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fix utils::ctz() when builtins are not available

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Our fallback 64-bits ctz() was completely wrong. 
The unit tests were wrong too.
This commit is contained in:
Pixelflinger
2020-03-09 23:12:29 -07:00
committed by Mathias Agopian
parent a2c6c34739
commit 8fb65a19b4
2 changed files with 47 additions and 41 deletions
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62
libs/utils/include/utils/algorithm.h
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@@ -19,7 +19,8 @@
#include <utils/compiler.h>
#include <functional>
#include <functional> // for std::less
#include <type_traits> // for std::enable_if
#include <limits.h>
#include <stdint.h>
@@ -27,46 +28,51 @@
namespace utils {
namespace details {
template<typename T>
constexpr inline T popcount(T v) noexcept {
static_assert(sizeof(T) * 8 <= 128, "details::popcount() only support up to 128 bits");
static_assert(sizeof(T) * CHAR_BIT <= 128, "details::popcount() only support up to 128 bits");
constexpr T ONES = ~T(0);
v = v - ((v >> 1) & ONES / 3);
v = (v & ONES / 15 * 3) + ((v >> 2) & ONES / 15 * 3);
v = (v + (v >> 4)) & ONES / 255 * 15;
v = v - ((v >> 1u) & ONES / 3);
v = (v & ONES / 15 * 3) + ((v >> 2u) & ONES / 15 * 3);
v = (v + (v >> 4u)) & ONES / 255 * 15;
return (T) (v * (ONES / 255)) >> (sizeof(T) - 1) * CHAR_BIT;
}
template<typename T>
template<typename T, typename = std::enable_if_t<std::is_unsigned<T>::value>>
constexpr inline T clz(T x) noexcept {
static_assert(sizeof(T) <= sizeof(uint64_t), "details::clz() only support up to 64 bits");
x |= (x >> 1);
x |= (x >> 2);
x |= (x >> 4);
x |= (x >> 8);
x |= (x >> 16);
if (sizeof(T) * 8 > 32) { // if() only needed to quash compiler warnings
x |= (x >> 32);
static_assert(sizeof(T) * CHAR_BIT <= 128, "details::clz() only support up to 128 bits");
x |= (x >> 1u);
x |= (x >> 2u);
x |= (x >> 4u);
x |= (x >> 8u);
x |= (x >> 16u);
if (sizeof(T) * CHAR_BIT >= 64) { // just to silence compiler warning
x |= (x >> 32u);
}
return (sizeof(T) * CHAR_BIT) - details::popcount(x);
if (sizeof(T) * CHAR_BIT >= 128) { // just to silence compiler warning
x |= (x >> 64u);
}
return T(sizeof(T) * CHAR_BIT) - details::popcount(x);
}
template<typename T>
template<typename T, typename = std::enable_if_t<std::is_unsigned<T>::value>>
constexpr inline T ctz(T x) noexcept {
static_assert(sizeof(T) <= sizeof(uint64_t), "details::ctz() only support up to 64 bits");
T c = sizeof(T) * 8;
x &= -signed(x);
static_assert(sizeof(T) * CHAR_BIT <= 64, "details::ctz() only support up to 64 bits");
T c = sizeof(T) * CHAR_BIT;
x &= -x; // equivalent to x & (~x + 1)
if (x) c--;
if (sizeof(T) * 8 > 32) { // if() only needed to quash compiler warnings
if (x & 0x00000000FFFFFFFF) c -= 32;
if (sizeof(T) * CHAR_BIT >= 64) {
if (x & T(0x00000000FFFFFFFF)) c -= 32;
}
if (x & 0x0000FFFF) c -= 16;
if (x & 0x00FF00FF) c -= 8;
if (x & 0x0F0F0F0F) c -= 4;
if (x & 0x33333333) c -= 2;
if (x & 0x55555555) c -= 1;
if (x & T(0x0000FFFF0000FFFF)) c -= 16;
if (x & T(0x00FF00FF00FF00FF)) c -= 8;
if (x & T(0x0F0F0F0F0F0F0F0F)) c -= 4;
if (x & T(0x3333333333333333)) c -= 2;
if (x & T(0x5555555555555555)) c -= 1;
return c;
}
} // namespace details
constexpr inline UTILS_PUBLIC UTILS_PURE
@@ -249,8 +255,8 @@ RandomAccessIterator partition_point(
// The number of repetitions here doesn't affect the result. We manually unroll the loop
// twice, to guarantee we have at least two iterations without branches (for the case
// where the size is not known at compile time
first += pred(first[len>>=1]) ? len : 0;
first += pred(first[len>>=1]) ? len : 0;
first += pred(first[len>>=1u]) ? len : 0;
first += pred(first[len>>=1u]) ? len : 0;
}
first += pred(*first);
return first;