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[Stephen Warren <2122352@xxxxxxxxxxxxxxxxxx>]

In case GitHub goes away, minimal repro is:

    #include <boost/multiprecision/cpp_int.hpp>
    
    using BigInt = boost::multiprecision::cpp_int;
    
    BigInt calcm(unsigned ls) {
        return BigInt(1) << ls;
    }


docker run -it --rm -v $(pwd):/data ubuntu:24.04
apt update
apt -y install libboost-dev g++
cd /data
g++ -O0 -fdiagnostics-color=always -Wall -Wextra -Wno-overloaded-virtual \
    -Wno-deprecated -Wno-deprecated-declarations -pedantic -O3 -DNDEBUG \
    -Wall -Wextra -Wno-overloaded-virtual -Wno-deprecated \
    -Wno-deprecated-declarations -pedantic -Werror -Wall -Wextra -Wswitch-enum \
    -std=gnu++20 \
    -c -o repro repro.cpp


Failure mode:

        inlined from 'BigInt calcm(size_t, size_t)' at repro.cpp:46:24,
        inlined from 'void f(S*, S*)' at repro.cpp:51:36:
    /usr/include/boost/multiprecision/cpp_int/bitwise.hpp:441:26: error: iteration 2305843009213693951 invokes undefined behavior [-Werror=aggressive-loop-optimizations]
      441 |       pr[rs - 1 - i] = pr[rs - 1 - i - offset] << shift;
          |                        ~~^
    /usr/include/boost/multiprecision/cpp_int/bitwise.hpp:439:18: note: within this loop
      439 |    for (; rs - i >= 2 + offset; ++i)
      |           ~~~~~~~^~~~~~~~~~~~~

Fix:

include/boost/multiprecision/cpp_int/bitwise.hpp
@@ -436,7 +436,7 @@ inline BOOST_MP_CXX14_CONSTEXPR void left_shift_generic(Int& result, double_limb
          ++i;
       }
    }
-   for (; rs - i >= static_cast<std::size_t>(static_cast<std::size_t>(2u) + offset); ++i)
+   for (; static_cast<std::ptrdiff_t>(static_cast<std::ptrdiff_t>(rs) - static_cast<std::ptrdiff_t>(i)) >= static_cast<std::ptrdiff_t>(2 + static_cast<std::ptrdiff_t>(offset)); ++i)

-- 
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Debcrafters packages, which is subscribed to boost-defaults in Ubuntu.
https://bugs.launchpad.net/bugs/2122352

Title:
  Noble/1.83: compile error re: undefined behavior (fix available)

Status in boost-defaults package in Ubuntu:
  New

Bug description:
  https://github.com/boostorg/multiprecision/issues/733

  Using a specific simple Boost feature yields a compile error re:
  undefined behavior. See the GitHub bug for full details and a simple
  patch that solves the issue. Is it possible to rebuild the Ubuntu
  package so that applications can be compiled on Noble? Thanks.

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