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474c90156c
gcc-7 has an "optimization" pass that completely screws up, and generates the code expansion for the (impossible) case of calling ilog2() with a zero constant, even when the code gcc compiles does not actually have a zero constant. And we try to generate a compile-time error for anybody doing ilog2() on a constant where that doesn't make sense (be it zero or negative). So now gcc7 will fail the build due to our sanity checking, because it created that constant-zero case that didn't actually exist in the source code. There's a whole long discussion on the kernel mailing about how to work around this gcc bug. The gcc people themselevs have discussed their "feature" in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72785 but it's all water under the bridge, because while it looked at one point like it would be solved by the time gcc7 was released, that was not to be. So now we have to deal with this compiler braindamage. And the only simple approach seems to be to just delete the code that tries to warn about bad uses of ilog2(). So now "ilog2()" will just return 0 not just for the value 1, but for any non-positive value too. It's not like I can recall anybody having ever actually tried to use this function on any invalid value, but maybe the sanity check just meant that such code never made it out in public. Reported-by: Laura Abbott <labbott@redhat.com> Cc: John Stultz <john.stultz@linaro.org>, Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
177 lines
4.6 KiB
C
177 lines
4.6 KiB
C
/* Integer base 2 logarithm calculation
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*
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* Copyright (C) 2006 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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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version
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* 2 of the License, or (at your option) any later version.
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*/
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#ifndef _TOOLS_LINUX_LOG2_H
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#define _TOOLS_LINUX_LOG2_H
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/*
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* non-constant log of base 2 calculators
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* - the arch may override these in asm/bitops.h if they can be implemented
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* more efficiently than using fls() and fls64()
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* - the arch is not required to handle n==0 if implementing the fallback
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*/
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static inline __attribute__((const))
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int __ilog2_u32(u32 n)
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{
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return fls(n) - 1;
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}
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static inline __attribute__((const))
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int __ilog2_u64(u64 n)
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{
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return fls64(n) - 1;
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}
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/*
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* Determine whether some value is a power of two, where zero is
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* *not* considered a power of two.
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*/
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static inline __attribute__((const))
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bool is_power_of_2(unsigned long n)
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{
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return (n != 0 && ((n & (n - 1)) == 0));
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}
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/*
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* round up to nearest power of two
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*/
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static inline __attribute__((const))
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unsigned long __roundup_pow_of_two(unsigned long n)
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{
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return 1UL << fls_long(n - 1);
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}
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/*
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* round down to nearest power of two
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*/
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static inline __attribute__((const))
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unsigned long __rounddown_pow_of_two(unsigned long n)
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{
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return 1UL << (fls_long(n) - 1);
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}
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/**
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* ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value
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* @n - parameter
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*
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* constant-capable log of base 2 calculation
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* - this can be used to initialise global variables from constant data, hence
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* the massive ternary operator construction
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*
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* selects the appropriately-sized optimised version depending on sizeof(n)
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*/
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#define ilog2(n) \
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( \
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__builtin_constant_p(n) ? ( \
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(n) < 2 ? 0 : \
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(n) & (1ULL << 63) ? 63 : \
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(n) & (1ULL << 62) ? 62 : \
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(n) & (1ULL << 61) ? 61 : \
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(n) & (1ULL << 60) ? 60 : \
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(n) & (1ULL << 59) ? 59 : \
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(n) & (1ULL << 58) ? 58 : \
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(n) & (1ULL << 57) ? 57 : \
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(n) & (1ULL << 56) ? 56 : \
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(n) & (1ULL << 55) ? 55 : \
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(n) & (1ULL << 54) ? 54 : \
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(n) & (1ULL << 53) ? 53 : \
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(n) & (1ULL << 52) ? 52 : \
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(n) & (1ULL << 51) ? 51 : \
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(n) & (1ULL << 50) ? 50 : \
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(n) & (1ULL << 49) ? 49 : \
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(n) & (1ULL << 48) ? 48 : \
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(n) & (1ULL << 47) ? 47 : \
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(n) & (1ULL << 46) ? 46 : \
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(n) & (1ULL << 45) ? 45 : \
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(n) & (1ULL << 44) ? 44 : \
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(n) & (1ULL << 43) ? 43 : \
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(n) & (1ULL << 42) ? 42 : \
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(n) & (1ULL << 41) ? 41 : \
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(n) & (1ULL << 40) ? 40 : \
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(n) & (1ULL << 39) ? 39 : \
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(n) & (1ULL << 38) ? 38 : \
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(n) & (1ULL << 37) ? 37 : \
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(n) & (1ULL << 36) ? 36 : \
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(n) & (1ULL << 35) ? 35 : \
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(n) & (1ULL << 34) ? 34 : \
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(n) & (1ULL << 33) ? 33 : \
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(n) & (1ULL << 32) ? 32 : \
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(n) & (1ULL << 31) ? 31 : \
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(n) & (1ULL << 30) ? 30 : \
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(n) & (1ULL << 29) ? 29 : \
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(n) & (1ULL << 28) ? 28 : \
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(n) & (1ULL << 27) ? 27 : \
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(n) & (1ULL << 26) ? 26 : \
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(n) & (1ULL << 25) ? 25 : \
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(n) & (1ULL << 24) ? 24 : \
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(n) & (1ULL << 23) ? 23 : \
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(n) & (1ULL << 22) ? 22 : \
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(n) & (1ULL << 21) ? 21 : \
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(n) & (1ULL << 20) ? 20 : \
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(n) & (1ULL << 19) ? 19 : \
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(n) & (1ULL << 18) ? 18 : \
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(n) & (1ULL << 17) ? 17 : \
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(n) & (1ULL << 16) ? 16 : \
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(n) & (1ULL << 15) ? 15 : \
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(n) & (1ULL << 14) ? 14 : \
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(n) & (1ULL << 13) ? 13 : \
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(n) & (1ULL << 12) ? 12 : \
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(n) & (1ULL << 11) ? 11 : \
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(n) & (1ULL << 10) ? 10 : \
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(n) & (1ULL << 9) ? 9 : \
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(n) & (1ULL << 8) ? 8 : \
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(n) & (1ULL << 7) ? 7 : \
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(n) & (1ULL << 6) ? 6 : \
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(n) & (1ULL << 5) ? 5 : \
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(n) & (1ULL << 4) ? 4 : \
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(n) & (1ULL << 3) ? 3 : \
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(n) & (1ULL << 2) ? 2 : \
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1 ) : \
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(sizeof(n) <= 4) ? \
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__ilog2_u32(n) : \
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__ilog2_u64(n) \
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)
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/**
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* roundup_pow_of_two - round the given value up to nearest power of two
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* @n - parameter
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*
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* round the given value up to the nearest power of two
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* - the result is undefined when n == 0
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* - this can be used to initialise global variables from constant data
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*/
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#define roundup_pow_of_two(n) \
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( \
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__builtin_constant_p(n) ? ( \
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(n == 1) ? 1 : \
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(1UL << (ilog2((n) - 1) + 1)) \
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) : \
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__roundup_pow_of_two(n) \
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)
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/**
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* rounddown_pow_of_two - round the given value down to nearest power of two
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* @n - parameter
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*
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* round the given value down to the nearest power of two
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* - the result is undefined when n == 0
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* - this can be used to initialise global variables from constant data
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*/
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#define rounddown_pow_of_two(n) \
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( \
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__builtin_constant_p(n) ? ( \
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(1UL << ilog2(n))) : \
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__rounddown_pow_of_two(n) \
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)
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#endif /* _TOOLS_LINUX_LOG2_H */
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