[libcxx-commits] [libcxx] linear_congruential_engine: add using more precision to prevent overflow (PR #81583)

[Mark de Wever via libcxx-commits]

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@@ -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;
+#ifdef _LIBCPP_HAS_NO_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
 
+#ifndef _LIBCPP_HAS_NO_INT128
----------------
mordante wrote:

What kind of diagnostics do we get when 128-bit is not available?

https://github.com/llvm/llvm-project/pull/81583