how achieve split of unsigned4 into two signed2's using bit-wise instead of union?

This is a discussion on how achieve split of unsigned4 into two signed2's using bit-wise instead of union? within the C++ Programming forums, part of the General Programming Boards category; I know this code is completely valid but in the interest of learning, how would I achieve the same thing ...

  1. #1
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    how achieve split of unsigned4 into two signed2's using bit-wise instead of union?

    I know this code is completely valid but in the interest of learning, how would I achieve the same thing as below using bitwise operations on the unsigned4 value instead of the union "trick"?

    Code:
    void value(unsigned4 v)
    {
        union 
        {
            struct
            {
            signed2 left;
            signed2 right;
            } split;
     
            unsigned4 combined;
     
        } trick;
     
        //Set the base value
        trick.combined = v;
     
        //"Split" the value into left and right values
        left = trick.split.left;
        right = trick.split.right;
    }

  2. #2
    Banned master5001's Avatar
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    What are signed2 and unsigned4?

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    Code:
    left = (v & 0xFFFF0000) >> 16;
    right = (v & 0x0000FFFF);
    perhaps?

  4. #4
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    Quote Originally Posted by master5001 View Post
    What are signed2 and unsigned4?
    As best I can gather from the code:

    unsigned4 = unsigned int
    signed2 = signed short int

  5. #5
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    Quote Originally Posted by Waterbottle View Post
    Code:
    left = (v & 0xFFFF0000) >> 16;
    right = (v & 0x0000FFFF);
    perhaps?
    It is sometimes better to do
    Code:
    left = (v  >> 16) & 0x0000FFFF;
    right = v & 0x0000FFFF;
    because using smaller constants and put the and operation last makes it possible for the compiler to figure out that those operations are meaningless if the processor has native support for 16-bit integers.

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  6. #6
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by matsp View Post
    It is sometimes better to do
    Code:
    left = (v  >> 16) & 0x0000FFFF;
    right = v & 0x0000FFFF;
    because using smaller constants and put the and operation last makes it possible for the compiler to figure out that those operations are meaningless if the processor has native support for 16-bit integers.
    Yeah I recommend always doing a right shift before the masking. On CISC systems, it takes fewer bytes of compiled code to store the smaller constant in the compiled instructions. (assuming it isn't exactly the case matsp mentioned)

    Nothing wrong either way of course, it's just a micro optimisation.
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