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Kernel lacks for a function that searches for Nth bit in a bitmap. Usually people do it like this: for_each_set_bit(bit, mask, size) if (n-- == 0) return bit; We can do it more efficiently, if we: 1. find a word containing Nth bit, using hweight(); and 2. find the bit, using a helper fns(), that works similarly to __ffs() and ffz(). fns() is implemented as a simple loop. For x86_64, there's PDEP instruction to do that: ret = clz(pdep(1 << idx, num)). However, for large bitmaps the most of improvement comes from using hweight(), so I kept fns() simple. New find_nth_bit() is ~70 times faster on x86_64/kvm in find_bit benchmark: find_nth_bit: 7154190 ns, 16411 iterations for_each_bit: 505493126 ns, 16315 iterations With all that, a family of 3 new functions is added, and used where appropriate in the following patches. Signed-off-by: Yury Norov <yury.norov@gmail.com>
244 lines
6.3 KiB
C
244 lines
6.3 KiB
C
// SPDX-License-Identifier: GPL-2.0-or-later
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/* bit search implementation
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*
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* Copyright (C) 2004 Red Hat, Inc. All Rights Reserved.
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* Written by David Howells (dhowells@redhat.com)
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*
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* Copyright (C) 2008 IBM Corporation
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* 'find_last_bit' is written by Rusty Russell <rusty@rustcorp.com.au>
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* (Inspired by David Howell's find_next_bit implementation)
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*
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* Rewritten by Yury Norov <yury.norov@gmail.com> to decrease
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* size and improve performance, 2015.
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*/
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#include <linux/bitops.h>
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#include <linux/bitmap.h>
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#include <linux/export.h>
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#include <linux/math.h>
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#include <linux/minmax.h>
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#include <linux/swab.h>
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/*
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* Common helper for find_bit() function family
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* @FETCH: The expression that fetches and pre-processes each word of bitmap(s)
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* @MUNGE: The expression that post-processes a word containing found bit (may be empty)
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* @size: The bitmap size in bits
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*/
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#define FIND_FIRST_BIT(FETCH, MUNGE, size) \
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({ \
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unsigned long idx, val, sz = (size); \
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\
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for (idx = 0; idx * BITS_PER_LONG < sz; idx++) { \
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val = (FETCH); \
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if (val) { \
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sz = min(idx * BITS_PER_LONG + __ffs(MUNGE(val)), sz); \
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break; \
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} \
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} \
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\
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sz; \
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})
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/*
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* Common helper for find_next_bit() function family
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* @FETCH: The expression that fetches and pre-processes each word of bitmap(s)
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* @MUNGE: The expression that post-processes a word containing found bit (may be empty)
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* @size: The bitmap size in bits
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* @start: The bitnumber to start searching at
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*/
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#define FIND_NEXT_BIT(FETCH, MUNGE, size, start) \
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({ \
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unsigned long mask, idx, tmp, sz = (size), __start = (start); \
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\
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if (unlikely(__start >= sz)) \
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goto out; \
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\
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mask = MUNGE(BITMAP_FIRST_WORD_MASK(__start)); \
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idx = __start / BITS_PER_LONG; \
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\
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for (tmp = (FETCH) & mask; !tmp; tmp = (FETCH)) { \
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if ((idx + 1) * BITS_PER_LONG >= sz) \
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goto out; \
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idx++; \
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} \
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\
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sz = min(idx * BITS_PER_LONG + __ffs(MUNGE(tmp)), sz); \
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out: \
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sz; \
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})
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#define FIND_NTH_BIT(FETCH, size, num) \
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({ \
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unsigned long sz = (size), nr = (num), idx, w, tmp; \
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\
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for (idx = 0; (idx + 1) * BITS_PER_LONG <= sz; idx++) { \
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if (idx * BITS_PER_LONG + nr >= sz) \
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goto out; \
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\
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tmp = (FETCH); \
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w = hweight_long(tmp); \
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if (w > nr) \
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goto found; \
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\
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nr -= w; \
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} \
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\
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if (sz % BITS_PER_LONG) \
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tmp = (FETCH) & BITMAP_LAST_WORD_MASK(sz); \
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found: \
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sz = min(idx * BITS_PER_LONG + fns(tmp, nr), sz); \
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out: \
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sz; \
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})
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#ifndef find_first_bit
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/*
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* Find the first set bit in a memory region.
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*/
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unsigned long _find_first_bit(const unsigned long *addr, unsigned long size)
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{
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return FIND_FIRST_BIT(addr[idx], /* nop */, size);
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}
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EXPORT_SYMBOL(_find_first_bit);
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#endif
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#ifndef find_first_and_bit
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/*
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* Find the first set bit in two memory regions.
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*/
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unsigned long _find_first_and_bit(const unsigned long *addr1,
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const unsigned long *addr2,
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unsigned long size)
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{
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return FIND_FIRST_BIT(addr1[idx] & addr2[idx], /* nop */, size);
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}
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EXPORT_SYMBOL(_find_first_and_bit);
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#endif
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#ifndef find_first_zero_bit
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/*
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* Find the first cleared bit in a memory region.
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*/
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unsigned long _find_first_zero_bit(const unsigned long *addr, unsigned long size)
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{
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return FIND_FIRST_BIT(~addr[idx], /* nop */, size);
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}
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EXPORT_SYMBOL(_find_first_zero_bit);
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#endif
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#ifndef find_next_bit
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unsigned long _find_next_bit(const unsigned long *addr, unsigned long nbits, unsigned long start)
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{
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return FIND_NEXT_BIT(addr[idx], /* nop */, nbits, start);
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}
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EXPORT_SYMBOL(_find_next_bit);
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#endif
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unsigned long __find_nth_bit(const unsigned long *addr, unsigned long size, unsigned long n)
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{
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return FIND_NTH_BIT(addr[idx], size, n);
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}
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EXPORT_SYMBOL(__find_nth_bit);
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unsigned long __find_nth_and_bit(const unsigned long *addr1, const unsigned long *addr2,
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unsigned long size, unsigned long n)
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{
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return FIND_NTH_BIT(addr1[idx] & addr2[idx], size, n);
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}
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EXPORT_SYMBOL(__find_nth_and_bit);
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unsigned long __find_nth_andnot_bit(const unsigned long *addr1, const unsigned long *addr2,
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unsigned long size, unsigned long n)
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{
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return FIND_NTH_BIT(addr1[idx] & ~addr2[idx], size, n);
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}
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EXPORT_SYMBOL(__find_nth_andnot_bit);
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#ifndef find_next_and_bit
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unsigned long _find_next_and_bit(const unsigned long *addr1, const unsigned long *addr2,
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unsigned long nbits, unsigned long start)
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{
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return FIND_NEXT_BIT(addr1[idx] & addr2[idx], /* nop */, nbits, start);
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}
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EXPORT_SYMBOL(_find_next_and_bit);
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#endif
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#ifndef find_next_zero_bit
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unsigned long _find_next_zero_bit(const unsigned long *addr, unsigned long nbits,
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unsigned long start)
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{
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return FIND_NEXT_BIT(~addr[idx], /* nop */, nbits, start);
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}
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EXPORT_SYMBOL(_find_next_zero_bit);
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#endif
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#ifndef find_last_bit
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unsigned long _find_last_bit(const unsigned long *addr, unsigned long size)
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{
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if (size) {
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unsigned long val = BITMAP_LAST_WORD_MASK(size);
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unsigned long idx = (size-1) / BITS_PER_LONG;
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do {
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val &= addr[idx];
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if (val)
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return idx * BITS_PER_LONG + __fls(val);
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val = ~0ul;
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} while (idx--);
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}
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return size;
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}
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EXPORT_SYMBOL(_find_last_bit);
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#endif
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unsigned long find_next_clump8(unsigned long *clump, const unsigned long *addr,
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unsigned long size, unsigned long offset)
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{
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offset = find_next_bit(addr, size, offset);
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if (offset == size)
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return size;
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offset = round_down(offset, 8);
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*clump = bitmap_get_value8(addr, offset);
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return offset;
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}
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EXPORT_SYMBOL(find_next_clump8);
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#ifdef __BIG_ENDIAN
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#ifndef find_first_zero_bit_le
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/*
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* Find the first cleared bit in an LE memory region.
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*/
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unsigned long _find_first_zero_bit_le(const unsigned long *addr, unsigned long size)
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{
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return FIND_FIRST_BIT(~addr[idx], swab, size);
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}
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EXPORT_SYMBOL(_find_first_zero_bit_le);
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#endif
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#ifndef find_next_zero_bit_le
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unsigned long _find_next_zero_bit_le(const unsigned long *addr,
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unsigned long size, unsigned long offset)
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{
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return FIND_NEXT_BIT(~addr[idx], swab, size, offset);
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}
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EXPORT_SYMBOL(_find_next_zero_bit_le);
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#endif
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#ifndef find_next_bit_le
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unsigned long _find_next_bit_le(const unsigned long *addr,
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unsigned long size, unsigned long offset)
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{
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return FIND_NEXT_BIT(addr[idx], swab, size, offset);
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}
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EXPORT_SYMBOL(_find_next_bit_le);
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#endif
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#endif /* __BIG_ENDIAN */
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