Messages in this thread Patch in this message | | | From | David Howells <> | Subject | [PATCH 4/6] Implement a general log2 facility in the kernel | Date | Wed, 13 Sep 2006 14:03:00 +0100 |
| |
From: David Howells <dhowells@redhat.com>
This facility provides three entry points:
log2() Log base 2 of u32 ll_log2() Log base 2 of u64 long_log2() Log base 2 of unsigned long
These facilities can either be used inside functions on dynamic data:
int do_something(long q) { ...; y = log2(x) ...; }
Or can be used to statically initialise global variables with constant values:
unsigned n = log2(27);
When performing static initialisation, the compiler will report "error: initializer element is not constant" if asked to take a log of zero or of something not reducible to a constant. They treat negative numbers as unsigned.
When not dealing with a constant, they fall back to using fls() which permits them to use arch-specific log calculation instructions - such as BSR on x86/x86_64 or SCAN on FRV - if available.
Signed-Off-By: David Howells <dhowells@redhat.com> ---
fs/ext2/super.c | 2 - fs/ext3/super.c | 2 - include/asm-frv/bitops.h | 44 +++++++++++ include/linux/kernel.h | 9 -- include/linux/log2.h | 179 ++++++++++++++++++++++++++++++++++++++++++++++ 5 files changed, 224 insertions(+), 12 deletions(-)
diff --git a/fs/ext2/super.c b/fs/ext2/super.c index 681dea8..a4573b6 100644 --- a/fs/ext2/super.c +++ b/fs/ext2/super.c @@ -548,8 +548,6 @@ static int ext2_check_descriptors (struc return 1; } -#define log2(n) ffz(~(n)) - /* * Maximal file size. There is a direct, and {,double-,triple-}indirect * block limit, and also a limit of (2^32 - 1) 512-byte sectors in i_blocks. diff --git a/fs/ext3/super.c b/fs/ext3/super.c index 813d589..af1424d 100644 --- a/fs/ext3/super.c +++ b/fs/ext3/super.c @@ -1294,8 +1294,6 @@ #endif sb->s_flags = s_flags; /* Restore MS_RDONLY status */ } -#define log2(n) ffz(~(n)) - /* * Maximal file size. There is a direct, and {,double-,triple-}indirect * block limit, and also a limit of (2^32 - 1) 512-byte sectors in i_blocks. diff --git a/include/asm-frv/bitops.h b/include/asm-frv/bitops.h index 1f70d47..41a5866 100644 --- a/include/asm-frv/bitops.h +++ b/include/asm-frv/bitops.h @@ -256,6 +256,50 @@ int __ffs(unsigned long x) return 31 - bit; } +/* + * special slimline version of fls() for calculating log2() + * - note: no protection against n == 0 + */ +#define ARCH_HAS_LOG2 +static inline __attribute__((const)) +int __log2(u32 n) +{ + int bit; + asm("scan %1,gr0,%0" : "=r"(bit) : "r"(n)); + return 31 - bit; +} + +/* + * special slimline version of fls64() for calculating ll_log2() + * - note: no protection against n == 0 + */ +#define ARCH_HAS_LL_LOG2 +static inline __attribute__((const)) +int __ll_log2(u64 n) +{ + union { + u64 ll; + struct { u32 h, l; }; + } _; + int bit, x, y; + + _.ll = n; + + asm(" subcc %3,gr0,gr0,icc0 \n" + " ckeq icc0,cc4 \n" + " cscan.p %3,gr0,%0 ,cc4,0 \n" + " setlos #63,%1 \n" + " cscan.p %4,gr0,%0 ,cc4,1 \n" + " setlos #31,%2 \n" + " csub.p %1,%0,%0 ,cc4,0 \n" + " csub %2,%0,%0 ,cc4,1 \n" + : "=&r"(bit), "=r"(x), "=r"(y) + : "0r"(_.h), "r"(_.l) + : "icc0", "cc4" + ); + return bit; +} + #include <asm-generic/bitops/sched.h> #include <asm-generic/bitops/hweight.h> diff --git a/include/linux/kernel.h b/include/linux/kernel.h index 851aa1b..e8c6c66 100644 --- a/include/linux/kernel.h +++ b/include/linux/kernel.h @@ -13,6 +13,7 @@ #include <linux/stddef.h> #include <linux/types.h> #include <linux/compiler.h> #include <linux/bitops.h> +#include <linux/log2.h> #include <asm/byteorder.h> #include <asm/bug.h> @@ -154,14 +155,6 @@ #endif unsigned long int_sqrt(unsigned long); -static inline int __attribute_pure__ long_log2(unsigned long x) -{ - int r = 0; - for (x >>= 1; x > 0; x >>= 1) - r++; - return r; -} - static inline unsigned long __attribute_const__ roundup_pow_of_two(unsigned long x) { diff --git a/include/linux/log2.h b/include/linux/log2.h new file mode 100644 index 0000000..9bef055 --- /dev/null +++ b/include/linux/log2.h @@ -0,0 +1,179 @@ +/* Log base 2 calculation + * + * Copyright (C) 2006 Red Hat, Inc. All Rights Reserved. + * Written by David Howells (dhowells@redhat.com) + * + * This program is free software; you can redistribute it and/or + * modify it under the terms of the GNU General Public License + * as published by the Free Software Foundation; either version + * 2 of the License, or (at your option) any later version. + */ + +#ifndef _LINUX_LOG2_H +#define _LINUX_LOG2_H + +#include <linux/types.h> +#include <linux/bitops.h> + +/* + * deal with unrepresentable constant logarithms + */ +extern __attribute__((const, noreturn)) +int ____log2_NaN(void); + +/* + * non-constant log of base 2 calculation + * - the arch may override these in asm/bitops.h if they can be implemented + * more efficiently than using fls() and fls64() + */ +#ifndef ARCH_HAS_LOG2 +static inline __attribute__((const)) +int __log2(u32 n) +{ + return fls(n) - 1; +} +#endif + +#ifndef ARCH_HAS_LL_LOG2 +static inline __attribute__((const)) +int __ll_log2(u64 n) +{ + return fls64(n) - 1; +} +#endif + +/* + * constant-capable 32-bit log of base 2 calculation + * - this can be used to initialise global variables from constant data, hence + * the massive ternary operator construction + */ +#define log2(n) \ +( \ + __builtin_constant_p(n) ? ( \ + n < 1 ? ____log2_NaN() : \ + n >= (1ULL << 31) ? 31 : \ + n >= (1ULL << 30) ? 30 : \ + n >= (1ULL << 29) ? 29 : \ + n >= (1ULL << 28) ? 28 : \ + n >= (1ULL << 27) ? 27 : \ + n >= (1ULL << 26) ? 26 : \ + n >= (1ULL << 25) ? 25 : \ + n >= (1ULL << 24) ? 24 : \ + n >= (1ULL << 23) ? 23 : \ + n >= (1ULL << 22) ? 22 : \ + n >= (1ULL << 21) ? 21 : \ + n >= (1ULL << 20) ? 20 : \ + n >= (1ULL << 19) ? 19 : \ + n >= (1ULL << 18) ? 18 : \ + n >= (1ULL << 17) ? 17 : \ + n >= (1ULL << 16) ? 16 : \ + n >= (1ULL << 15) ? 15 : \ + n >= (1ULL << 14) ? 14 : \ + n >= (1ULL << 13) ? 13 : \ + n >= (1ULL << 12) ? 12 : \ + n >= (1ULL << 11) ? 11 : \ + n >= (1ULL << 10) ? 10 : \ + n >= (1ULL << 9) ? 9 : \ + n >= (1ULL << 8) ? 8 : \ + n >= (1ULL << 7) ? 7 : \ + n >= (1ULL << 6) ? 6 : \ + n >= (1ULL << 5) ? 5 : \ + n >= (1ULL << 4) ? 4 : \ + n >= (1ULL << 3) ? 3 : \ + n >= (1ULL << 2) ? 2 : \ + n >= (1ULL << 1) ? 1 : \ + n >= (1ULL << 0) ? 0 : \ + ____log2_NaN() \ + ) : \ + __log2(n) \ + ) + +/* + * constant-capable 64-bit log of base 2 calculation + * - this can be used to initialise global variables from constant data, hence + * the massive ternary operator construction + */ +#define ll_log2(n) \ +( \ + __builtin_constant_p(n) ? ( \ + n < 1 ? ____log2_NaN() : \ + n >= (1ULL << 63) ? 63 : \ + n >= (1ULL << 62) ? 62 : \ + n >= (1ULL << 61) ? 61 : \ + n >= (1ULL << 60) ? 60 : \ + n >= (1ULL << 59) ? 59 : \ + n >= (1ULL << 58) ? 58 : \ + n >= (1ULL << 57) ? 57 : \ + n >= (1ULL << 56) ? 56 : \ + n >= (1ULL << 55) ? 55 : \ + n >= (1ULL << 54) ? 54 : \ + n >= (1ULL << 53) ? 53 : \ + n >= (1ULL << 52) ? 52 : \ + n >= (1ULL << 51) ? 51 : \ + n >= (1ULL << 50) ? 50 : \ + n >= (1ULL << 49) ? 49 : \ + n >= (1ULL << 48) ? 48 : \ + n >= (1ULL << 47) ? 47 : \ + n >= (1ULL << 46) ? 46 : \ + n >= (1ULL << 45) ? 45 : \ + n >= (1ULL << 44) ? 44 : \ + n >= (1ULL << 43) ? 43 : \ + n >= (1ULL << 42) ? 42 : \ + n >= (1ULL << 41) ? 41 : \ + n >= (1ULL << 40) ? 40 : \ + n >= (1ULL << 39) ? 39 : \ + n >= (1ULL << 38) ? 38 : \ + n >= (1ULL << 37) ? 37 : \ + n >= (1ULL << 36) ? 36 : \ + n >= (1ULL << 35) ? 35 : \ + n >= (1ULL << 34) ? 34 : \ + n >= (1ULL << 33) ? 33 : \ + n >= (1ULL << 32) ? 32 : \ + n >= (1ULL << 31) ? 31 : \ + n >= (1ULL << 30) ? 30 : \ + n >= (1ULL << 29) ? 29 : \ + n >= (1ULL << 28) ? 28 : \ + n >= (1ULL << 27) ? 27 : \ + n >= (1ULL << 26) ? 26 : \ + n >= (1ULL << 25) ? 25 : \ + n >= (1ULL << 24) ? 24 : \ + n >= (1ULL << 23) ? 23 : \ + n >= (1ULL << 22) ? 22 : \ + n >= (1ULL << 21) ? 21 : \ + n >= (1ULL << 20) ? 20 : \ + n >= (1ULL << 19) ? 19 : \ + n >= (1ULL << 18) ? 18 : \ + n >= (1ULL << 17) ? 17 : \ + n >= (1ULL << 16) ? 16 : \ + n >= (1ULL << 15) ? 15 : \ + n >= (1ULL << 14) ? 14 : \ + n >= (1ULL << 13) ? 13 : \ + n >= (1ULL << 12) ? 12 : \ + n >= (1ULL << 11) ? 11 : \ + n >= (1ULL << 10) ? 10 : \ + n >= (1ULL << 9) ? 9 : \ + n >= (1ULL << 8) ? 8 : \ + n >= (1ULL << 7) ? 7 : \ + n >= (1ULL << 6) ? 6 : \ + n >= (1ULL << 5) ? 5 : \ + n >= (1ULL << 4) ? 4 : \ + n >= (1ULL << 3) ? 3 : \ + n >= (1ULL << 2) ? 2 : \ + n >= (1ULL << 1) ? 1 : \ + n >= (1ULL << 0) ? 0 : \ + ____log2_NaN() \ + ) : \ + __ll_log2(n) \ + ) + +/* + * constant-capable unsigned long log of base 2 calculation + * - this can be used to initialise global variables from constant data + */ +#if BITS_PER_LONG == 32 +#define long_log2(n) log2(n) +#else +#define long_log2(n) ll_log2(n) +#endif + +#endif /* _LINUX_LOG2_H */ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |