[Boost-commit] svn:boost r77775 - sandbox/big_number/boost/multiprecision

Subject: [Boost-commit] svn:boost r77775 - sandbox/big_number/boost/multiprecision
From: john_at_[hidden]
Date: 2012-04-05 04:22:24

Author: johnmaddock
Date: 2012-04-05 04:22:20 EDT (Thu, 05 Apr 2012)
New Revision: 77775
URL: http://svn.boost.org/trac/boost/changeset/77775

Log:
Add static assert to protect multiplication algorithm from excessive limb counts.
Text files modified: 
   sandbox/big_number/boost/multiprecision/cpp_dec_float.hpp |     8 ++++++++                                
   1 files changed, 8 insertions(+), 0 deletions(-)

Modified: sandbox/big_number/boost/multiprecision/cpp_dec_float.hpp
==============================================================================
--- sandbox/big_number/boost/multiprecision/cpp_dec_float.hpp	(original)
+++ sandbox/big_number/boost/multiprecision/cpp_dec_float.hpp	2012-04-05 04:22:20 EDT (Thu, 05 Apr 2012)
@@ -2208,6 +2208,14 @@
 template <unsigned Digits10>
 void cpp_dec_float<Digits10>::mul_loop_uv(const boost::uint32_t* const u, const boost::uint32_t* const v, boost::uint32_t* const w, const boost::int32_t p)
 {
+   //
+   // There is a limit on how many limbs this algorithm can handle without dropping digits 
+   // due to overflow in the carry, it is: 
+   //
+   // FLOOR( (2^64 - 1) / (10^8 * 10^8) )  ==  1844
+   //
+   BOOST_STATIC_ASSERT_MSG(mp_elem_number < 1800, "Too many limbs in the data type for the multiplication algorithm - unsupported precision in cpp_dec_float.");
+
    boost::uint64_t carry = static_cast<boost::uint64_t>(0u);
 
    for(boost::int32_t j = static_cast<boost::int32_t>(p - 1u); j >= static_cast<boost::int32_t>(0); j--)