id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
13503	Boost.Multiprecision generates incorrect code	Sam Lunt <samuel.j.lunt@…>	John Maddock	"Comparing two identical functions, one using `boost::multiprecision::uint128_t` and the other using GCC/Clang builtin  `unsigned __int128`:

{{{
auto foo(std::uint64_t lhs, std::uint64_t rhs) -> std::uint64_t
{
    boost::multiprecision::uint128_t a(lhs);
    a *= rhs;
    return static_cast<std::uint64_t>(a >> 64) 
         + static_cast<std::uint64_t>(a);
}

auto bar(std::uint64_t lhs, std::uint64_t rhs) -> std::uint64_t
{
    unsingned __int128 a(lhs);
    a *= rhs;
    return static_cast<std::uint64_t>(a >> 64) 
         + static_cast<std::uint64_t>(a);
}
}}}

Comparing the outputs of each function, they return different results for `(-1ULL, -1ULL)`, `(-1ULL, 2)`, `(-1ULL, 3)`, and presumably others. (`foo` returns `18446744073709551613`, `0`, and `1`, respectively).

Comparing the results to Python's big int type (the builtin `int`), it seems that the `foo` results are not correct and the `bar` results are (this can also be confirmed by hand). This is also confirmed by looking at the disassembly for the each function:

{{{
0000000000000000 <foo(unsigned long, unsigned long)>:
   0:	48 89 f0             	mov    %rsi,%rax
   3:	48 f7 e7             	mul    %rdi
   6:	48 83 f8 ff          	cmp    $0xffffffffffffffff,%rax
   a:	48 89 d1             	mov    %rdx,%rcx
   d:	48 83 d9 00          	sbb    $0x0,%rcx
  11:	48 c7 c1 ff ff ff ff 	mov    $0xffffffffffffffff,%rcx
  18:	48 0f 43 c1          	cmovae %rcx,%rax
  1c:	48 01 d0             	add    %rdx,%rax
  1f:	c3                   	retq   

0000000000000020 <bar(unsigned long, unsigned long)>:
  20:	48 89 f0             	mov    %rsi,%rax
  23:	48 f7 e7             	mul    %rdi
  26:	48 01 d0             	add    %rdx,%rax
  29:	c3                   	retq   
}}}

These results were consistent when compiling with clang++ (5.0.0) and g++ (6.3.1) and when compiling with optimization enabled and disabled."	Bugs	closed	To Be Determined	multiprecision	Boost 1.64.0	Problem	fixed