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serge-sans-pailleserge-sans-paille
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[libc++] Implement std::gcd using the binary version
The binary version is four times faster than current implementation in my setup, and generally considered a better implementation. Code inspired by https://en.algorithmica.org/hpc/algorithms/gcd/ which itself is inspired by https://lemire.me/blog/2013/12/26/fastest-way-to-compute-the-greatest-common-divisor/ Fix #77648
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10 files changed

+176
-4
lines changed

10 files changed

+176
-4
lines changed

‎libcxx/benchmarks/CMakeLists.txt

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -122,7 +122,7 @@ endif()
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add_library( cxx-benchmarks-flags-libcxx INTERFACE)
123123
target_link_libraries( cxx-benchmarks-flags-libcxx INTERFACE cxx-benchmarks-flags)
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target_compile_options(cxx-benchmarks-flags-libcxx INTERFACE ${SANITIZER_FLAGS} -Wno-user-defined-literals -Wno-suggest-override)
125-
target_link_options( cxx-benchmarks-flags-libcxx INTERFACE -nostdlib++ "-L${BENCHMARK_LIBCXX_INSTALL}/lib" "-L${BENCHMARK_LIBCXX_INSTALL}/lib64" ${SANITIZER_FLAGS})
125+
target_link_options( cxx-benchmarks-flags-libcxx INTERFACE -lm -nostdlib++ "-L${BENCHMARK_LIBCXX_INSTALL}/lib" "-L${BENCHMARK_LIBCXX_INSTALL}/lib64" ${SANITIZER_FLAGS})
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127127
set(libcxx_benchmark_targets)
128128

@@ -220,6 +220,7 @@ set(BENCHMARK_TESTS
220220
lexicographical_compare_three_way.bench.cpp
221221
map.bench.cpp
222222
monotonic_buffer.bench.cpp
223+
numeric/gcd.bench.cpp
223224
ordered_set.bench.cpp
224225
shared_mutex_vs_mutex.bench.cpp
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stop_token.bench.cpp

‎libcxx/benchmarks/numeric/gcd.bench.cpp

Lines changed: 33 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,33 @@
1+
//===----------------------------------------------------------------------===//
2+
//
3+
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4+
// See https://llvm.org/LICENSE.txt for license information.
5+
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6+
//
7+
//===----------------------------------------------------------------------===//
8+
9+
#include <array>
10+
#include <benchmark/benchmark.h>
11+
#include <cstring>
12+
#include <numeric>
13+
#include <random>
14+
15+
template <class T>
16+
static std::array<T, 1000> generate(std::uniform_int_distribution<T> distribution = std::uniform_int_distribution<T>{
17+
std::numeric_limits<T>::min(), std::numeric_limits<T>::max()}) {
18+
std::mt19937 generator;
19+
std::array<T, 1000> result;
20+
std::generate_n(result.begin(), result.size(), [&] { return distribution(generator); });
21+
return result;
22+
}
23+
24+
static void bm_gcd(benchmark::State& state) {
25+
std::array data = generate<int>();
26+
while (state.KeepRunningBatch(data.size()))
27+
for (auto v0 : data)
28+
for (auto v1 : data)
29+
benchmark::DoNotOptimize(std::gcd(v0, v1));
30+
}
31+
BENCHMARK(bm_gcd);
32+
33+
BENCHMARK_MAIN();

‎libcxx/include/__numeric/gcd_lcm.h

Lines changed: 31 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -10,7 +10,9 @@
1010
#ifndef _LIBCPP___NUMERIC_GCD_LCM_H
1111
#define _LIBCPP___NUMERIC_GCD_LCM_H
1212

13+
#include <__algorithm/min.h>
1314
#include <__assert>
15+
#include <__bit/countr.h>
1416
#include <__config>
1517
#include <__type_traits/common_type.h>
1618
#include <__type_traits/is_integral.h>
@@ -50,9 +52,36 @@ struct __ct_abs<_Result, _Source, false> {
5052
};
5153

5254
template <class _Tp>
53-
_LIBCPP_CONSTEXPR _LIBCPP_HIDDEN _Tp __gcd(_Tp __m, _Tp __n) {
55+
_LIBCPP_CONSTEXPR _LIBCPP_HIDDEN _Tp __gcd(_Tp __a, _Tp __b) {
5456
static_assert((!is_signed<_Tp>::value), "");
55-
return __n == 0 ? __m : std::__gcd<_Tp>(__n, __m % __n);
57+
if (__a < __b) {
58+
_Tp __tmp = __b;
59+
__b = __a;
60+
__a = __tmp;
61+
}
62+
if (__b == 0)
63+
return __a;
64+
__a %= __b; // Make both argument of the same size, and early result in the easy case.
65+
if (__a == 0)
66+
return __b;
67+
68+
int __az = std::__countr_zero(__a);
69+
int __bz = std::__countr_zero(__b);
70+
int __shift = std::min(__az, __bz);
71+
__a >>= __az;
72+
__b >>= __bz;
73+
do {
74+
_Tp __diff = __a - __b;
75+
if (__a > __b) {
76+
__a = __b;
77+
__b = __diff;
78+
} else {
79+
__b = __b - __a;
80+
}
81+
if (__diff != 0)
82+
__b >>= std::__countr_zero(__diff);
83+
} while (__b != 0);
84+
return __a << __shift;
5685
}
5786

5887
template <class _Tp, class _Up>

‎libcxx/test/libcxx/transitive_includes/cxx03.csv

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Original file line numberDiff line numberDiff line change
@@ -558,6 +558,7 @@ numeric cstddef
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numeric cstdint
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numeric execution
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numeric functional
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numeric initializer_list
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numeric iterator
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numeric limits
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numeric new

‎libcxx/test/libcxx/transitive_includes/cxx11.csv

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@@ -563,6 +563,7 @@ numeric cstddef
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numeric cstdint
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numeric execution
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numeric functional
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numeric initializer_list
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numeric iterator
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numeric limits
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‎libcxx/test/libcxx/transitive_includes/cxx14.csv

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@@ -566,6 +566,7 @@ numeric cstddef
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numeric cstdint
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numeric execution
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numeric functional
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numeric initializer_list
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numeric iterator
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numeric limits
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‎libcxx/test/libcxx/transitive_includes/cxx17.csv

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@@ -566,6 +566,7 @@ numeric cstddef
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numeric cstdint
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numeric execution
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numeric functional
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numeric initializer_list
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numeric iterator
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numeric limits
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numeric new

‎libcxx/test/libcxx/transitive_includes/cxx20.csv

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@@ -576,6 +576,7 @@ numeric cstddef
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numeric cstdint
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numeric execution
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numeric functional
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numeric initializer_list
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numeric iterator
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numeric limits
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numeric new

‎libcxx/test/libcxx/transitive_includes/cxx26.csv

Lines changed: 23 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -176,6 +176,29 @@ experimental/simd limits
176176
experimental/type_traits initializer_list
177177
experimental/type_traits type_traits
178178
experimental/utility utility
179+
experimental/vector experimental/memory_resource
180+
experimental/vector vector
181+
ext/hash_map algorithm
182+
ext/hash_map cmath
183+
ext/hash_map cstddef
184+
ext/hash_map cstdint
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ext/hash_map cstring
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ext/hash_map functional
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ext/hash_map initializer_list
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ext/hash_map limits
189+
ext/hash_map new
190+
ext/hash_map stdexcept
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ext/hash_map string
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ext/hash_set algorithm
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ext/hash_set cmath
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ext/hash_set cstddef
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ext/hash_set cstdint
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ext/hash_set cstring
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ext/hash_set functional
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ext/hash_set initializer_list
199+
ext/hash_set limits
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ext/hash_set new
201+
ext/hash_set string
179202
filesystem compare
180203
filesystem cstddef
181204
filesystem cstdint

‎libcxx/test/std/numerics/numeric.ops/numeric.ops.gcd/gcd.pass.cpp

Lines changed: 82 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -17,6 +17,7 @@
1717
#include <cassert>
1818
#include <climits>
1919
#include <cstdint>
20+
#include <random>
2021
#include <type_traits>
2122

2223
#include "test_macros.h"
@@ -48,6 +49,68 @@ constexpr bool test0(int in1, int in2, int out)
4849
return true;
4950
}
5051

52+
template <typename T>
53+
T basic_gcd_(T m, T n) {
54+
return n == 0 ? m : basic_gcd_<T>(n, m % n);
55+
}
56+
57+
template <typename T>
58+
T basic_gcd(T m, T n) {
59+
using Tp = std::make_unsigned_t<T>;
60+
if (m < 0 && m != std::numeric_limits<T>::min())
61+
m = -m;
62+
if (n < 0 && n != std::numeric_limits<T>::min())
63+
n = -n;
64+
return basic_gcd_(static_cast<Tp>(m), static_cast<Tp>(n));
65+
}
66+
67+
template <typename Input>
68+
void do_fuzzy_tests() {
69+
std::mt19937 gen(1938);
70+
std::uniform_int_distribution<Input> distrib;
71+
72+
constexpr int nb_rounds = 10000;
73+
for (int i = 0; i < nb_rounds; ++i) {
74+
Input n = distrib(gen);
75+
Input m = distrib(gen);
76+
assert(std::gcd(n, m) == basic_gcd(n, m));
77+
}
78+
}
79+
80+
template <typename Input>
81+
void do_limit_tests() {
82+
Input inputs[] = {
83+
std::numeric_limits<Input>::min(),
84+
std::numeric_limits<Input>::max(),
85+
0,
86+
1,
87+
2,
88+
3,
89+
4,
90+
5,
91+
6,
92+
7,
93+
8,
94+
9,
95+
10,
96+
(Input)-1,
97+
(Input)-2,
98+
(Input)-3,
99+
(Input)-4,
100+
(Input)-5,
101+
(Input)-6,
102+
(Input)-7,
103+
(Input)-8,
104+
(Input)-9,
105+
(Input)-10,
106+
};
107+
108+
for (auto n : inputs) {
109+
for (auto m : inputs) {
110+
assert(std::gcd(n, m) == basic_gcd(n, m));
111+
}
112+
}
113+
}
51114

52115
template <typename Input1, typename Input2 = Input1>
53116
constexpr bool do_test(int = 0)
@@ -143,5 +206,23 @@ int main(int argc, char**)
143206
assert(res == 2);
144207
}
145208

146-
return 0;
209+
do_fuzzy_tests<std::int8_t>();
210+
do_fuzzy_tests<std::int16_t>();
211+
do_fuzzy_tests<std::int32_t>();
212+
do_fuzzy_tests<std::int64_t>();
213+
do_fuzzy_tests<std::uint8_t>();
214+
do_fuzzy_tests<std::uint16_t>();
215+
do_fuzzy_tests<std::uint32_t>();
216+
do_fuzzy_tests<std::uint64_t>();
217+
218+
do_limit_tests<std::int8_t>();
219+
do_limit_tests<std::int16_t>();
220+
do_limit_tests<std::int32_t>();
221+
do_limit_tests<std::int64_t>();
222+
do_limit_tests<std::uint8_t>();
223+
do_limit_tests<std::uint16_t>();
224+
do_limit_tests<std::uint32_t>();
225+
do_limit_tests<std::uint64_t>();
226+
227+
return 0;
147228
}

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