Commit 586952f4 authored by Jorg Brown's avatar Jorg Brown
Browse files

Optimize std::midpoint for integers 

Same idea as the current algorithm, that is, add (half of the difference between a and b) to a.

But we use a different technique for computing the difference: we compute b - a into a pair of integers that are named "sign_bit" and "diff". We have to use a pair because subtracting two 32-bit integers produces a 33-bit result.

Computing half of that is a simple matter of shifting diff right by 1, and adding sign_bit shifted left by 31. llvm knows how to do that with one instruction: shld.

The only tricky part is that if the difference is odd and negative, then shifting it by one isn't the same as dividing it by two - shifting a negative one produces a negative one, for example. So there's one more adjustment: if the sign bit and the low bit of diff are one, we add one.

For a demonstration of the codegen difference, see https://godbolt.org/z/7ar3K9 , which also has a built-in test.

Differential Revision: https://reviews.llvm.org/D69459
parent 610f80f7

libcxx/include/numeric

+7 −10
Original line number Diff line number Diff line
@@ -532,17 +532,14 @@ midpoint(_Tp __a, _Tp __b) noexcept 
_LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK

{

    using _Up = std::make_unsigned_t<_Tp>;

    constexpr _Up __bitshift = std::numeric_limits<_Up>::digits - 1;



    int __sign = 1;

    _Up __m = __a;

    _Up __M = __b;

    if (__a > __b)

    {

        __sign = -1;

        __m = __b;

        __M = __a;

    }

     return __a + __sign * _Tp(_Up(__M-__m) >> 1);

    _Up __diff = _Up(__b) - _Up(__a);

    _Up __sign_bit = __b < __a;



    _Up __half_diff = (__diff / 2) + (__sign_bit << __bitshift) + (__sign_bit & __diff);



    return __a + __half_diff;

}