linux/lib/hweight.c

69 lines
1.9 KiB
C
Raw Normal View History

License cleanup: add SPDX GPL-2.0 license identifier to files with no license Many source files in the tree are missing licensing information, which makes it harder for compliance tools to determine the correct license. By default all files without license information are under the default license of the kernel, which is GPL version 2. Update the files which contain no license information with the 'GPL-2.0' SPDX license identifier. The SPDX identifier is a legally binding shorthand, which can be used instead of the full boiler plate text. This patch is based on work done by Thomas Gleixner and Kate Stewart and Philippe Ombredanne. How this work was done: Patches were generated and checked against linux-4.14-rc6 for a subset of the use cases: - file had no licensing information it it. - file was a */uapi/* one with no licensing information in it, - file was a */uapi/* one with existing licensing information, Further patches will be generated in subsequent months to fix up cases where non-standard license headers were used, and references to license had to be inferred by heuristics based on keywords. The analysis to determine which SPDX License Identifier to be applied to a file was done in a spreadsheet of side by side results from of the output of two independent scanners (ScanCode & Windriver) producing SPDX tag:value files created by Philippe Ombredanne. Philippe prepared the base worksheet, and did an initial spot review of a few 1000 files. The 4.13 kernel was the starting point of the analysis with 60,537 files assessed. Kate Stewart did a file by file comparison of the scanner results in the spreadsheet to determine which SPDX license identifier(s) to be applied to the file. She confirmed any determination that was not immediately clear with lawyers working with the Linux Foundation. Criteria used to select files for SPDX license identifier tagging was: - Files considered eligible had to be source code files. - Make and config files were included as candidates if they contained >5 lines of source - File already had some variant of a license header in it (even if <5 lines). All documentation files were explicitly excluded. The following heuristics were used to determine which SPDX license identifiers to apply. - when both scanners couldn't find any license traces, file was considered to have no license information in it, and the top level COPYING file license applied. For non */uapi/* files that summary was: SPDX license identifier # files ---------------------------------------------------|------- GPL-2.0 11139 and resulted in the first patch in this series. If that file was a */uapi/* path one, it was "GPL-2.0 WITH Linux-syscall-note" otherwise it was "GPL-2.0". Results of that was: SPDX license identifier # files ---------------------------------------------------|------- GPL-2.0 WITH Linux-syscall-note 930 and resulted in the second patch in this series. - if a file had some form of licensing information in it, and was one of the */uapi/* ones, it was denoted with the Linux-syscall-note if any GPL family license was found in the file or had no licensing in it (per prior point). Results summary: SPDX license identifier # files ---------------------------------------------------|------ GPL-2.0 WITH Linux-syscall-note 270 GPL-2.0+ WITH Linux-syscall-note 169 ((GPL-2.0 WITH Linux-syscall-note) OR BSD-2-Clause) 21 ((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause) 17 LGPL-2.1+ WITH Linux-syscall-note 15 GPL-1.0+ WITH Linux-syscall-note 14 ((GPL-2.0+ WITH Linux-syscall-note) OR BSD-3-Clause) 5 LGPL-2.0+ WITH Linux-syscall-note 4 LGPL-2.1 WITH Linux-syscall-note 3 ((GPL-2.0 WITH Linux-syscall-note) OR MIT) 3 ((GPL-2.0 WITH Linux-syscall-note) AND MIT) 1 and that resulted in the third patch in this series. - when the two scanners agreed on the detected license(s), that became the concluded license(s). - when there was disagreement between the two scanners (one detected a license but the other didn't, or they both detected different licenses) a manual inspection of the file occurred. - In most cases a manual inspection of the information in the file resulted in a clear resolution of the license that should apply (and which scanner probably needed to revisit its heuristics). - When it was not immediately clear, the license identifier was confirmed with lawyers working with the Linux Foundation. - If there was any question as to the appropriate license identifier, the file was flagged for further research and to be revisited later in time. In total, over 70 hours of logged manual review was done on the spreadsheet to determine the SPDX license identifiers to apply to the source files by Kate, Philippe, Thomas and, in some cases, confirmation by lawyers working with the Linux Foundation. Kate also obtained a third independent scan of the 4.13 code base from FOSSology, and compared selected files where the other two scanners disagreed against that SPDX file, to see if there was new insights. The Windriver scanner is based on an older version of FOSSology in part, so they are related. Thomas did random spot checks in about 500 files from the spreadsheets for the uapi headers and agreed with SPDX license identifier in the files he inspected. For the non-uapi files Thomas did random spot checks in about 15000 files. In initial set of patches against 4.14-rc6, 3 files were found to have copy/paste license identifier errors, and have been fixed to reflect the correct identifier. Additionally Philippe spent 10 hours this week doing a detailed manual inspection and review of the 12,461 patched files from the initial patch version early this week with: - a full scancode scan run, collecting the matched texts, detected license ids and scores - reviewing anything where there was a license detected (about 500+ files) to ensure that the applied SPDX license was correct - reviewing anything where there was no detection but the patch license was not GPL-2.0 WITH Linux-syscall-note to ensure that the applied SPDX license was correct This produced a worksheet with 20 files needing minor correction. This worksheet was then exported into 3 different .csv files for the different types of files to be modified. These .csv files were then reviewed by Greg. Thomas wrote a script to parse the csv files and add the proper SPDX tag to the file, in the format that the file expected. This script was further refined by Greg based on the output to detect more types of files automatically and to distinguish between header and source .c files (which need different comment types.) Finally Greg ran the script using the .csv files to generate the patches. Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org> Reviewed-by: Philippe Ombredanne <pombredanne@nexb.com> Reviewed-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2017-11-01 14:07:57 +00:00
// SPDX-License-Identifier: GPL-2.0
#include <linux/export.h>
#include <linux/bitops.h>
#include <asm/types.h>
/**
* hweightN - returns the hamming weight of a N-bit word
* @x: the word to weigh
*
* The Hamming Weight of a number is the total number of bits set in it.
*/
unsigned int __sw_hweight32(unsigned int w)
{
#ifdef CONFIG_ARCH_HAS_FAST_MULTIPLIER
w -= (w >> 1) & 0x55555555;
w = (w & 0x33333333) + ((w >> 2) & 0x33333333);
w = (w + (w >> 4)) & 0x0f0f0f0f;
return (w * 0x01010101) >> 24;
#else
[PATCH] bitops: hweight() speedup <linux@horizon.com> wrote: This is an extremely well-known technique. You can see a similar version that uses a multiply for the last few steps at http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel whch refers to "Software Optimization Guide for AMD Athlon 64 and Opteron Processors" http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/25112.PDF It's section 8.6, "Efficient Implementation of Population-Count Function in 32-bit Mode", pages 179-180. It uses the name that I am more familiar with, "popcount" (population count), although "Hamming weight" also makes sense. Anyway, the proof of correctness proceeds as follows: b = a - ((a >> 1) & 0x55555555); c = (b & 0x33333333) + ((b >> 2) & 0x33333333); d = (c + (c >> 4)) & 0x0f0f0f0f; #if SLOW_MULTIPLY e = d + (d >> 8) f = e + (e >> 16); return f & 63; #else /* Useful if multiply takes at most 4 cycles */ return (d * 0x01010101) >> 24; #endif The input value a can be thought of as 32 1-bit fields each holding their own hamming weight. Now look at it as 16 2-bit fields. Each 2-bit field a1..a0 has the value 2*a1 + a0. This can be converted into the hamming weight of the 2-bit field a1+a0 by subtracting a1. That's what the (a >> 1) & mask subtraction does. Since there can be no borrows, you can just do it all at once. Enumerating the 4 possible cases: 0b00 = 0 -> 0 - 0 = 0 0b01 = 1 -> 1 - 0 = 1 0b10 = 2 -> 2 - 1 = 1 0b11 = 3 -> 3 - 1 = 2 The next step consists of breaking up b (made of 16 2-bir fields) into even and odd halves and adding them into 4-bit fields. Since the largest possible sum is 2+2 = 4, which will not fit into a 4-bit field, the 2-bit ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ "which will not fit into a 2-bit field" fields have to be masked before they are added. After this point, the masking can be delayed. Each 4-bit field holds a population count from 0..4, taking at most 3 bits. These numbers can be added without overflowing a 4-bit field, so we can compute c + (c >> 4), and only then mask off the unwanted bits. This produces d, a number of 4 8-bit fields, each in the range 0..8. From this point, we can shift and add d multiple times without overflowing an 8-bit field, and only do a final mask at the end. The number to mask with has to be at least 63 (so that 32 on't be truncated), but can also be 128 or 255. The x86 has a special encoding for signed immediate byte values -128..127, so the value of 255 is slower. On other processors, a special "sign extend byte" instruction might be faster. On a processor with fast integer multiplies (Athlon but not P4), you can reduce the final few serially dependent instructions to a single integer multiply. Consider d to be 3 8-bit values d3, d2, d1 and d0, each in the range 0..8. The multiply forms the partial products: d3 d2 d1 d0 d3 d2 d1 d0 d3 d2 d1 d0 + d3 d2 d1 d0 ---------------------- e3 e2 e1 e0 Where e3 = d3 + d2 + d1 + d0. e2, e1 and e0 obviously cannot generate any carries. Signed-off-by: Akinobu Mita <mita@miraclelinux.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-03-26 09:40:00 +00:00
unsigned int res = w - ((w >> 1) & 0x55555555);
res = (res & 0x33333333) + ((res >> 2) & 0x33333333);
[PATCH] bitops: hweight() speedup <linux@horizon.com> wrote: This is an extremely well-known technique. You can see a similar version that uses a multiply for the last few steps at http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel whch refers to "Software Optimization Guide for AMD Athlon 64 and Opteron Processors" http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/25112.PDF It's section 8.6, "Efficient Implementation of Population-Count Function in 32-bit Mode", pages 179-180. It uses the name that I am more familiar with, "popcount" (population count), although "Hamming weight" also makes sense. Anyway, the proof of correctness proceeds as follows: b = a - ((a >> 1) & 0x55555555); c = (b & 0x33333333) + ((b >> 2) & 0x33333333); d = (c + (c >> 4)) & 0x0f0f0f0f; #if SLOW_MULTIPLY e = d + (d >> 8) f = e + (e >> 16); return f & 63; #else /* Useful if multiply takes at most 4 cycles */ return (d * 0x01010101) >> 24; #endif The input value a can be thought of as 32 1-bit fields each holding their own hamming weight. Now look at it as 16 2-bit fields. Each 2-bit field a1..a0 has the value 2*a1 + a0. This can be converted into the hamming weight of the 2-bit field a1+a0 by subtracting a1. That's what the (a >> 1) & mask subtraction does. Since there can be no borrows, you can just do it all at once. Enumerating the 4 possible cases: 0b00 = 0 -> 0 - 0 = 0 0b01 = 1 -> 1 - 0 = 1 0b10 = 2 -> 2 - 1 = 1 0b11 = 3 -> 3 - 1 = 2 The next step consists of breaking up b (made of 16 2-bir fields) into even and odd halves and adding them into 4-bit fields. Since the largest possible sum is 2+2 = 4, which will not fit into a 4-bit field, the 2-bit ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ "which will not fit into a 2-bit field" fields have to be masked before they are added. After this point, the masking can be delayed. Each 4-bit field holds a population count from 0..4, taking at most 3 bits. These numbers can be added without overflowing a 4-bit field, so we can compute c + (c >> 4), and only then mask off the unwanted bits. This produces d, a number of 4 8-bit fields, each in the range 0..8. From this point, we can shift and add d multiple times without overflowing an 8-bit field, and only do a final mask at the end. The number to mask with has to be at least 63 (so that 32 on't be truncated), but can also be 128 or 255. The x86 has a special encoding for signed immediate byte values -128..127, so the value of 255 is slower. On other processors, a special "sign extend byte" instruction might be faster. On a processor with fast integer multiplies (Athlon but not P4), you can reduce the final few serially dependent instructions to a single integer multiply. Consider d to be 3 8-bit values d3, d2, d1 and d0, each in the range 0..8. The multiply forms the partial products: d3 d2 d1 d0 d3 d2 d1 d0 d3 d2 d1 d0 + d3 d2 d1 d0 ---------------------- e3 e2 e1 e0 Where e3 = d3 + d2 + d1 + d0. e2, e1 and e0 obviously cannot generate any carries. Signed-off-by: Akinobu Mita <mita@miraclelinux.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-03-26 09:40:00 +00:00
res = (res + (res >> 4)) & 0x0F0F0F0F;
res = res + (res >> 8);
return (res + (res >> 16)) & 0x000000FF;
#endif
}
EXPORT_SYMBOL(__sw_hweight32);
unsigned int __sw_hweight16(unsigned int w)
{
[PATCH] bitops: hweight() speedup <linux@horizon.com> wrote: This is an extremely well-known technique. You can see a similar version that uses a multiply for the last few steps at http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel whch refers to "Software Optimization Guide for AMD Athlon 64 and Opteron Processors" http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/25112.PDF It's section 8.6, "Efficient Implementation of Population-Count Function in 32-bit Mode", pages 179-180. It uses the name that I am more familiar with, "popcount" (population count), although "Hamming weight" also makes sense. Anyway, the proof of correctness proceeds as follows: b = a - ((a >> 1) & 0x55555555); c = (b & 0x33333333) + ((b >> 2) & 0x33333333); d = (c + (c >> 4)) & 0x0f0f0f0f; #if SLOW_MULTIPLY e = d + (d >> 8) f = e + (e >> 16); return f & 63; #else /* Useful if multiply takes at most 4 cycles */ return (d * 0x01010101) >> 24; #endif The input value a can be thought of as 32 1-bit fields each holding their own hamming weight. Now look at it as 16 2-bit fields. Each 2-bit field a1..a0 has the value 2*a1 + a0. This can be converted into the hamming weight of the 2-bit field a1+a0 by subtracting a1. That's what the (a >> 1) & mask subtraction does. Since there can be no borrows, you can just do it all at once. Enumerating the 4 possible cases: 0b00 = 0 -> 0 - 0 = 0 0b01 = 1 -> 1 - 0 = 1 0b10 = 2 -> 2 - 1 = 1 0b11 = 3 -> 3 - 1 = 2 The next step consists of breaking up b (made of 16 2-bir fields) into even and odd halves and adding them into 4-bit fields. Since the largest possible sum is 2+2 = 4, which will not fit into a 4-bit field, the 2-bit ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ "which will not fit into a 2-bit field" fields have to be masked before they are added. After this point, the masking can be delayed. Each 4-bit field holds a population count from 0..4, taking at most 3 bits. These numbers can be added without overflowing a 4-bit field, so we can compute c + (c >> 4), and only then mask off the unwanted bits. This produces d, a number of 4 8-bit fields, each in the range 0..8. From this point, we can shift and add d multiple times without overflowing an 8-bit field, and only do a final mask at the end. The number to mask with has to be at least 63 (so that 32 on't be truncated), but can also be 128 or 255. The x86 has a special encoding for signed immediate byte values -128..127, so the value of 255 is slower. On other processors, a special "sign extend byte" instruction might be faster. On a processor with fast integer multiplies (Athlon but not P4), you can reduce the final few serially dependent instructions to a single integer multiply. Consider d to be 3 8-bit values d3, d2, d1 and d0, each in the range 0..8. The multiply forms the partial products: d3 d2 d1 d0 d3 d2 d1 d0 d3 d2 d1 d0 + d3 d2 d1 d0 ---------------------- e3 e2 e1 e0 Where e3 = d3 + d2 + d1 + d0. e2, e1 and e0 obviously cannot generate any carries. Signed-off-by: Akinobu Mita <mita@miraclelinux.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-03-26 09:40:00 +00:00
unsigned int res = w - ((w >> 1) & 0x5555);
res = (res & 0x3333) + ((res >> 2) & 0x3333);
[PATCH] bitops: hweight() speedup <linux@horizon.com> wrote: This is an extremely well-known technique. You can see a similar version that uses a multiply for the last few steps at http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel whch refers to "Software Optimization Guide for AMD Athlon 64 and Opteron Processors" http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/25112.PDF It's section 8.6, "Efficient Implementation of Population-Count Function in 32-bit Mode", pages 179-180. It uses the name that I am more familiar with, "popcount" (population count), although "Hamming weight" also makes sense. Anyway, the proof of correctness proceeds as follows: b = a - ((a >> 1) & 0x55555555); c = (b & 0x33333333) + ((b >> 2) & 0x33333333); d = (c + (c >> 4)) & 0x0f0f0f0f; #if SLOW_MULTIPLY e = d + (d >> 8) f = e + (e >> 16); return f & 63; #else /* Useful if multiply takes at most 4 cycles */ return (d * 0x01010101) >> 24; #endif The input value a can be thought of as 32 1-bit fields each holding their own hamming weight. Now look at it as 16 2-bit fields. Each 2-bit field a1..a0 has the value 2*a1 + a0. This can be converted into the hamming weight of the 2-bit field a1+a0 by subtracting a1. That's what the (a >> 1) & mask subtraction does. Since there can be no borrows, you can just do it all at once. Enumerating the 4 possible cases: 0b00 = 0 -> 0 - 0 = 0 0b01 = 1 -> 1 - 0 = 1 0b10 = 2 -> 2 - 1 = 1 0b11 = 3 -> 3 - 1 = 2 The next step consists of breaking up b (made of 16 2-bir fields) into even and odd halves and adding them into 4-bit fields. Since the largest possible sum is 2+2 = 4, which will not fit into a 4-bit field, the 2-bit ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ "which will not fit into a 2-bit field" fields have to be masked before they are added. After this point, the masking can be delayed. Each 4-bit field holds a population count from 0..4, taking at most 3 bits. These numbers can be added without overflowing a 4-bit field, so we can compute c + (c >> 4), and only then mask off the unwanted bits. This produces d, a number of 4 8-bit fields, each in the range 0..8. From this point, we can shift and add d multiple times without overflowing an 8-bit field, and only do a final mask at the end. The number to mask with has to be at least 63 (so that 32 on't be truncated), but can also be 128 or 255. The x86 has a special encoding for signed immediate byte values -128..127, so the value of 255 is slower. On other processors, a special "sign extend byte" instruction might be faster. On a processor with fast integer multiplies (Athlon but not P4), you can reduce the final few serially dependent instructions to a single integer multiply. Consider d to be 3 8-bit values d3, d2, d1 and d0, each in the range 0..8. The multiply forms the partial products: d3 d2 d1 d0 d3 d2 d1 d0 d3 d2 d1 d0 + d3 d2 d1 d0 ---------------------- e3 e2 e1 e0 Where e3 = d3 + d2 + d1 + d0. e2, e1 and e0 obviously cannot generate any carries. Signed-off-by: Akinobu Mita <mita@miraclelinux.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-03-26 09:40:00 +00:00
res = (res + (res >> 4)) & 0x0F0F;
return (res + (res >> 8)) & 0x00FF;
}
EXPORT_SYMBOL(__sw_hweight16);
unsigned int __sw_hweight8(unsigned int w)
{
[PATCH] bitops: hweight() speedup <linux@horizon.com> wrote: This is an extremely well-known technique. You can see a similar version that uses a multiply for the last few steps at http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel whch refers to "Software Optimization Guide for AMD Athlon 64 and Opteron Processors" http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/25112.PDF It's section 8.6, "Efficient Implementation of Population-Count Function in 32-bit Mode", pages 179-180. It uses the name that I am more familiar with, "popcount" (population count), although "Hamming weight" also makes sense. Anyway, the proof of correctness proceeds as follows: b = a - ((a >> 1) & 0x55555555); c = (b & 0x33333333) + ((b >> 2) & 0x33333333); d = (c + (c >> 4)) & 0x0f0f0f0f; #if SLOW_MULTIPLY e = d + (d >> 8) f = e + (e >> 16); return f & 63; #else /* Useful if multiply takes at most 4 cycles */ return (d * 0x01010101) >> 24; #endif The input value a can be thought of as 32 1-bit fields each holding their own hamming weight. Now look at it as 16 2-bit fields. Each 2-bit field a1..a0 has the value 2*a1 + a0. This can be converted into the hamming weight of the 2-bit field a1+a0 by subtracting a1. That's what the (a >> 1) & mask subtraction does. Since there can be no borrows, you can just do it all at once. Enumerating the 4 possible cases: 0b00 = 0 -> 0 - 0 = 0 0b01 = 1 -> 1 - 0 = 1 0b10 = 2 -> 2 - 1 = 1 0b11 = 3 -> 3 - 1 = 2 The next step consists of breaking up b (made of 16 2-bir fields) into even and odd halves and adding them into 4-bit fields. Since the largest possible sum is 2+2 = 4, which will not fit into a 4-bit field, the 2-bit ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ "which will not fit into a 2-bit field" fields have to be masked before they are added. After this point, the masking can be delayed. Each 4-bit field holds a population count from 0..4, taking at most 3 bits. These numbers can be added without overflowing a 4-bit field, so we can compute c + (c >> 4), and only then mask off the unwanted bits. This produces d, a number of 4 8-bit fields, each in the range 0..8. From this point, we can shift and add d multiple times without overflowing an 8-bit field, and only do a final mask at the end. The number to mask with has to be at least 63 (so that 32 on't be truncated), but can also be 128 or 255. The x86 has a special encoding for signed immediate byte values -128..127, so the value of 255 is slower. On other processors, a special "sign extend byte" instruction might be faster. On a processor with fast integer multiplies (Athlon but not P4), you can reduce the final few serially dependent instructions to a single integer multiply. Consider d to be 3 8-bit values d3, d2, d1 and d0, each in the range 0..8. The multiply forms the partial products: d3 d2 d1 d0 d3 d2 d1 d0 d3 d2 d1 d0 + d3 d2 d1 d0 ---------------------- e3 e2 e1 e0 Where e3 = d3 + d2 + d1 + d0. e2, e1 and e0 obviously cannot generate any carries. Signed-off-by: Akinobu Mita <mita@miraclelinux.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-03-26 09:40:00 +00:00
unsigned int res = w - ((w >> 1) & 0x55);
res = (res & 0x33) + ((res >> 2) & 0x33);
[PATCH] bitops: hweight() speedup <linux@horizon.com> wrote: This is an extremely well-known technique. You can see a similar version that uses a multiply for the last few steps at http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel whch refers to "Software Optimization Guide for AMD Athlon 64 and Opteron Processors" http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/25112.PDF It's section 8.6, "Efficient Implementation of Population-Count Function in 32-bit Mode", pages 179-180. It uses the name that I am more familiar with, "popcount" (population count), although "Hamming weight" also makes sense. Anyway, the proof of correctness proceeds as follows: b = a - ((a >> 1) & 0x55555555); c = (b & 0x33333333) + ((b >> 2) & 0x33333333); d = (c + (c >> 4)) & 0x0f0f0f0f; #if SLOW_MULTIPLY e = d + (d >> 8) f = e + (e >> 16); return f & 63; #else /* Useful if multiply takes at most 4 cycles */ return (d * 0x01010101) >> 24; #endif The input value a can be thought of as 32 1-bit fields each holding their own hamming weight. Now look at it as 16 2-bit fields. Each 2-bit field a1..a0 has the value 2*a1 + a0. This can be converted into the hamming weight of the 2-bit field a1+a0 by subtracting a1. That's what the (a >> 1) & mask subtraction does. Since there can be no borrows, you can just do it all at once. Enumerating the 4 possible cases: 0b00 = 0 -> 0 - 0 = 0 0b01 = 1 -> 1 - 0 = 1 0b10 = 2 -> 2 - 1 = 1 0b11 = 3 -> 3 - 1 = 2 The next step consists of breaking up b (made of 16 2-bir fields) into even and odd halves and adding them into 4-bit fields. Since the largest possible sum is 2+2 = 4, which will not fit into a 4-bit field, the 2-bit ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ "which will not fit into a 2-bit field" fields have to be masked before they are added. After this point, the masking can be delayed. Each 4-bit field holds a population count from 0..4, taking at most 3 bits. These numbers can be added without overflowing a 4-bit field, so we can compute c + (c >> 4), and only then mask off the unwanted bits. This produces d, a number of 4 8-bit fields, each in the range 0..8. From this point, we can shift and add d multiple times without overflowing an 8-bit field, and only do a final mask at the end. The number to mask with has to be at least 63 (so that 32 on't be truncated), but can also be 128 or 255. The x86 has a special encoding for signed immediate byte values -128..127, so the value of 255 is slower. On other processors, a special "sign extend byte" instruction might be faster. On a processor with fast integer multiplies (Athlon but not P4), you can reduce the final few serially dependent instructions to a single integer multiply. Consider d to be 3 8-bit values d3, d2, d1 and d0, each in the range 0..8. The multiply forms the partial products: d3 d2 d1 d0 d3 d2 d1 d0 d3 d2 d1 d0 + d3 d2 d1 d0 ---------------------- e3 e2 e1 e0 Where e3 = d3 + d2 + d1 + d0. e2, e1 and e0 obviously cannot generate any carries. Signed-off-by: Akinobu Mita <mita@miraclelinux.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-03-26 09:40:00 +00:00
return (res + (res >> 4)) & 0x0F;
}
EXPORT_SYMBOL(__sw_hweight8);
unsigned long __sw_hweight64(__u64 w)
{
#if BITS_PER_LONG == 32
return __sw_hweight32((unsigned int)(w >> 32)) +
__sw_hweight32((unsigned int)w);
#elif BITS_PER_LONG == 64
#ifdef CONFIG_ARCH_HAS_FAST_MULTIPLIER
w -= (w >> 1) & 0x5555555555555555ul;
w = (w & 0x3333333333333333ul) + ((w >> 2) & 0x3333333333333333ul);
w = (w + (w >> 4)) & 0x0f0f0f0f0f0f0f0ful;
return (w * 0x0101010101010101ul) >> 56;
#else
[PATCH] bitops: hweight() speedup <linux@horizon.com> wrote: This is an extremely well-known technique. You can see a similar version that uses a multiply for the last few steps at http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel whch refers to "Software Optimization Guide for AMD Athlon 64 and Opteron Processors" http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/25112.PDF It's section 8.6, "Efficient Implementation of Population-Count Function in 32-bit Mode", pages 179-180. It uses the name that I am more familiar with, "popcount" (population count), although "Hamming weight" also makes sense. Anyway, the proof of correctness proceeds as follows: b = a - ((a >> 1) & 0x55555555); c = (b & 0x33333333) + ((b >> 2) & 0x33333333); d = (c + (c >> 4)) & 0x0f0f0f0f; #if SLOW_MULTIPLY e = d + (d >> 8) f = e + (e >> 16); return f & 63; #else /* Useful if multiply takes at most 4 cycles */ return (d * 0x01010101) >> 24; #endif The input value a can be thought of as 32 1-bit fields each holding their own hamming weight. Now look at it as 16 2-bit fields. Each 2-bit field a1..a0 has the value 2*a1 + a0. This can be converted into the hamming weight of the 2-bit field a1+a0 by subtracting a1. That's what the (a >> 1) & mask subtraction does. Since there can be no borrows, you can just do it all at once. Enumerating the 4 possible cases: 0b00 = 0 -> 0 - 0 = 0 0b01 = 1 -> 1 - 0 = 1 0b10 = 2 -> 2 - 1 = 1 0b11 = 3 -> 3 - 1 = 2 The next step consists of breaking up b (made of 16 2-bir fields) into even and odd halves and adding them into 4-bit fields. Since the largest possible sum is 2+2 = 4, which will not fit into a 4-bit field, the 2-bit ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ "which will not fit into a 2-bit field" fields have to be masked before they are added. After this point, the masking can be delayed. Each 4-bit field holds a population count from 0..4, taking at most 3 bits. These numbers can be added without overflowing a 4-bit field, so we can compute c + (c >> 4), and only then mask off the unwanted bits. This produces d, a number of 4 8-bit fields, each in the range 0..8. From this point, we can shift and add d multiple times without overflowing an 8-bit field, and only do a final mask at the end. The number to mask with has to be at least 63 (so that 32 on't be truncated), but can also be 128 or 255. The x86 has a special encoding for signed immediate byte values -128..127, so the value of 255 is slower. On other processors, a special "sign extend byte" instruction might be faster. On a processor with fast integer multiplies (Athlon but not P4), you can reduce the final few serially dependent instructions to a single integer multiply. Consider d to be 3 8-bit values d3, d2, d1 and d0, each in the range 0..8. The multiply forms the partial products: d3 d2 d1 d0 d3 d2 d1 d0 d3 d2 d1 d0 + d3 d2 d1 d0 ---------------------- e3 e2 e1 e0 Where e3 = d3 + d2 + d1 + d0. e2, e1 and e0 obviously cannot generate any carries. Signed-off-by: Akinobu Mita <mita@miraclelinux.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-03-26 09:40:00 +00:00
__u64 res = w - ((w >> 1) & 0x5555555555555555ul);
res = (res & 0x3333333333333333ul) + ((res >> 2) & 0x3333333333333333ul);
[PATCH] bitops: hweight() speedup <linux@horizon.com> wrote: This is an extremely well-known technique. You can see a similar version that uses a multiply for the last few steps at http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel whch refers to "Software Optimization Guide for AMD Athlon 64 and Opteron Processors" http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/25112.PDF It's section 8.6, "Efficient Implementation of Population-Count Function in 32-bit Mode", pages 179-180. It uses the name that I am more familiar with, "popcount" (population count), although "Hamming weight" also makes sense. Anyway, the proof of correctness proceeds as follows: b = a - ((a >> 1) & 0x55555555); c = (b & 0x33333333) + ((b >> 2) & 0x33333333); d = (c + (c >> 4)) & 0x0f0f0f0f; #if SLOW_MULTIPLY e = d + (d >> 8) f = e + (e >> 16); return f & 63; #else /* Useful if multiply takes at most 4 cycles */ return (d * 0x01010101) >> 24; #endif The input value a can be thought of as 32 1-bit fields each holding their own hamming weight. Now look at it as 16 2-bit fields. Each 2-bit field a1..a0 has the value 2*a1 + a0. This can be converted into the hamming weight of the 2-bit field a1+a0 by subtracting a1. That's what the (a >> 1) & mask subtraction does. Since there can be no borrows, you can just do it all at once. Enumerating the 4 possible cases: 0b00 = 0 -> 0 - 0 = 0 0b01 = 1 -> 1 - 0 = 1 0b10 = 2 -> 2 - 1 = 1 0b11 = 3 -> 3 - 1 = 2 The next step consists of breaking up b (made of 16 2-bir fields) into even and odd halves and adding them into 4-bit fields. Since the largest possible sum is 2+2 = 4, which will not fit into a 4-bit field, the 2-bit ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ "which will not fit into a 2-bit field" fields have to be masked before they are added. After this point, the masking can be delayed. Each 4-bit field holds a population count from 0..4, taking at most 3 bits. These numbers can be added without overflowing a 4-bit field, so we can compute c + (c >> 4), and only then mask off the unwanted bits. This produces d, a number of 4 8-bit fields, each in the range 0..8. From this point, we can shift and add d multiple times without overflowing an 8-bit field, and only do a final mask at the end. The number to mask with has to be at least 63 (so that 32 on't be truncated), but can also be 128 or 255. The x86 has a special encoding for signed immediate byte values -128..127, so the value of 255 is slower. On other processors, a special "sign extend byte" instruction might be faster. On a processor with fast integer multiplies (Athlon but not P4), you can reduce the final few serially dependent instructions to a single integer multiply. Consider d to be 3 8-bit values d3, d2, d1 and d0, each in the range 0..8. The multiply forms the partial products: d3 d2 d1 d0 d3 d2 d1 d0 d3 d2 d1 d0 + d3 d2 d1 d0 ---------------------- e3 e2 e1 e0 Where e3 = d3 + d2 + d1 + d0. e2, e1 and e0 obviously cannot generate any carries. Signed-off-by: Akinobu Mita <mita@miraclelinux.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-03-26 09:40:00 +00:00
res = (res + (res >> 4)) & 0x0F0F0F0F0F0F0F0Ful;
res = res + (res >> 8);
res = res + (res >> 16);
return (res + (res >> 32)) & 0x00000000000000FFul;
#endif
#endif
}
EXPORT_SYMBOL(__sw_hweight64);