Commit 6aa05bb9 authored by Philip Pronin's avatar Philip Pronin Committed by Jordan DeLong

constexpr-ize folly/Bits.h

Summary:
Want to be able to call them from other constexpr functions
and from within static_assert()s.

Test Plan: compiled and ran folly tests

Reviewed By: tudorb@fb.com

FB internal diff: D627413
parent 91d0ef0e
...@@ -17,24 +17,24 @@ ...@@ -17,24 +17,24 @@
/** /**
* Various low-level, bit-manipulation routines. * Various low-level, bit-manipulation routines.
* *
* findFirstSet(x) * findFirstSet(x) [constexpr]
* find first (least significant) bit set in a value of an integral type, * find first (least significant) bit set in a value of an integral type,
* 1-based (like ffs()). 0 = no bits are set (x == 0) * 1-based (like ffs()). 0 = no bits are set (x == 0)
* *
* findLastSet(x) * findLastSet(x) [constexpr]
* find last (most significant) bit set in a value of an integral type, * find last (most significant) bit set in a value of an integral type,
* 1-based. 0 = no bits are set (x == 0) * 1-based. 0 = no bits are set (x == 0)
* for x != 0, findLastSet(x) == 1 + floor(log2(x)) * for x != 0, findLastSet(x) == 1 + floor(log2(x))
* *
* popcount(x) * nextPowTwo(x) [constexpr]
* return the number of 1 bits in x
*
* nextPowTwo(x)
* Finds the next power of two >= x. * Finds the next power of two >= x.
* *
* isPowTwo(x) * isPowTwo(x) [constexpr]
* return true iff x is a power of two * return true iff x is a power of two
* *
* popcount(x)
* return the number of 1 bits in x
*
* Endian * Endian
* convert between native, big, and little endian representation * convert between native, big, and little endian representation
* Endian::big(x) big <-> native * Endian::big(x) big <-> native
...@@ -84,6 +84,7 @@ namespace folly { ...@@ -84,6 +84,7 @@ namespace folly {
// Generate overloads for findFirstSet as wrappers around // Generate overloads for findFirstSet as wrappers around
// appropriate ffs, ffsl, ffsll gcc builtins // appropriate ffs, ffsl, ffsll gcc builtins
template <class T> template <class T>
inline constexpr
typename std::enable_if< typename std::enable_if<
(std::is_integral<T>::value && (std::is_integral<T>::value &&
std::is_unsigned<T>::value && std::is_unsigned<T>::value &&
...@@ -94,6 +95,7 @@ typename std::enable_if< ...@@ -94,6 +95,7 @@ typename std::enable_if<
} }
template <class T> template <class T>
inline constexpr
typename std::enable_if< typename std::enable_if<
(std::is_integral<T>::value && (std::is_integral<T>::value &&
std::is_unsigned<T>::value && std::is_unsigned<T>::value &&
...@@ -105,6 +107,7 @@ typename std::enable_if< ...@@ -105,6 +107,7 @@ typename std::enable_if<
} }
template <class T> template <class T>
inline constexpr
typename std::enable_if< typename std::enable_if<
(std::is_integral<T>::value && (std::is_integral<T>::value &&
std::is_unsigned<T>::value && std::is_unsigned<T>::value &&
...@@ -116,6 +119,7 @@ typename std::enable_if< ...@@ -116,6 +119,7 @@ typename std::enable_if<
} }
template <class T> template <class T>
inline constexpr
typename std::enable_if< typename std::enable_if<
(std::is_integral<T>::value && std::is_signed<T>::value), (std::is_integral<T>::value && std::is_signed<T>::value),
unsigned int>::type unsigned int>::type
...@@ -129,6 +133,7 @@ typename std::enable_if< ...@@ -129,6 +133,7 @@ typename std::enable_if<
// findLastSet: return the 1-based index of the highest bit set // findLastSet: return the 1-based index of the highest bit set
// for x > 0, findLastSet(x) == 1 + floor(log2(x)) // for x > 0, findLastSet(x) == 1 + floor(log2(x))
template <class T> template <class T>
inline constexpr
typename std::enable_if< typename std::enable_if<
(std::is_integral<T>::value && (std::is_integral<T>::value &&
std::is_unsigned<T>::value && std::is_unsigned<T>::value &&
...@@ -139,6 +144,7 @@ typename std::enable_if< ...@@ -139,6 +144,7 @@ typename std::enable_if<
} }
template <class T> template <class T>
inline constexpr
typename std::enable_if< typename std::enable_if<
(std::is_integral<T>::value && (std::is_integral<T>::value &&
std::is_unsigned<T>::value && std::is_unsigned<T>::value &&
...@@ -150,6 +156,7 @@ typename std::enable_if< ...@@ -150,6 +156,7 @@ typename std::enable_if<
} }
template <class T> template <class T>
inline constexpr
typename std::enable_if< typename std::enable_if<
(std::is_integral<T>::value && (std::is_integral<T>::value &&
std::is_unsigned<T>::value && std::is_unsigned<T>::value &&
...@@ -161,6 +168,7 @@ typename std::enable_if< ...@@ -161,6 +168,7 @@ typename std::enable_if<
} }
template <class T> template <class T>
inline constexpr
typename std::enable_if< typename std::enable_if<
(std::is_integral<T>::value && (std::is_integral<T>::value &&
std::is_signed<T>::value), std::is_signed<T>::value),
...@@ -170,24 +178,21 @@ typename std::enable_if< ...@@ -170,24 +178,21 @@ typename std::enable_if<
} }
template <class T> template <class T>
inline inline constexpr
typename std::enable_if< typename std::enable_if<
std::is_integral<T>::value && std::is_unsigned<T>::value, std::is_integral<T>::value && std::is_unsigned<T>::value,
T>::type T>::type
nextPowTwo(T v) { nextPowTwo(T v) {
if (UNLIKELY(v == 0)) { return v ? (1ul << findLastSet(v - 1)) : 1;
return 1;
}
return 1ul << findLastSet(v - 1);
} }
template <class T> template <class T>
inline inline constexpr
typename std::enable_if< typename std::enable_if<
std::is_integral<T>::value && std::is_unsigned<T>::value, std::is_integral<T>::value && std::is_unsigned<T>::value,
bool>::type bool>::type
isPowTwo(T v) { isPowTwo(T v) {
return ((v != 0) && !(v & (v-1))); // yes, this is endian-agnostic return (v != 0) && !(v & (v - 1));
} }
/** /**
......
...@@ -23,6 +23,12 @@ ...@@ -23,6 +23,12 @@
using namespace folly; using namespace folly;
// Test constexpr-ness.
static_assert(findFirstSet(2u) == 2, "findFirstSet");
static_assert(findLastSet(2u) == 2, "findLastSet");
static_assert(nextPowTwo(2u) == 2, "nextPowTwo");
static_assert(isPowTwo(2u), "isPowTwo");
namespace { namespace {
template <class INT> template <class INT>
......
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