[libcxx-commits] [libcxx] linear_congruential_engine: add using more precision to prevent overflow (PR #81583)
via libcxx-commits
libcxx-commits at lists.llvm.org
Fri Feb 16 18:32:02 PST 2024
https://github.com/LRFLEW updated https://github.com/llvm/llvm-project/pull/81583
>From 3cae6fa11dd0bc7aeb4b1f870148aa2af528da73 Mon Sep 17 00:00:00 2001
From: LRFLEW <LRFLEW at aol.com>
Date: Tue, 13 Feb 2024 01:19:04 -0600
Subject: [PATCH] linear_congruential_engine: add using more precision to
prevent overflow
---
.../__random/linear_congruential_engine.h | 86 +++++++++++++++----
.../rand/rand.eng/rand.eng.lcong/alg.pass.cpp | 65 ++++++++++----
.../rand.eng/rand.eng.lcong/assign.pass.cpp | 15 +++-
.../rand.eng/rand.eng.lcong/copy.pass.cpp | 15 +++-
.../rand.eng/rand.eng.lcong/default.pass.cpp | 15 +++-
.../rand.eng/rand.eng.lcong/values.pass.cpp | 15 +++-
6 files changed, 169 insertions(+), 42 deletions(-)
diff --git a/libcxx/include/__random/linear_congruential_engine.h b/libcxx/include/__random/linear_congruential_engine.h
index fe9cb909b74d21..26f3b820f7a844 100644
--- a/libcxx/include/__random/linear_congruential_engine.h
+++ b/libcxx/include/__random/linear_congruential_engine.h
@@ -30,28 +30,45 @@ template <unsigned long long __a,
unsigned long long __c,
unsigned long long __m,
unsigned long long _Mp,
- bool _MightOverflow = (__a != 0 && __m != 0 && __m - 1 > (_Mp - __c) / __a),
- bool _OverflowOK = ((__m & (__m - 1)) == 0ull), // m = 2^n
- bool _SchrageOK = (__a != 0 && __m != 0 && __m % __a <= __m / __a)> // r <= q
+ bool _HasOverflow = (__a != 0ull && (__m & (__m - 1ull)) != 0ull), // a != 0, m != 0, m != 2^n
+ bool _Full = (!_HasOverflow || __m - 1ull <= (_Mp - __c) / __a), // (a * x + c) % m works
+ bool _Part = (!_HasOverflow || __m - 1ull <= _Mp / __a), // (a * x) % m works
+ bool _Schrage = (_HasOverflow && __m % __a <= __m / __a)> // r <= q
struct __lce_alg_picker {
- static_assert(!_MightOverflow || _OverflowOK || _SchrageOK,
- "The current values of a, c, and m cannot generate a number "
- "within bounds of linear_congruential_engine.");
+ static _LIBCPP_CONSTEXPR const int __mode = _Full ? 0 : _Part ? 1 : _Schrage ? 2 : 3;
- static _LIBCPP_CONSTEXPR const bool __use_schrage = _MightOverflow && !_OverflowOK && _SchrageOK;
+#ifndef __SIZEOF_INT128__
+ static_assert(_Mp != (unsigned long long)(~0) || _Full || _Part || _Schrage,
+ "The current values for a, c, and m are not currently supported on platforms without __int128");
+#endif
};
template <unsigned long long __a,
unsigned long long __c,
unsigned long long __m,
unsigned long long _Mp,
- bool _UseSchrage = __lce_alg_picker<__a, __c, __m, _Mp>::__use_schrage>
+ int _Mode = __lce_alg_picker<__a, __c, __m, _Mp>::__mode>
struct __lce_ta;
// 64
+#ifdef __SIZEOF_INT128__
+template <unsigned long long _Ap, unsigned long long _Cp, unsigned long long _Mp>
+struct __lce_ta<_Ap, _Cp, _Mp, (unsigned long long)(~0), 3> {
+ typedef unsigned long long result_type;
+ _LIBCPP_HIDE_FROM_ABI static result_type next(result_type _Xp) {
+ __extension__ typedef unsigned __int128 calc_type;
+ const calc_type __a = static_cast<calc_type>(_Ap);
+ const calc_type __c = static_cast<calc_type>(_Cp);
+ const calc_type __m = static_cast<calc_type>(_Mp);
+ const calc_type __x = static_cast<calc_type>(_Xp);
+ return static_cast<result_type>((__a * __x + __c) % __m);
+ }
+};
+#endif
+
template <unsigned long long __a, unsigned long long __c, unsigned long long __m>
-struct __lce_ta<__a, __c, __m, (unsigned long long)(~0), true> {
+struct __lce_ta<__a, __c, __m, (unsigned long long)(~0), 2> {
typedef unsigned long long result_type;
_LIBCPP_HIDE_FROM_ABI static result_type next(result_type __x) {
// Schrage's algorithm
@@ -66,7 +83,7 @@ struct __lce_ta<__a, __c, __m, (unsigned long long)(~0), true> {
};
template <unsigned long long __a, unsigned long long __m>
-struct __lce_ta<__a, 0, __m, (unsigned long long)(~0), true> {
+struct __lce_ta<__a, 0ull, __m, (unsigned long long)(~0), 2> {
typedef unsigned long long result_type;
_LIBCPP_HIDE_FROM_ABI static result_type next(result_type __x) {
// Schrage's algorithm
@@ -80,21 +97,40 @@ struct __lce_ta<__a, 0, __m, (unsigned long long)(~0), true> {
};
template <unsigned long long __a, unsigned long long __c, unsigned long long __m>
-struct __lce_ta<__a, __c, __m, (unsigned long long)(~0), false> {
+struct __lce_ta<__a, __c, __m, (unsigned long long)(~0), 1> {
+ typedef unsigned long long result_type;
+ _LIBCPP_HIDE_FROM_ABI static result_type next(result_type __x) {
+ // Use (((a*x) % m) + c) % m
+ __x = (__a * __x) % __m;
+ __x += __c - (__x >= __m - __c) * __m;
+ return __x;
+ }
+};
+
+template <unsigned long long __a, unsigned long long __c, unsigned long long __m>
+struct __lce_ta<__a, __c, __m, (unsigned long long)(~0), 0> {
typedef unsigned long long result_type;
_LIBCPP_HIDE_FROM_ABI static result_type next(result_type __x) { return (__a * __x + __c) % __m; }
};
template <unsigned long long __a, unsigned long long __c>
-struct __lce_ta<__a, __c, 0, (unsigned long long)(~0), false> {
+struct __lce_ta<__a, __c, 0ull, (unsigned long long)(~0), 0> {
typedef unsigned long long result_type;
_LIBCPP_HIDE_FROM_ABI static result_type next(result_type __x) { return __a * __x + __c; }
};
// 32
+template <unsigned long long __a, unsigned long long __c, unsigned long long __m>
+struct __lce_ta<__a, __c, __m, unsigned(~0), 3> {
+ typedef unsigned result_type;
+ _LIBCPP_HIDE_FROM_ABI static result_type next(result_type __x) {
+ return static_cast<result_type>(__lce_ta<__a, __c, __m, (unsigned long long)(~0)>::next(__x));
+ }
+};
+
template <unsigned long long _Ap, unsigned long long _Cp, unsigned long long _Mp>
-struct __lce_ta<_Ap, _Cp, _Mp, unsigned(~0), true> {
+struct __lce_ta<_Ap, _Cp, _Mp, unsigned(~0), 2> {
typedef unsigned result_type;
_LIBCPP_HIDE_FROM_ABI static result_type next(result_type __x) {
const result_type __a = static_cast<result_type>(_Ap);
@@ -112,7 +148,7 @@ struct __lce_ta<_Ap, _Cp, _Mp, unsigned(~0), true> {
};
template <unsigned long long _Ap, unsigned long long _Mp>
-struct __lce_ta<_Ap, 0, _Mp, unsigned(~0), true> {
+struct __lce_ta<_Ap, 0ull, _Mp, unsigned(~0), 2> {
typedef unsigned result_type;
_LIBCPP_HIDE_FROM_ABI static result_type next(result_type __x) {
const result_type __a = static_cast<result_type>(_Ap);
@@ -128,7 +164,21 @@ struct __lce_ta<_Ap, 0, _Mp, unsigned(~0), true> {
};
template <unsigned long long _Ap, unsigned long long _Cp, unsigned long long _Mp>
-struct __lce_ta<_Ap, _Cp, _Mp, unsigned(~0), false> {
+struct __lce_ta<_Ap, _Cp, _Mp, unsigned(~0), 1> {
+ typedef unsigned result_type;
+ _LIBCPP_HIDE_FROM_ABI static result_type next(result_type __x) {
+ const result_type __a = static_cast<result_type>(_Ap);
+ const result_type __c = static_cast<result_type>(_Cp);
+ const result_type __m = static_cast<result_type>(_Mp);
+ // Use (((a*x) % m) + c) % m
+ __x = (__a * __x) % __m;
+ __x += __c - (__x >= __m - __c) * __m;
+ return __x;
+ }
+};
+
+template <unsigned long long _Ap, unsigned long long _Cp, unsigned long long _Mp>
+struct __lce_ta<_Ap, _Cp, _Mp, unsigned(~0), 0> {
typedef unsigned result_type;
_LIBCPP_HIDE_FROM_ABI static result_type next(result_type __x) {
const result_type __a = static_cast<result_type>(_Ap);
@@ -139,7 +189,7 @@ struct __lce_ta<_Ap, _Cp, _Mp, unsigned(~0), false> {
};
template <unsigned long long _Ap, unsigned long long _Cp>
-struct __lce_ta<_Ap, _Cp, 0, unsigned(~0), false> {
+struct __lce_ta<_Ap, _Cp, 0ull, unsigned(~0), 0> {
typedef unsigned result_type;
_LIBCPP_HIDE_FROM_ABI static result_type next(result_type __x) {
const result_type __a = static_cast<result_type>(_Ap);
@@ -150,8 +200,8 @@ struct __lce_ta<_Ap, _Cp, 0, unsigned(~0), false> {
// 16
-template <unsigned long long __a, unsigned long long __c, unsigned long long __m, bool __b>
-struct __lce_ta<__a, __c, __m, (unsigned short)(~0), __b> {
+template <unsigned long long __a, unsigned long long __c, unsigned long long __m, int __mode>
+struct __lce_ta<__a, __c, __m, (unsigned short)(~0), __mode> {
typedef unsigned short result_type;
_LIBCPP_HIDE_FROM_ABI static result_type next(result_type __x) {
return static_cast<result_type>(__lce_ta<__a, __c, __m, unsigned(~0)>::next(__x));
diff --git a/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/alg.pass.cpp b/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/alg.pass.cpp
index 8a9cae0e610c35..fff93a895f8955 100644
--- a/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/alg.pass.cpp
+++ b/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/alg.pass.cpp
@@ -38,12 +38,12 @@ int main(int, char**)
// m might overflow. The overflow is not OK and result will be in bounds
// so we should use Schrage's algorithm
- typedef std::linear_congruential_engine<T, (1ull << 32), 0, (1ull << 63) + 1> E2;
+ typedef std::linear_congruential_engine<T, 0x100000000ull, 0, (1ull << 63) + 1ull> E2;
E2 e2;
// make sure Schrage's algorithm is used (it would be 0s after the first otherwise)
- assert(e2() == (1ull << 32));
+ assert(e2() == 0x100000000ull);
assert(e2() == (1ull << 63) - 1ull);
- assert(e2() == (1ull << 63) - (1ull << 33) + 1ull);
+ assert(e2() == (1ull << 63) - 0x1ffffffffull);
// make sure result is in bounds
assert(e2() < (1ull << 63) + 1);
assert(e2() < (1ull << 63) + 1);
@@ -56,9 +56,9 @@ int main(int, char**)
typedef std::linear_congruential_engine<T, 0x18000001ull, 0x12347ull, (3ull << 56)> E3;
E3 e3;
// make sure Schrage's algorithm is used
- assert(e3() == 402727752ull);
- assert(e3() == 162159612030764687ull);
- assert(e3() == 108176466184989142ull);
+ assert(e3() == 0x18012348ull);
+ assert(e3() == 0x2401b4ed802468full);
+ assert(e3() == 0x18051ec400369d6ull);
// make sure result is in bounds
assert(e3() < (3ull << 56));
assert(e3() < (3ull << 56));
@@ -66,19 +66,52 @@ int main(int, char**)
assert(e3() < (3ull << 56));
assert(e3() < (3ull << 56));
- // m will not overflow so we should not use Schrage's algorithm
- typedef std::linear_congruential_engine<T, 1ull, 1, (1ull << 48)> E4;
+ // 32-bit case:
+ // m might overflow. The overflow is not OK, result will be in bounds,
+ // and Schrage's algorithm is incompatible here. Need to use 64 bit arithmetic.
+ typedef std::linear_congruential_engine<unsigned, 0x10009u, 0u, 0x7fffffffu> E4;
E4 e4;
+ // make sure enough precision is used
+ assert(e4() == 0x10009u);
+ assert(e4() == 0x120053u);
+ assert(e4() == 0xf5030fu);
+ // make sure result is in bounds
+ assert(e4() < 0x7fffffffu);
+ assert(e4() < 0x7fffffffu);
+ assert(e4() < 0x7fffffffu);
+ assert(e4() < 0x7fffffffu);
+ assert(e4() < 0x7fffffffu);
+
+#ifdef __SIZEOF_INT128__
+ // m might overflow. The overflow is not OK, result will be in bounds,
+ // and Schrage's algorithm is incompatible here. Need to use 128 bit arithmetic.
+ typedef std::linear_congruential_engine<T, 0x100000001ull, 0ull, (1ull << 61) - 1ull> E5;
+ E5 e5;
+ // make sure enough precision is used
+ assert(e5() == 0x100000001ull);
+ assert(e5() == 0x200000009ull);
+ assert(e5() == 0xb00000019ull);
+ // make sure result is in bounds
+ assert(e5() < (1ull << 61) - 1ull);
+ assert(e5() < (1ull << 61) - 1ull);
+ assert(e5() < (1ull << 61) - 1ull);
+ assert(e5() < (1ull << 61) - 1ull);
+ assert(e5() < (1ull << 61) - 1ull);
+#endif
+
+ // m will not overflow so we should not use Schrage's algorithm
+ typedef std::linear_congruential_engine<T, 1ull, 1, (1ull << 48)> E6;
+ E6 e6;
// make sure the correct algorithm was used
- assert(e4() == 2ull);
- assert(e4() == 3ull);
- assert(e4() == 4ull);
+ assert(e6() == 2ull);
+ assert(e6() == 3ull);
+ assert(e6() == 4ull);
// make sure result is in bounds
- assert(e4() < (1ull << 48));
- assert(e4() < (1ull << 48));
- assert(e4() < (1ull << 48));
- assert(e4() < (1ull << 48));
- assert(e4() < (1ull << 48));
+ assert(e6() < (1ull << 48));
+ assert(e6() < (1ull << 48));
+ assert(e6() < (1ull << 48));
+ assert(e6() < (1ull << 48));
+ assert(e6() < (1ull << 48));
return 0;
}
\ No newline at end of file
diff --git a/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/assign.pass.cpp b/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/assign.pass.cpp
index 5317f171a98a79..96aac34c31f376 100644
--- a/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/assign.pass.cpp
+++ b/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/assign.pass.cpp
@@ -61,8 +61,14 @@ test()
test1<T, A, 0, M>();
test1<T, A, M - 2, M>();
test1<T, A, M - 1, M>();
+}
+
+template <class T>
+void
+test_ext()
+{
+ const T M(static_cast<T>(-1));
- /*
// Cases where m is odd and m % a > m / a (not implemented)
test1<T, M - 2, 0, M>();
test1<T, M - 2, M - 2, M>();
@@ -70,15 +76,20 @@ test()
test1<T, M - 1, 0, M>();
test1<T, M - 1, M - 2, M>();
test1<T, M - 1, M - 1, M>();
- */
}
int main(int, char**)
{
test<unsigned short>();
+ test_ext<unsigned short>();
test<unsigned int>();
+ test_ext<unsigned int>();
test<unsigned long>();
+ test_ext<unsigned long>();
test<unsigned long long>();
+#ifdef __SIZEOF_INT128__
+ test_ext<unsigned long long>();
+#endif
return 0;
}
diff --git a/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/copy.pass.cpp b/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/copy.pass.cpp
index 8e950043d594f9..f734cf1cdc33c4 100644
--- a/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/copy.pass.cpp
+++ b/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/copy.pass.cpp
@@ -60,8 +60,14 @@ test()
test1<T, A, 0, M>();
test1<T, A, M - 2, M>();
test1<T, A, M - 1, M>();
+}
+
+template <class T>
+void
+test_ext()
+{
+ const T M(static_cast<T>(-1));
- /*
// Cases where m is odd and m % a > m / a (not implemented)
test1<T, M - 2, 0, M>();
test1<T, M - 2, M - 2, M>();
@@ -69,15 +75,20 @@ test()
test1<T, M - 1, 0, M>();
test1<T, M - 1, M - 2, M>();
test1<T, M - 1, M - 1, M>();
- */
}
int main(int, char**)
{
test<unsigned short>();
+ test_ext<unsigned short>();
test<unsigned int>();
+ test_ext<unsigned int>();
test<unsigned long>();
+ test_ext<unsigned long>();
test<unsigned long long>();
+#ifdef __SIZEOF_INT128__
+ test_ext<unsigned long long>();
+#endif
return 0;
}
diff --git a/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/default.pass.cpp b/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/default.pass.cpp
index 52126f7a200dbe..01377e0d683dc5 100644
--- a/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/default.pass.cpp
+++ b/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/default.pass.cpp
@@ -58,8 +58,14 @@ test()
test1<T, A, 0, M>();
test1<T, A, M - 2, M>();
test1<T, A, M - 1, M>();
+}
+
+template <class T>
+void
+test_ext()
+{
+ const T M(static_cast<T>(-1));
- /*
// Cases where m is odd and m % a > m / a (not implemented)
test1<T, M - 2, 0, M>();
test1<T, M - 2, M - 2, M>();
@@ -67,15 +73,20 @@ test()
test1<T, M - 1, 0, M>();
test1<T, M - 1, M - 2, M>();
test1<T, M - 1, M - 1, M>();
- */
}
int main(int, char**)
{
test<unsigned short>();
+ test_ext<unsigned short>();
test<unsigned int>();
+ test_ext<unsigned int>();
test<unsigned long>();
+ test_ext<unsigned long>();
test<unsigned long long>();
+#ifdef __SIZEOF_INT128__
+ test_ext<unsigned long long>();
+#endif
return 0;
}
diff --git a/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/values.pass.cpp b/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/values.pass.cpp
index 28d8dfea01fab3..cacae03138557d 100644
--- a/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/values.pass.cpp
+++ b/libcxx/test/std/numerics/rand/rand.eng/rand.eng.lcong/values.pass.cpp
@@ -91,8 +91,14 @@ test()
test1<T, A, 0, M>();
test1<T, A, M - 2, M>();
test1<T, A, M - 1, M>();
+}
+
+template <class T>
+void
+test_ext()
+{
+ const T M(static_cast<T>(-1));
- /*
// Cases where m is odd and m % a > m / a (not implemented)
test1<T, M - 2, 0, M>();
test1<T, M - 2, M - 2, M>();
@@ -100,15 +106,20 @@ test()
test1<T, M - 1, 0, M>();
test1<T, M - 1, M - 2, M>();
test1<T, M - 1, M - 1, M>();
- */
}
int main(int, char**)
{
test<unsigned short>();
+ test_ext<unsigned short>();
test<unsigned int>();
+ test_ext<unsigned int>();
test<unsigned long>();
+ test_ext<unsigned long>();
test<unsigned long long>();
+#ifdef __SIZEOF_INT128__
+ test_ext<unsigned long long>();
+#endif
return 0;
}
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