re PR libstdc++/51795 (linear_congruential_engine doesn't work correctly)

2012-01-27  Paolo Carlini  <paolo.carlini@oracle.com>

	PR libstdc++/51795
	* include/bits/random.h (linear_congruential_generator): Add
	static_assert preventing instantiation for values of 'a' and 'm'
	currently handled incorrectly but _Mod::__calc.
	* include/bits/random.tcc (seed_seq::generate): Avoid unsafe
	uses of _Mod::__calc.

From-SVN: r183655

libstdc++-v3/ChangeLog

+2012-01-27  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	PR libstdc++/51795
+	* include/bits/random.h (linear_congruential_generator): Add
+	static_assert preventing instantiation for values of 'a' and 'm'
+	currently handled incorrectly but _Mod::__calc.
+	* include/bits/random.tcc (seed_seq::generate): Avoid unsafe
+	uses of _Mod::__calc.
+
 2012-01-27  Jakub Jelinek  <jakub@redhat.com>
 
 	PR libstdc++/51798

libstdc++-v3/include/bits/random.h

 // random number generation -*- C++ -*-
 
-// Copyright (C) 2009, 2010, 2011 Free Software Foundation, Inc.
+// Copyright (C) 2009, 2010, 2011, 2012 Free Software Foundation, Inc.
 //
 // This file is part of the GNU ISO C++ Library.  This library is free
 // software; you can redistribute it and/or modify it under the
@@ -174,6 +174,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static_assert(__m == 0u || (__a < __m && __c < __m),
 		    "template argument substituting __m out of bounds");
 
+      // XXX FIXME:
+      // _Mod::__calc should handle correctly __m % __a >= __m / __a too.
+      static_assert(__m % __a < __m / __a,
+		    "sorry, not implemented yet: try a smaller 'a' constant");
+
     public:
       /** The type of the generated random value. */
       typedef _UIntType result_type;

libstdc++-v3/include/bits/random.tcc

 // random number generation (out of line) -*- C++ -*-
 
-// Copyright (C) 2009, 2010, 2011 Free Software Foundation, Inc.
+// Copyright (C) 2009, 2010, 2011, 2012 Free Software Foundation, Inc.
 //
 // This file is part of the GNU ISO C++ Library.  This library is free
 // software; you can redistribute it and/or modify it under the
@@ -49,6 +49,8 @@ namespace std _GLIBCXX_VISIBILITY(default)
     //
     // Preconditions:  a > 0, m > 0.
     //
+    // XXX FIXME: as-is, only works correctly for __m % __a < __m / __a. 
+    //
     template<typename _Tp, _Tp __m, _Tp __a, _Tp __c, bool>
       struct _Mod
       {
@@ -2771,8 +2773,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 			 ^ __begin[(__k + __p) % __n]
 			 ^ __begin[(__k - 1) % __n]);
 	  _Type __r1 = __arg ^ (__arg >> 27);
-	  __r1 = __detail::__mod<_Type, __detail::_Shift<_Type, 32>::__value,
-	                         1664525u, 0u>(__r1);
+	  __r1 = __detail::__mod<_Type,
+		    __detail::_Shift<_Type, 32>::__value>(1664525u * __r1);
 	  _Type __r2 = __r1;
 	  if (__k == 0)
 	    __r2 += __s;
@@ -2793,8 +2795,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 			 + __begin[(__k + __p) % __n]
 			 + __begin[(__k - 1) % __n]);
 	  _Type __r3 = __arg ^ (__arg >> 27);
-	  __r3 = __detail::__mod<_Type, __detail::_Shift<_Type, 32>::__value,
-	                         1566083941u, 0u>(__r3);
+	  __r3 = __detail::__mod<_Type,
+		   __detail::_Shift<_Type, 32>::__value>(1566083941u * __r3);
 	  _Type __r4 = __r3 - __k % __n;
 	  __r4 = __detail::__mod<_Type,
 	           __detail::_Shift<_Type, 32>::__value>(__r4);