arch/x86/lib/string_64.c line 27
Quoted 9 lines
arch/x86/lib/string_64.c new file +9
-0+return (void *)src; + src++; + } +if (length >=LBLOCKSIZE ) { +unsigned long mask = d <<8 | d; +unsigned int i =32 ; +long xor, data; +const long consta =0xFEFEFEFEFEFEFEFF , + constb =0x8080808080808080 ;Two magic number..
Quoted 20 lines
arch/x86/lib/string_64.c new file +20
-0+/* +* Create a 8-bytes mask for word-wise comparing. +* For example, a mask for 'a' is 0x6161616161616161. +*/ + + mask |= mask <<16 ; +for (i =32 ; i <LBLOCKSIZE *8 ; i <<=1 ) + mask |= mask << i; +/* +* We perform word-wise comparing with following operation: +* 1. Perform xor on the long word @src and @mask +* and put into @xor. +* 2. Add @xor with @consta. +* 3. ~@xor & @constb. +* 4. Perform & with the result of step 2 and 3. +* +* Step 1 creates a byte which is 0 in the long word if +* there is at least one target byte in it. +* +* Step 2 to Step 4 find if there is a byte with 0 in arch/x86/lib/string_64.c line 58
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+* the long word. +*/ + asmvolatile ("1:\n\t" +"movq (%0),%1\n\t" +"xorq %6,%1\n\t" +"lea (%1,%4), %2\n\t" +"notq %1\n\t" +"andq %5,%1\n\t" +"testq %1,%2\n\t" + "jne 2f\n\t"s/jne/jnz/
Lack much here from me. But I give example that should check the CF flag is zero.
1) contain matching byte.
1111111011111111(consta)
+add
0000000001101100(xor)
------------------------
1111111101101011 (%1)1111111110010100(~xor)
&and
1000000010000000(constb)
------------------------
1000000010000000 (%2)the logical and of %1 and %2 is
1000000000000000 that is not zero.2) not contain matching byte
1111111011111111
+0110111011011100
----------------
0110110111011011(%1)1001000100100011
&1000000010000000
----------------
1000000000000000(%2)
Quoted 8 lines
%1 and %2 is
0000000000000000 that is zero.I guess that here should use jump instruction jnz instead. Even though, I do not know why that two magic number is so magical..
Thanks,
Tao
According to the Intel® 64 and IA-32 Architectures Developer's Manual: Vol. 2A page 3-484 and 3-485, instruction jne and jnz jump according to zero flag. They both jump when ZF = 0. Here I believe that using jnz is easier to understand the assembly code.
consta is the 2's complement of 0x0101010101010101. That is, we perform subtraction here. As a result, it is not necessary to check the CF flags.
Here I explain how these two magic numbers work..
After we perform xor to the long word and the mask, the match byte will
be 0x00. The goal to subtract 0x0101010101010101 is turning the zero byte into 0xFF. The following step, ~xor & constb, transforms the zero byte into 0x80. Then, 0x80 & 0xFF = 0x80.
On the other hand, for a none zero byte, let k (0 <= k <=7) be the position of the lowest 1 in this byte. The lowest k bits are 0. After the addition, the byte ends in a single bit of value 0 and k bits of value 1. To explain clearly,
we perform & (~xor) at first. This byte turns into k bits of 1, which is 2^k - 1.
Then, we perform & constb with the previous result and get 0. Here we just do (~xor) & 0x8080808080808080 at first.
Thanks,
Yu-Jen Chang
Quoted 23 lines
Quoted 23 lines
arch/x86/lib/string_64.c new file +20
-1+ "add $8,%0\n\t" + "sub $8,%3\n\t" + "cmp $7,%3\n\t" + "ja 1b\n\t" + "2:\n\t" + : "=D"(src), "=r"(xor), "=r"(data), "=r"(length) + : "r"(consta), "r"(constb), "r"(mask), "0"(src), + "1"(xor), "2"(data), "3"(length) + : "memory", "cc"); + } + +while (length--) { +if (*src == d) +return (void *)src; + src++; + } +return NULL ; +} +EXPORT_SYMBOL (memchr); +#endif --2.25.1
RE: [PATCH 1/2] x86/lib: Optimize memchr()
In reply to Yu-Jen ChangFrom: Yu-Jen Chang
Quoted 26 lines
Sent: 28 May 2022 09:13
The original assembly version of
memchr()is implemented with the byte-wise comparing technique, which does not fully use 64-bits registers inx86_64CPU. We use word-wide comparing so that 8 characters can be compared at the same time onx86_64CPU. First we align the input and then use word-wise comparing to find the first 64-bit word that contain the target. Secondly, we compare every byte in the word and get the output.We create two files to measure the performance. The first file contains on average 10 characters ahead the target character. The second file contains at least 1000 characters ahead the target character. Our implementation of “memchr()” is slightly better in the first test and nearly 4x faster than the orginal implementation in the second test.
Signed-off-by Yu-Jen Chang <[email protected]>Signed-off-by Ching-Chun (Jim) Huang <[email protected]>--- arch/x86/include/asm/string_64.h | 3 ++ arch/x86/lib/Makefile | 1 + arch/x86/lib/string_64.c | 78 ++++++++++++++++++++++++++++++++ 3 files changed, 82 insertions(+) create mode 100644 arch/x86/lib/string_64.c
...
Quoted 9 lines
arch/x86/lib/string_64.c new file +3
-0@@ -0,0 +1,78 @@ +// SPDX-License-Identifier: GPL-2.0 +#include <linux/string.h> +#include <linux/export.h>
arch/x86/lib/string_64.c line 13
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+#include <linux/align.h> + +/* How many bytes are loaded each iteration of the word copy loop */ +#define LBLOCKSIZE (sizeof(long)) + +#ifdef __HAVE_ARCH_MEMCHR + +void *memchr (const void *cs,int c,size_t length) +{ +const unsigned char *src = (const unsigned char *)cs, d = c;
You don't need the cast.
arch/x86/lib/string_64.c line 21
Quoted 8 lines
arch/x86/lib/string_64.c new file +8
-0+ +while (!IS_ALIGNED ((long )src,sizeof (long ))) { +if (!length--) +return NULL ; +if (*src == d) +return (void *)src; + src++; + }
There is no point aligning the address.
On tests I've done misaligned reads don't even take an extra clock - even if you get the cpu doing two reads/clock. Even if they did the code isn't memory limited.
Quoted 5 lines
arch/x86/lib/string_64.c new file +5
-0+if (length >=LBLOCKSIZE ) { +unsigned long mask = d <<8 | d; +unsigned int i =32 ; +long xor, data; + const long consta = 0xFEFEFEFEFEFEFEFF,
arch/x86/lib/string_64.c line 36
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+ constb =0x8080808080808080 ; + +/* +* Create a 8-bytes mask for word-wise comparing. +* For example, a mask for 'a' is 0x6161616161616161. +*/ + + mask |= mask <<16 ; +for (i =32 ; i <LBLOCKSIZE *8 ; i <<=1 ) + mask |= mask << i;
Given that consta/b only support 64 bit why the loop. Just do mask |= mask << 32.
I'd also put all 3 calculations together - not hide one in the initialiser.
Quoted 18 lines
arch/x86/lib/string_64.c new file +18
-0+/* +* We perform word-wise comparing with following operation: +* 1. Perform xor on the long word @src and @mask +* and put into @xor. +* 2. Add @xor with @consta. +* 3. ~@xor & @constb. +* 4. Perform & with the result of step 2 and 3. +* +* Step 1 creates a byte which is 0 in the long word if +* there is at least one target byte in it. +* +* Step 2 to Step 4 find if there is a byte with 0 in +* the long word. +*/ + asmvolatile ("1:\n\t" +"movq (%0),%1\n\t" +"xorq %6,%1\n\t" + "lea (%1,%4), %2\n\t"
arch/x86/lib/string_64.c line 64
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+ "notq %1\n\t" + "andq %5,%1\n\t" + "testq %1,%2\n\t" + "jne 2f\n\t" + "add $8,%0\n\t" + "sub $8,%3\n\t" + "cmp $7,%3\n\t" + "ja 1b\n\t" + "2:\n\t" + : "=D"(src), "=r"(xor), "=r"(data), "=r"(length)
Why constrain src to %rdi?
arch/x86/lib/string_64.c line 66
Quoted 2 lines
arch/x86/lib/string_64.c new file +2
-0+ : "r"(consta), "r"(constb), "r"(mask), "0"(src), +"1" (xor),"2" (data),"3" (length)
Use "+r" in the outputs instead of respecifying the args. I'd also suggest using named arguments - much easier to read.
arch/x86/lib/string_64.c line 67
Quoted 1 line
arch/x86/lib/string_64.c new file +1
-0+ : "memory", "cc");
Doesn't the compiler generate much the same code?
You should also be able to code without needing add, sub and cmp at the end of the loop.
If you use negative offsets from the end of the buffer the loop can be a single add and jnz.
David
Quoted 12 lines
arch/x86/lib/string_64.c new file +11
-1+ } + +while (length--) { +if (*src == d) +return (void *)src; + src++; + } +return NULL ; +} +EXPORT_SYMBOL (memchr); +#endif --
Quoted 1 line
2.25.1
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
Re: [PATCH 1/2] x86/lib: Optimize memchr()
David Laight <[email protected]> 於 2022年5月30日 週一 下午4:10寫道：
Quoted 115 lines
From: Yu-Jen Chang
Quoted 26 lines
Sent: 28 May 2022 09:13
The original assembly version of
memchr()is implemented with the byte-wise comparing technique, which does not fully use 64-bits registers inx86_64CPU. We use word-wide comparing so that 8 characters can be compared at the same time onx86_64CPU. First we align the input and then use word-wise comparing to find the first 64-bit word that contain the target. Secondly, we compare every byte in the word and get the output.We create two files to measure the performance. The first file contains on average 10 characters ahead the target character. The second file contains at least 1000 characters ahead the target character. Our implementation of “memchr()” is slightly better in the first test and nearly 4x faster than the orginal implementation in the second test.
Signed-off-by Yu-Jen Chang <[email protected]>Signed-off-by Ching-Chun (Jim) Huang <[email protected]>--- arch/x86/include/asm/string_64.h | 3 ++ arch/x86/lib/Makefile | 1 + arch/x86/lib/string_64.c | 78 ++++++++++++++++++++++++++++++++ 3 files changed, 82 insertions(+) create mode 100644 arch/x86/lib/string_64.c...
Quoted 9 lines
arch/x86/lib/string_64.c new file +3
-0@@ -0,0 +1,78 @@ +// SPDX-License-Identifier: GPL-2.0 +#include <linux/string.h> +#include <linux/export.h> arch/x86/lib/string_64.c line 13
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+#include <linux/align.h> + +/* How many bytes are loaded each iteration of the word copy loop */ +#define LBLOCKSIZE (sizeof(long)) + +#ifdef __HAVE_ARCH_MEMCHR + +void *memchr (const void *cs,int c,size_t length) +{ +const unsigned char *src = (const unsigned char *)cs, d = c;You don't need the cast.
arch/x86/lib/string_64.c line 21
Quoted 8 lines
arch/x86/lib/string_64.c new file +8
-0+ +while (!IS_ALIGNED ((long )src,sizeof (long ))) { +if (!length--) +return NULL ; +if (*src == d) +return (void *)src; + src++; + }There is no point aligning the address.
On tests I've done misaligned reads don't even take an extra clock - even if you get the cpu doing two reads/clock. Even if they did the code isn't memory limited.Quoted 5 lines
arch/x86/lib/string_64.c new file +5
-0+if (length >=LBLOCKSIZE ) { +unsigned long mask = d <<8 | d; +unsigned int i =32 ; +long xor, data; + const long consta = 0xFEFEFEFEFEFEFEFF,arch/x86/lib/string_64.c line 36
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+ constb =0x8080808080808080 ; + +/* +* Create a 8-bytes mask for word-wise comparing. +* For example, a mask for 'a' is 0x6161616161616161. +*/ + + mask |= mask <<16 ; +for (i =32 ; i <LBLOCKSIZE *8 ; i <<=1 ) + mask |= mask << i;Given that consta/b only support 64 bit why the loop. Just do mask |= mask << 32.
I'd also put all 3 calculations together - not hide one in the initialiser.Quoted 18 lines
arch/x86/lib/string_64.c new file +18
-0+/* +* We perform word-wise comparing with following operation: +* 1. Perform xor on the long word @src and @mask +* and put into @xor. +* 2. Add @xor with @consta. +* 3. ~@xor & @constb. +* 4. Perform & with the result of step 2 and 3. +* +* Step 1 creates a byte which is 0 in the long word if +* there is at least one target byte in it. +* +* Step 2 to Step 4 find if there is a byte with 0 in +* the long word. +*/ + asmvolatile ("1:\n\t" +"movq (%0),%1\n\t" +"xorq %6,%1\n\t" + "lea (%1,%4), %2\n\t"arch/x86/lib/string_64.c line 64
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+ "notq %1\n\t" + "andq %5,%1\n\t" + "testq %1,%2\n\t" + "jne 2f\n\t" + "add $8,%0\n\t" + "sub $8,%3\n\t" + "cmp $7,%3\n\t" + "ja 1b\n\t" + "2:\n\t" + : "=D"(src), "=r"(xor), "=r"(data), "=r"(length)
Quoted 1 line
Why constrain src to %rdi?
At first I try to use some instructions related to %rdi, but I realize that I won't use these instructions. It is unnecessary to constrain src to %rdi.
Quoted 26 lines
arch/x86/lib/string_64.c line 66
Quoted 3 lines
arch/x86/lib/string_64.c new file +2
-0+ : "r"(consta), "r"(constb), "r"(mask), "0"(src), +"1" (xor),"2" (data),"3" (length)Use "+r" in the outputs instead of respecifying the args. I'd also suggest using named arguments - much easier to read.
arch/x86/lib/string_64.c line 67
Quoted 1 line
arch/x86/lib/string_64.c new file +1
-0+ : "memory", "cc");Doesn't the compiler generate much the same code?
You should also be able to code without needing add, sub and cmp at the end of the loop.
If you use negative offsets from the end of the buffer the loop can be a single add and jnz.David
Quoted 9 lines
arch/x86/lib/string_64.c new file +9
-0+ } + +while (length--) { +if (*src == d) +return (void *)src; + src++; + } +return NULL ; +}
Quoted 8 lines
Quoted 4 lines
arch/x86/lib/string_64.c new file +2
-1+EXPORT_SYMBOL (memchr); +#endif --2.25.1
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
I remove the aligning address part. On my tests the performance are similar. Here I rewrite the assembly using named arguments and I reduce one instruction in the loop by adding two parameters, which are 'end' and 'dst'. 'end' stores the
address of the end of the string. 'dst' stores the address of the end of word-wise
comparison. As a result, when 'src' is equal to 'dst', the number of remaining characters is less than 8. The following while loop will find if the target character is
in these remaining characters.
On my test the performance is similar with the my original implementation. Only a little bit fast when going through a very long string, which contains 128*1024 characters and the target character is near the end of the string.
I also explain how consta and constb work clearly in the comments. Hope that it helps understanding.
The following code is what I change.
void *memchr(const void *cs, int c, size_t length)
{
const unsigned char *src = (const unsigned char *)cs;
const unsigned char *end = src + length;
if (length >= LBLOCKSIZE) {
unsigned long mask = c << 8 | c;
long xor, data;
const long consta = 0xFEFEFEFEFEFEFEFF,
constb = 0x8080808080808080;
const unsigned char *dst = (const unsigned char *)src +
(length & 0xFFFFFFFFFFFFFFF8);
/*
* Create a 8-bytes mask for word-wise comparing.
* For example, a mask for 'a' is 0x6161616161616161.
*/
mask |= mask << 16;
mask |= mask << 32;
/*
* We perform word-wise comparing with following operation:
* 1. Perform xor on the long word @src and @mask
* and put into @xor.
* 2. Add @xor with @consta.
* 3. ~@xor & @constb.
* 4. Perform & with the result of step 2 and 3.
*
* If there is a zero byte in @xor, step 2 turns it into
* 0xFF. Then step 3 and 4 turn it into 0x80.
*
* If there is a none-zero byte in @xor, let k
* (0 <= k <= 7) be the lowest 1 in this byte. The lowest
* k bits are 0. After step 2, the byte ends in a single
* bit of value 0. Step 3 and 4 turns this byte into k
* bits of 1, which is 2^k - 1, at first. Then & @constb
* makes it into 0.
*
* Step 2 to Step 4 find if there is a byte with 0 in
* the long word.
*/
asm volatile("1:\n\t"
"movq (%[src]),%[xor]\n\t"
"xorq %[mask],%[xor]\n\t"
"lea (%[xor],%[const_a]), %[tmp]\n\t"
"notq %[xor]\n\t"
"andq %[const_b],%[xor]\n\t"
"testq %[xor],%[tmp]\n\t"
"jnz 2f\n\t"
"add $8,%[src]\n\t"
"cmp %[src], %[dst]\n\t"
"ja 1b\n\t"
"2:\n\t"
:
[src] "+r"(src), [xor] "+r"(xor), [tmp] "+r"(data)
: [const_a] "r"(consta), [const_b] "r"(constb),
[mask] "r"(mask), [dst] "r"(dst)
: "memory", "cc");
}
while (src <= end) {
if (*src == d)
return (void *)src;
src++;
}
return NULL;
}Thanks,
Yu-Jen Chang
_______________________________________________
linux-um mailing list
[email protected]
http://lists.infradead.org/mailman/listinfo/linux-um
RE: [PATCH 1/2] x86/lib: Optimize memchr()
From: Yu-Jen Chang
Quoted 179 lines
Sent: 01 June 2022 06:59
David Laight <[email protected]> 於 2022年5月30日 週一 下午4:10寫道：
Quoted 115 lines
From: Yu-Jen Chang
Quoted 26 lines
Sent: 28 May 2022 09:13
The original assembly version of
memchr()is implemented with the byte-wise comparing technique, which does not fully use 64-bits registers inx86_64CPU. We use word-wide comparing so that 8 characters can be compared at the same time onx86_64CPU. First we align the input and then use word-wise comparing to find the first 64-bit word that contain the target. Secondly, we compare every byte in the word and get the output.We create two files to measure the performance. The first file contains on average 10 characters ahead the target character. The second file contains at least 1000 characters ahead the target character. Our implementation of “memchr()” is slightly better in the first test and nearly 4x faster than the orginal implementation in the second test.
Signed-off-by Yu-Jen Chang <[email protected]>Signed-off-by Ching-Chun (Jim) Huang <[email protected]>--- arch/x86/include/asm/string_64.h | 3 ++ arch/x86/lib/Makefile | 1 + arch/x86/lib/string_64.c | 78 ++++++++++++++++++++++++++++++++ 3 files changed, 82 insertions(+) create mode 100644 arch/x86/lib/string_64.c...
Quoted 9 lines
arch/x86/lib/string_64.c new file +3
-0@@ -0,0 +1,78 @@ +// SPDX-License-Identifier: GPL-2.0 +#include <linux/string.h> +#include <linux/export.h> arch/x86/lib/string_64.c line 13
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+#include <linux/align.h> + +/* How many bytes are loaded each iteration of the word copy loop */ +#define LBLOCKSIZE (sizeof(long)) + +#ifdef __HAVE_ARCH_MEMCHR + +void *memchr (const void *cs,int c,size_t length) +{ +const unsigned char *src = (const unsigned char *)cs, d = c;You don't need the cast.
arch/x86/lib/string_64.c line 21
Quoted 8 lines
arch/x86/lib/string_64.c new file +8
-0+ +while (!IS_ALIGNED ((long )src,sizeof (long ))) { +if (!length--) +return NULL ; +if (*src == d) +return (void *)src; + src++; + }There is no point aligning the address.
On tests I've done misaligned reads don't even take an extra clock - even if you get the cpu doing two reads/clock. Even if they did the code isn't memory limited.Quoted 5 lines
arch/x86/lib/string_64.c new file +5
-0+if (length >=LBLOCKSIZE ) { +unsigned long mask = d <<8 | d; +unsigned int i =32 ; +long xor, data; + const long consta = 0xFEFEFEFEFEFEFEFF,arch/x86/lib/string_64.c line 36
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+ constb =0x8080808080808080 ; + +/* +* Create a 8-bytes mask for word-wise comparing. +* For example, a mask for 'a' is 0x6161616161616161. +*/ + + mask |= mask <<16 ; +for (i =32 ; i <LBLOCKSIZE *8 ; i <<=1 ) + mask |= mask << i;Given that consta/b only support 64 bit why the loop. Just do mask |= mask << 32.
I'd also put all 3 calculations together - not hide one in the initialiser.Quoted 18 lines
arch/x86/lib/string_64.c new file +18
-0+/* +* We perform word-wise comparing with following operation: +* 1. Perform xor on the long word @src and @mask +* and put into @xor. +* 2. Add @xor with @consta. +* 3. ~@xor & @constb. +* 4. Perform & with the result of step 2 and 3. +* +* Step 1 creates a byte which is 0 in the long word if +* there is at least one target byte in it. +* +* Step 2 to Step 4 find if there is a byte with 0 in +* the long word. +*/ + asmvolatile ("1:\n\t" +"movq (%0),%1\n\t" +"xorq %6,%1\n\t" + "lea (%1,%4), %2\n\t"arch/x86/lib/string_64.c line 64
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+ "notq %1\n\t" + "andq %5,%1\n\t" + "testq %1,%2\n\t" + "jne 2f\n\t" + "add $8,%0\n\t" + "sub $8,%3\n\t" + "cmp $7,%3\n\t" + "ja 1b\n\t" + "2:\n\t" + : "=D"(src), "=r"(xor), "=r"(data), "=r"(length)Quoted 1 line
Why constrain src to %rdi?
At first I try to use some instructions related to %rdi, but I realize that I won't use these instructions. It is unnecessary to constrain src to %rdi.
Quoted 26 lines
arch/x86/lib/string_64.c line 66
Quoted 3 lines
arch/x86/lib/string_64.c new file +2
-0+ : "r"(consta), "r"(constb), "r"(mask), "0"(src), +"1" (xor),"2" (data),"3" (length)Use "+r" in the outputs instead of respecifying the args. I'd also suggest using named arguments - much easier to read.
arch/x86/lib/string_64.c line 67
Quoted 1 line
arch/x86/lib/string_64.c new file +1
-0+ : "memory", "cc");Doesn't the compiler generate much the same code?
You should also be able to code without needing add, sub and cmp at the end of the loop.
If you use negative offsets from the end of the buffer the loop can be a single add and jnz.David
Quoted 9 lines
arch/x86/lib/string_64.c new file +9
-0+ } + +while (length--) { +if (*src == d) +return (void *)src; + src++; + } +return NULL ; +}Quoted 8 lines
Quoted 4 lines
arch/x86/lib/string_64.c new file +2
-1+EXPORT_SYMBOL (memchr); +#endif --2.25.1
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)I remove the aligning address part. On my tests the performance are similar. Here I rewrite the assembly using named arguments and I reduce one instruction in the loop by adding two parameters, which are 'end' and 'dst'. 'end' stores the
address of the end of the string. 'dst' stores the address of the end of word-wise
comparison. As a result, when 'src' is equal to 'dst', the number of remaining characters is less than 8. The following while loop will find if the target character is
in these remaining characters.On my test the performance is similar with the my original implementation. Only a little bit fast when going through a very long string, which contains 128*1024 characters and the target character is near the end of the string.
I also explain how consta and constb work clearly in the comments. Hope that it helps understanding.
The following code is what I change.
Quoted 7 lines
void *memchr(const void *cs, int c,
size_tlength){ const unsigned char *src = (const unsigned char *)cs; const unsigned char *end = src + length; if (length >= LBLOCKSIZE) { unsigned long mask = c << 8 | c;
That is wrong if 'c' is outside 0..255.
I suspect it is best to at least allow -128..-1.
Quoted 47 lines
long xor, data;
const long consta = 0xFEFEFEFEFEFEFEFF,
constb = 0x8080808080808080;
const unsigned char *dst = (const unsigned char *)src +
(length & 0xFFFFFFFFFFFFFFF8);
/*
* Create a 8-bytes mask for word-wise comparing.
* For example, a mask for 'a' is 0x6161616161616161.
*/
mask |= mask << 16;
mask |= mask << 32;
/*
* We perform word-wise comparing with following operation:
* 1. Perform xor on the long word @src and @mask
* and put into @xor.
* 2. Add @xor with @consta.
* 3. ~@xor & @constb.
* 4. Perform & with the result of step 2 and 3.
*
* If there is a zero byte in @xor, step 2 turns it into
* 0xFF. Then step 3 and 4 turn it into 0x80.
*
* If there is a none-zero byte in @xor, let k
* (0 <= k <= 7) be the lowest 1 in this byte. The lowest
* k bits are 0. After step 2, the byte ends in a single
* bit of value 0. Step 3 and 4 turns this byte into k
* bits of 1, which is 2^k - 1, at first. Then & @constb
* makes it into 0.
*
* Step 2 to Step 4 find if there is a byte with 0 in
* the long word.
*/
asm volatile("1:\n\t"
"movq (%[src]),%[xor]\n\t"
"xorq %[mask],%[xor]\n\t"
"lea (%[xor],%[const_a]), %[tmp]\n\t"
"notq %[xor]\n\t"
"andq %[const_b],%[xor]\n\t"
"testq %[xor],%[tmp]\n\t"
"jnz 2f\n\t"
"add $8,%[src]\n\t"
"cmp %[src], %[dst]\n\t"
"ja 1b\n\t"
"2:\n\t"
:Quoted 8 lines
[src] "+r"(src), [xor] "+r"(xor), [tmp] "+r"(data)
: [const_a] "r"(consta), [const_b] "r"(constb),
[mask] "r"(mask), [dst] "r"(dst)
: "memory", "cc");
}
while (src <= end) {
if (*src == d)I think you mean 'c'.
Quoted 8 lines
return (void *)src; src++; } return NULL; }Thanks,
Yu-Jen Chang
Gcc compiles this C to the same loop and is easier to read. Valid on all LE 64bit systems.
void *memchr(const void *p, int c, unsigned long length)
{
unsigned long mask, val;
const void *end = p + length;
c &= 0xff;
if (p <= end - 8) {
mask = c | c << 8;
mask |= mask << 16;
mask |= mask << 32;
for (; p <= end - 8; p += 8) {
val = *(unsigned long *)p ^ mask;
if ((val + 0xfefefefefefefeffu) & (~val & 0x8080808080808080u))
break;
}
}
for (; p < end; p++)
if (*(unsigned char *)p == c)
return p;
return NULL;
}See https://godbolt.org/z/6rqTqfEsx
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
Re: [PATCH 1/2] x86/lib: Optimize memchr()
David Laight <[email protected]> 於 2022年6月1日 週三 下午4:25寫道：
Quoted 275 lines
From: Yu-Jen Chang
Quoted 179 lines
Sent: 01 June 2022 06:59
David Laight <[email protected]> 於 2022年5月30日 週一 下午4:10寫道：
Quoted 115 lines
From: Yu-Jen Chang
Quoted 26 lines
Sent: 28 May 2022 09:13
The original assembly version of
memchr()is implemented with the byte-wise comparing technique, which does not fully use 64-bits registers inx86_64CPU. We use word-wide comparing so that 8 characters can be compared at the same time onx86_64CPU. First we align the input and then use word-wise comparing to find the first 64-bit word that contain the target. Secondly, we compare every byte in the word and get the output.We create two files to measure the performance. The first file contains on average 10 characters ahead the target character. The second file contains at least 1000 characters ahead the target character. Our implementation of “memchr()” is slightly better in the first test and nearly 4x faster than the orginal implementation in the second test.
Signed-off-by Yu-Jen Chang <[email protected]>Signed-off-by Ching-Chun (Jim) Huang <[email protected]>--- arch/x86/include/asm/string_64.h | 3 ++ arch/x86/lib/Makefile | 1 + arch/x86/lib/string_64.c | 78 ++++++++++++++++++++++++++++++++ 3 files changed, 82 insertions(+) create mode 100644 arch/x86/lib/string_64.c...
Quoted 9 lines
arch/x86/lib/string_64.c new file +3
-0@@ -0,0 +1,78 @@ +// SPDX-License-Identifier: GPL-2.0 +#include <linux/string.h> +#include <linux/export.h> arch/x86/lib/string_64.c line 13
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+#include <linux/align.h> + +/* How many bytes are loaded each iteration of the word copy loop */ +#define LBLOCKSIZE (sizeof(long)) + +#ifdef __HAVE_ARCH_MEMCHR + +void *memchr (const void *cs,int c,size_t length) +{ +const unsigned char *src = (const unsigned char *)cs, d = c;You don't need the cast.
arch/x86/lib/string_64.c line 21
Quoted 8 lines
arch/x86/lib/string_64.c new file +8
-0+ +while (!IS_ALIGNED ((long )src,sizeof (long ))) { +if (!length--) +return NULL ; +if (*src == d) +return (void *)src; + src++; + }There is no point aligning the address.
On tests I've done misaligned reads don't even take an extra clock - even if you get the cpu doing two reads/clock. Even if they did the code isn't memory limited.Quoted 5 lines
arch/x86/lib/string_64.c new file +5
-0+if (length >=LBLOCKSIZE ) { +unsigned long mask = d <<8 | d; +unsigned int i =32 ; +long xor, data; + const long consta = 0xFEFEFEFEFEFEFEFF,arch/x86/lib/string_64.c line 36
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+ constb =0x8080808080808080 ; + +/* +* Create a 8-bytes mask for word-wise comparing. +* For example, a mask for 'a' is 0x6161616161616161. +*/ + + mask |= mask <<16 ; +for (i =32 ; i <LBLOCKSIZE *8 ; i <<=1 ) + mask |= mask << i;Given that consta/b only support 64 bit why the loop. Just do mask |= mask << 32.
I'd also put all 3 calculations together - not hide one in the initialiser.Quoted 18 lines
arch/x86/lib/string_64.c new file +18
-0+/* +* We perform word-wise comparing with following operation: +* 1. Perform xor on the long word @src and @mask +* and put into @xor. +* 2. Add @xor with @consta. +* 3. ~@xor & @constb. +* 4. Perform & with the result of step 2 and 3. +* +* Step 1 creates a byte which is 0 in the long word if +* there is at least one target byte in it. +* +* Step 2 to Step 4 find if there is a byte with 0 in +* the long word. +*/ + asmvolatile ("1:\n\t" +"movq (%0),%1\n\t" +"xorq %6,%1\n\t" + "lea (%1,%4), %2\n\t"arch/x86/lib/string_64.c line 64
Quoted 10 lines
arch/x86/lib/string_64.c new file +10
-0+ "notq %1\n\t" + "andq %5,%1\n\t" + "testq %1,%2\n\t" + "jne 2f\n\t" + "add $8,%0\n\t" + "sub $8,%3\n\t" + "cmp $7,%3\n\t" + "ja 1b\n\t" + "2:\n\t" + : "=D"(src), "=r"(xor), "=r"(data), "=r"(length)Quoted 1 line
Why constrain src to %rdi?
At first I try to use some instructions related to %rdi, but I realize that I won't use these instructions. It is unnecessary to constrain src to %rdi.
Quoted 26 lines
arch/x86/lib/string_64.c line 66
Quoted 3 lines
arch/x86/lib/string_64.c new file +2
-0+ : "r"(consta), "r"(constb), "r"(mask), "0"(src), +"1" (xor),"2" (data),"3" (length)Use "+r" in the outputs instead of respecifying the args. I'd also suggest using named arguments - much easier to read.
arch/x86/lib/string_64.c line 67
Quoted 1 line
arch/x86/lib/string_64.c new file +1
-0+ : "memory", "cc");Doesn't the compiler generate much the same code?
You should also be able to code without needing add, sub and cmp at the end of the loop.
If you use negative offsets from the end of the buffer the loop can be a single add and jnz.David
Quoted 9 lines
arch/x86/lib/string_64.c new file +9
-0+ } + +while (length--) { +if (*src == d) +return (void *)src; + src++; + } +return NULL ; +}Quoted 8 lines
Quoted 4 lines
arch/x86/lib/string_64.c new file +2
-1+EXPORT_SYMBOL (memchr); +#endif --2.25.1
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)I remove the aligning address part. On my tests the performance are similar. Here I rewrite the assembly using named arguments and I reduce one instruction in the loop by adding two parameters, which are 'end' and 'dst'. 'end' stores the
address of the end of the string. 'dst' stores the address of the end of word-wise
comparison. As a result, when 'src' is equal to 'dst', the number of remaining characters is less than 8. The following while loop will find if the target character is
in these remaining characters.On my test the performance is similar with the my original implementation. Only a little bit fast when going through a very long string, which contains 128*1024 characters and the target character is near the end of the string.
I also explain how consta and constb work clearly in the comments. Hope that it helps understanding.
The following code is what I change.
Quoted 7 lines
void *memchr(const void *cs, int c,
size_tlength){ const unsigned char *src = (const unsigned char *)cs; const unsigned char *end = src + length; if (length >= LBLOCKSIZE) { unsigned long mask = c << 8 | c;That is wrong if 'c' is outside 0..255.
I suspect it is best to at least allow -128..-1.Quoted 47 lines
long xor, data; const long consta = 0xFEFEFEFEFEFEFEFF, constb = 0x8080808080808080; const unsigned char *dst = (const unsigned char *)src + (length & 0xFFFFFFFFFFFFFFF8); /* * Create a 8-bytes mask for word-wise comparing. * For example, a mask for 'a' is 0x6161616161616161. */ mask |= mask << 16; mask |= mask << 32; /* * We perform word-wise comparing with following operation: * 1. Perform xor on the long word @src and @mask * and put into @xor. * 2. Add @xor with @consta. * 3. ~@xor & @constb. * 4. Perform & with the result of step 2 and 3. * * If there is a zero byte in @xor, step 2 turns it into * 0xFF. Then step 3 and 4 turn it into 0x80. * * If there is a none-zero byte in @xor, let k * (0 <= k <= 7) be the lowest 1 in this byte. The lowest * k bits are 0. After step 2, the byte ends in a single * bit of value 0. Step 3 and 4 turns this byte into k * bits of 1, which is 2^k - 1, at first. Then & @constb * makes it into 0. * * Step 2 to Step 4 find if there is a byte with 0 in * the long word. */ asm volatile("1:\n\t" "movq (%[src]),%[xor]\n\t" "xorq %[mask],%[xor]\n\t" "lea (%[xor],%[const_a]), %[tmp]\n\t" "notq %[xor]\n\t" "andq %[const_b],%[xor]\n\t" "testq %[xor],%[tmp]\n\t" "jnz 2f\n\t" "add $8,%[src]\n\t" "cmp %[src], %[dst]\n\t" "ja 1b\n\t" "2:\n\t" :Quoted 8 lines
[src] "+r"(src), [xor] "+r"(xor), [tmp] "+r"(data) : [const_a] "r"(consta), [const_b] "r"(constb), [mask] "r"(mask), [dst] "r"(dst) : "memory", "cc"); } while (src <= end) { if (*src == d)I think you mean 'c'.
Quoted 8 lines
return (void *)src; src++; } return NULL; }Thanks,
Yu-Jen ChangGcc compiles this C to the same loop and is easier to read. Valid on all LE 64bit systems.
void *memchr(const void *p, int c, unsigned long length)
{ unsigned long mask, val; const void *end = p + length; c &= 0xff; if (p <= end - 8) { mask = c | c << 8; mask |= mask << 16; mask |= mask << 32; for (; p <= end - 8; p += 8) { val = *(unsigned long *)p ^ mask;
Quoted 8 lines
if ((val + 0xfefefefefefefeffu) & (~val & 0x8080808080808080u))
break;
}
}
for (; p < end; p++)
if (*(unsigned char *)p == c)
return p;Here I think we should return (void*) p. So that there is no compilation warning.
Quoted 11 lines
return NULL; }See https://godbolt.org/z/6rqTqfEsx
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
Your modification is easier to understand. I add the comments to explain how it work.
I will use it in the second version of the patch. Thanks for your advice.
Yu-Jen Chang
void *memchr(const void *p, int c, unsigned long length)
{
unsigned long mask, val;
const void *end = p + length;
c &= 0xff;
if (p <= end - 8) {
/*
* Create a 8-bytes mask for word-wise comparing.
* For example, a mask for 'a' is 0x6161616161616161.
*/
mask = c | c << 8;
mask |= mask << 16;
mask |= mask << 32;
/*
* We perform word-wise comparing with following operation:
* 1. Perform xor on the long word @p and @mask
* and put into @val.
* 2. Add @val with 0xfefefefefefefeff.
* 3. ~@val & 0x8080808080808080
* 4. Perform & with the result of step 2 and 3.
*
* If there is a zero byte in @val, step 2 turns it into
* 0xFF. Then step 3 and 4 turn it into 0x80.
*
* If there is a none-zero byte in @val, let k
* (0 <= k <= 7) be the lowest 1 in this byte. The lowest
* k bits are 0. After step 2, the byte ends in a single
* bit of value 0. Step 3 and 4 turns this byte into k
* bits of 1, which is 2^k - 1, at first. Then &
* 0x8080808080808080 makes it into 0
*/
for (; p <= end - 8; p += 8) {
val = *(unsigned long *)p ^ mask;
if ((val + 0xfefefefefefefeffu) & (~val & 0x8080808080808080u))
break;
}
}
for (; p < end; p++)
if (*(unsigned char *)p == c)
return (void *)p;
return NULL;
}PATCH 2/2 x86/um: Use x86_64-optimized memchr
In reply to Yu-Jen ChangAdd x86_64-optimized memchr, which is 4x faster
than the original implementation, into um.
--- arch/x86/um/Makefile | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
arch/x86/um/Makefile +1 -1
@@ -28,7 +28,7 @@ else obj-y += syscalls_64.o vdso/ -subarch-y = ../lib/csum-partial_64.o ../lib/memcpy_64.o ../entry/thunk_64.o +subarch-y = ../lib/csum-partial_64.o ../lib/memcpy_64.o ../lib/string_64.o ../entry/thunk_64.o endif
Signature · 7 lines
2.25.1 _______________________________________________ linux-um mailing list [email protected] http://lists.infradead.org/mailman/listinfo/linux-um
Quoted 10 lines · Yu-Jen Chang
*** BLURB HERE ***
These patch series add an optimized "memchr()" for x86-64 and USER-MODE LINUX (UML).There exists an assemebly implementation for x86-32. However, for x86-64, there isn't any optimized version. We implement word-wise comparison so that 8 characters can be compared at the same time on x86-64 CPU. The optimized “memchr()” is nearly 4x faster than the orginal implementation for long strings.
Quoted 6 lines
We test the optimized “memchr()” in UML and also recompile the 5.18 Kernel with the optimized “memchr()”. They run correctly.
In this patch we add a new file "
string_64.c", which only contains "memchr()". We can add more optimized string functions in it in the future.
Are there any workloads that care? From a quick grep I don't see any that look performance critical.
It would be good to describe what you optimized it for. For example optimization for small input strings is quite different than large strings. I don't know what is more common in the kernel.
I assume you ran it through some existing test suites for memchr (like glibc etc.) for correctness testing?
(bugs in optimized string functions are often subtle, it might be also worth trying some randomized testing comparing against a known reference)
-Andi