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Unsigned int64 to unsigned int32

Posted on 2013-01-13
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Last Modified: 2013-01-15
Fastest and efficient way to convert (unsigned) _uint64 to two (unsigned) _uint32

example

somefunc(in _uint64 , out _uint32, out _uint32)
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Question by:Rahul Gupta
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Assisted Solution

by:Kdo
Kdo earned 125 total points
ID: 38772088
Howdy....

The fastest way is to just let C do all of the work.

 _uint64 I64;
 _uint32 L32;
 _uint32 U32;

  L32 = I64;
  U32 = (I64 >> 32);

Depending on your compiler options you may wish to recast these items:

  L32 = (_uint32)I64;
  U32 = (_uint32)(I64 >> 32);


Good Luck,
Kent
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Author Comment

by:Rahul Gupta
ID: 38772153
does it support both little endian and big endian systems
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Expert Comment

by:Kdo
ID: 38772159
Yep.  The C implementation is specifically mindful of the underlying hardware requirements and all integer type operations MUST work correctly according to the rules.

Now if you were to store the 64-bit value and try to extract it from the memory address and memory address+4 locations, all bets are off.  In this case you'd be at the mercy of the big/little endian rules.

But with integer operations the value is already converted to its "native" value in one of the hardware registers so it's simply a matter of selecting the upper or lower 32 bits.


Kent
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Author Comment

by:Rahul Gupta
ID: 38773877
I had done in this way...

   _uint64= _uint64(_uint64(_uint32var1) << 32) | _uint32var2;

  _uint32var1 = _uint32(_uint64variable & &HFFFFFFFF);
  _uint32var2 = _uint32(_uint64variable >> 32);

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Expert Comment

by:Kdo
ID: 38774397
That's essentially the same.  The only significant different is that on line 3, you mask off the lower 32 bits of the 64 bit value before storing it in a 32-bit container.  The mask is unnecessary since the container can't possibly hold more than 32 bits.


Kent
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Accepted Solution

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developmentguru earned 125 total points
ID: 38774983
I use a bit of Delphi magic and do it like this:

procedure TfMain.FormDblClick(Sender: TObject);
type
  TDDWord = record
    Low, High : DWord;
  end;

var
  UI64 : UInt64;
  DDWord : TDDWord absolute UI64;

begin
  UI64 := 1;
  //DDWord.Low = 1
  //DDWord.High = 0
end;

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By simply assigning the UI64 variable, the breakout of the two 32 bit pieces is done, with no extra code, no extra time.  The absolute keyword is for instances just like this, where you want to simply view one area of memory in a different way.  If you run this code in Delphi (with debug info on and optimizations off) then you will be able to inspect that the low value is 1 and the high value is 0, after you do the assignment.  The splitting litterally take no time...
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Expert Comment

by:developmentguru
ID: 38774985
If you need that solution to be mindful of different hardware then you would change the type with conditional compilation and it would be just as fast...
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Expert Comment

by:developmentguru
ID: 38776169
In C/C++ you can get the same effect by having a struct for the two smaller pieces.

struct Two32s {
  _uint32 Low;
  _uint32 High;
};

union 64BitSplit {
  _uint64 Whole;
  Two32s Split;
};

I haven't done C/C++ in a long time, so the code is "as is"... just to get the gyst of it.  If someone else feels the need to correct, by all means do!

If you assign to a variable of type 64BitSplit as

MyVar.Whole = VALUE;
You can then read out MyValue.Low and MyValue.High.  The Union makes the two structures occupy the same memory.

Once again, if the CPU independence is important then you should use conditional defines to make the struct swap low for high as needed.
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Expert Comment

by:Kdo
ID: 38776216
That code will sometimes work, but it's hardware dependent.

It doesn't work for both big and little endian systems.  It should also have the appropriate #PRAGMA or command line option to force data alignment to the 32-bit boundary (or less).


Kent
0
 
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Expert Comment

by:developmentguru
ID: 38776682
Since you need to compile for little endian versus big endian, conditional compilation takes care of that... I thought I mentioned that already...
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