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- // 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 <limits.h>
- #include <stddef.h>
- #include <stdint.h>
- #include <type_traits>
- #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 <typename T, typename = std::enable_if_t<std::is_integral<T>::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 <typename T, typename = std::enable_if_t<std::is_integral_v<T>>>
- 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 <typename T, typename = typename std::enable_if<sizeof(T) == 1>::type>
- inline T* AlignDown(T* ptr, uintptr_t alignment) {
- return reinterpret_cast<T*>(
- AlignDown(reinterpret_cast<uintptr_t>(ptr), alignment));
- }
- // Round up |size| to a multiple of alignment, which must be a power of two.
- template <typename T, typename = std::enable_if_t<std::is_integral_v<T>>>
- 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 <typename T, typename = typename std::enable_if<sizeof(T) == 1>::type>
- inline T* AlignUp(T* ptr, uintptr_t alignment) {
- return reinterpret_cast<T*>(
- AlignUp(reinterpret_cast<uintptr_t>(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 <typename T, int bits = sizeof(T) * 8>
- ALWAYS_INLINE constexpr
- typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) <= 8,
- int>::type
- CountLeadingZeroBits(T value) {
- static_assert(bits > 0, "invalid instantiation");
- return LIKELY(value)
- ? bits == 64
- ? __builtin_clzll(static_cast<uint64_t>(value))
- : __builtin_clz(static_cast<uint32_t>(value)) - (32 - bits)
- : bits;
- }
- template <typename T, int bits = sizeof(T) * 8>
- ALWAYS_INLINE constexpr
- typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) <= 8,
- int>::type
- CountTrailingZeroBits(T value) {
- return LIKELY(value) ? bits == 64
- ? __builtin_ctzll(static_cast<uint64_t>(value))
- : __builtin_ctz(static_cast<uint32_t>(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 <typename T>
- constexpr T LeftmostBit() {
- static_assert(std::is_integral<T>::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_
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