Delphi 7: int64 size bug?

Question

I wrote a small function to convert MB to Bytes, however, there seems to be a bug in int64. According to the documentation, int64 ranges from -9223372036854775808 to 9223372036854775807, but my results differ...a lot:

Const FreeSpace = 67100;

var FreeSpaceConverted :int64;
.
.
.

FreeSpaceConverted := FreeSpace shl 20;

Using a value of 67100 for FreeSpace results in a value of 1639972864 instead of 70359449600. It's obvious that the conversion ran out of space and wrapped around. The actual size of int64 seems to be 70359449600 - 1639972864 = 68719476736 = 2^36 while it should be 2^63-1. An exponent of 36 looks rather strange. Could it be a number twist in the compiler itself??

Also, using the following alternative gives the error "Overflow in conversion or arithmetic operation" even though it shouldn't:

FreeSpaceConverted := FreeSpace * 1024 * 1024;

On the other hand, the following alternative does work:

FreeSpaceConverted := FreeSpace * 1024;
FreeSpaceConverted := FreeSpaceConverted * 1024;

Is this normal behavior and if so, what's the reason for all of this?

Answer 1

All the code you include in the question works fine in Delphi 7.

program Int64Test;

{$APPTYPE CONSOLE}

var
  FreeSpaceConverted, FreeSpace: Int64;

begin
  FreeSpace := 67100;
  FreeSpaceConverted := FreeSpace shl 20;
  Writeln(FreeSpaceConverted);
  FreeSpaceConverted := FreeSpace * 1024 * 1024;
  Writeln(FreeSpaceConverted);
  Readln;
end.

Output

70359449600
70359449600

---

Your actual code is different from what you have stated in the question. In fact, FreeSpace is declared as a 32 bit type in your code, probably Integer. For example, and I'm having to guess a little here:

program Int64Test;

{$APPTYPE CONSOLE}

var
  FreeSpace: Integer;
  FreeSpaceConverted: Int64;

begin
  FreeSpace := 67100;
  FreeSpaceConverted := FreeSpace shl 20;
  Writeln(FreeSpaceConverted);
  FreeSpaceConverted := FreeSpace * 1024 * 1024;
  Writeln(FreeSpaceConverted);
  Readln;
end.

Output

1639972864
1639972864

If we enable overflow checking then the multiplication code results in an overflow exception, as you report.

---

Now consider FreeSpace shl 20 when FreeSpace is an integer. The compiler interprets this as a 32 bit integer operation and shifts the more significant bits off the end of the 32 bit register. The fact that you are assigning to a 64 bit integer is not relevant. What matters are the data types in the expression. You can make the code behave the way you want by including a cast to Int64 on the right hand side.

program Int64Test;

{$APPTYPE CONSOLE}

var
  FreeSpace: Integer;
  FreeSpaceConverted: Int64;

begin
  FreeSpace := 67100;
  FreeSpaceConverted := Int64(FreeSpace) shl 20;
  Writeln(FreeSpaceConverted);
  FreeSpaceConverted := Int64(FreeSpace) * 1024 * 1024;
  Writeln(FreeSpaceConverted);
  Readln;
end.

Output

70359449600
70359449600

For a fuller discussion I refer you to Barry Kelly's answer to another question.

Answer 2

I don't happen to have D7 on this system but I do have Delphi 2007 and in that, this code:

var
  FreeSpaceConverted, FreeSpace:int64;
begin
  try
     FreeSpace := 67100;
     FreeSpaceConverted := FreeSpace shl 20;
     writeln(FreeSpaceConverted);
     readln;
  except
    on E:Exception do
      Writeln(E.Classname, ': ', E.Message);
  end;
end.

yields "70359449600".

Answer 3

I'm using Delphi XE7. I think the problem is with shifting a 32-bit integer into a 64-bit integer. If I use the your code, I get the same thing you do. But if I first typecast FreeSpace to int64, i.e.

Const FreeSpace : int64 = 67100;

I get the correct result.

Answer 4

Okay, so this line doesn't work -> FreeSpaceConverted := FreeSpace shl 20;

Perhaps you should watch this part -> FreeSpace shl 20;

Yes, FreeSpace shl 20, and you may want to look this -> const -> FreeSpace = 67100;

Perhaps D7 considers FreeSpace as Integer 32 constant? If so, you might want to change your code into something like this -> const -> FreeSpace: Int64 = 67100;

You don't need untyped constant here, unless you want to do something like this var A: array [0..FreeSpace] of Integer Other than that you don't need untyped constant.

Further information In Pascal untyped constant is like a text editor replacement. You can use untyped constant where you use number. It doesn't take any memory reference, it's just a TEXT EDITOR REPLACEMENT. You can't make any operation in that Example: const A: Integer = 10; B = 20;

begin Inc(A); // it works Inc(B); // not only it won't work, it also generates compile error end;

Comparison Untyped Constant Typed constant C #define A 20 long a = 20;
Pascal const A = 20; const A: Integer = 20;
Assembly A equ 20 A dd 20 ; double word, integer 32 ; equivalent in assembly