// Copyright (c) 2013 The Chromium Authors. All rights reserved. // Use of this source code is governed by a BSD-style license that can be // found in the LICENSE file. // This file defines some bit utilities. #ifndef BASE_BITS_H_ #define BASE_BITS_H_ #include #include #include #include #include "base/check.h" #include "base/compiler_specific.h" #include "build/build_config.h" namespace base { namespace bits { // Returns true iff |value| is a power of 2. // // TODO(pkasting): When C++20 is available, replace with std::has_single_bit(). template ::value>> constexpr bool IsPowerOfTwo(T value) { // From "Hacker's Delight": Section 2.1 Manipulating Rightmost Bits. // // Only positive integers with a single bit set are powers of two. If only one // bit is set in x (e.g. 0b00000100000000) then |x-1| will have that bit set // to zero and all bits to its right set to 1 (e.g. 0b00000011111111). Hence // |x & (x-1)| is 0 iff x is a power of two. return value > 0 && (value & (value - 1)) == 0; } // Round down |size| to a multiple of alignment, which must be a power of two. template >> constexpr T AlignDown(T size, T alignment) { DCHECK(IsPowerOfTwo(alignment)); return size & ~(alignment - 1); } // Move |ptr| back to the previous multiple of alignment, which must be a power // of two. Defined for types where sizeof(T) is one byte. template ::type> inline T* AlignDown(T* ptr, uintptr_t alignment) { return reinterpret_cast( AlignDown(reinterpret_cast(ptr), alignment)); } // Round up |size| to a multiple of alignment, which must be a power of two. template >> constexpr T AlignUp(T size, T alignment) { DCHECK(IsPowerOfTwo(alignment)); return (size + alignment - 1) & ~(alignment - 1); } // Advance |ptr| to the next multiple of alignment, which must be a power of // two. Defined for types where sizeof(T) is one byte. template ::type> inline T* AlignUp(T* ptr, uintptr_t alignment) { return reinterpret_cast( AlignUp(reinterpret_cast(ptr), alignment)); } // CountLeadingZeroBits(value) returns the number of zero bits following the // most significant 1 bit in |value| if |value| is non-zero, otherwise it // returns {sizeof(T) * 8}. // Example: 00100010 -> 2 // // CountTrailingZeroBits(value) returns the number of zero bits preceding the // least significant 1 bit in |value| if |value| is non-zero, otherwise it // returns {sizeof(T) * 8}. // Example: 00100010 -> 1 // // C does not have an operator to do this, but fortunately the various // compilers have built-ins that map to fast underlying processor instructions. // // TODO(pkasting): When C++20 is available, replace with std::countl_zero() and // similar. // __builtin_clz has undefined behaviour for an input of 0, even though there's // clearly a return value that makes sense, and even though some processor clz // instructions have defined behaviour for 0. We could drop to raw __asm__ to // do better, but we'll avoid doing that unless we see proof that we need to. template ALWAYS_INLINE constexpr typename std::enable_if::value && sizeof(T) <= 8, int>::type CountLeadingZeroBits(T value) { static_assert(bits > 0, "invalid instantiation"); return LIKELY(value) ? bits == 64 ? __builtin_clzll(static_cast(value)) : __builtin_clz(static_cast(value)) - (32 - bits) : bits; } template ALWAYS_INLINE constexpr typename std::enable_if::value && sizeof(T) <= 8, int>::type CountTrailingZeroBits(T value) { return LIKELY(value) ? bits == 64 ? __builtin_ctzll(static_cast(value)) : __builtin_ctz(static_cast(value)) : bits; } // Returns the integer i such as 2^i <= n < 2^(i+1). // // There is a common `BitLength` function, which returns the number of bits // required to represent a value. Rather than implement that function, // use `Log2Floor` and add 1 to the result. // // TODO(pkasting): When C++20 is available, replace with std::bit_xxx(). constexpr int Log2Floor(uint32_t n) { return 31 - CountLeadingZeroBits(n); } // Returns the integer i such as 2^(i-1) < n <= 2^i. constexpr int Log2Ceiling(uint32_t n) { // When n == 0, we want the function to return -1. // When n == 0, (n - 1) will underflow to 0xFFFFFFFF, which is // why the statement below starts with (n ? 32 : -1). return (n ? 32 : -1) - CountLeadingZeroBits(n - 1); } // Returns a value of type T with a single bit set in the left-most position. // Can be used instead of manually shifting a 1 to the left. template constexpr T LeftmostBit() { static_assert(std::is_integral::value, "This function can only be used with integral types."); T one(1u); return one << ((CHAR_BIT * sizeof(T) - 1)); } } // namespace bits } // namespace base #endif // BASE_BITS_H_