(-1 xor CONST) != (CONST xor -1)

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IronFelix

Joined: 09 Dec 2004
Posts: 141
Location: Russia, Murmansk region

IronFelix 12 Jan 2009, 14:47

Hi, all!
Please, tell me, is it a bug:

Code:

 dd -1 xor 3

is FFFFFFFF in memory, but

Code:

 dd 3 xor -1

is FFFFFFFC.
It seems to me that both must be 0xFFFFFFFC.
Thanks.

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12 Jan 2009, 14:47

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8356
Location: Kraków, Poland

Tomasz Grysztar 12 Jan 2009, 14:57

IronFelix wrote:

Please, tell me, is it a bug:
Code:
 dd -1 xor 3     
is FFFFFFFF in memory

This should be:

Code:

dd (-1) xor 3

because what you wrote is the same as:

Code:

dd -(1 xor 3)

Check out the operators precedence in manual.

12 Jan 2009, 14:57

IronFelix

Joined: 09 Dec 2004
Posts: 141
Location: Russia, Murmansk region

IronFelix 12 Jan 2009, 15:07

Thanks for so quick reply and sorry for wasting your time, Tomasz.

12 Jan 2009, 15:07

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20401
Location: In your JS exploiting you and your system

revolution 12 Jan 2009, 15:16

IronFelix wrote:

Code:
 dd -1 xor 3     
is FFFFFFFF in memory

Tomasz Grysztar wrote:

because what you wrote is the same as:
Code:
dd -(1 xor 3)    

Isn't -(1 xor 3) ===> -(2) ===> 0xFFFFFFFE?

12 Jan 2009, 15:16

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8356
Location: Kraków, Poland

Tomasz Grysztar 12 Jan 2009, 15:20

revolution wrote:

Isn't -(1 xor 3) ===> -(2) ===> 0xFFFFFFFE?

And this is how fasm assembles it. Perhaps IronFelix made a mistake there.

12 Jan 2009, 15:20

IronFelix

Joined: 09 Dec 2004
Posts: 141
Location: Russia, Murmansk region

IronFelix 12 Jan 2009, 15:46

Yes, it is my mistake, sorry for it too, please.

12 Jan 2009, 15:46

IceStudent

Joined: 19 Dec 2003
Posts: 60
Location: Ukraine

IceStudent 03 Feb 2009, 22:00

Tomasz Grysztar wrote:

Check out the operators precedence

But it seems that XOR have higher priority than unary minus. But, by manual, it is not (minus have zero priority and XOR have 3rd).

Is it a bug or feature?

Another example:
1 - 1 xor 1 != 1 xor 1 - 1

But in other languages this expressions are equal (1 - 1 ^ 1 == 1 ^ 1 - 1)

03 Feb 2009, 22:00

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8356
Location: Kraków, Poland

Tomasz Grysztar 03 Feb 2009, 22:23

IceStudent wrote:

But it seems that XOR have highest priority than unary minus. But, by manual, it is not (minus have zero priority and XOR have 3rd).

Yes, XOR has higher priority than any minus (unary or binary), and this is exactly what manual states. The higher "priority" value gives a higher priority to operator, therefore operator with this value higher is processed first.

IceStudent wrote:

Another example:
1 - 1 xor 1 != 1 xor 1 - 1

But in other languages this expressions are equal (1 - 1 ^ 1 == 1 ^ 1 - 1)

In some languages they may be equal, in some not. In fasm XOR is processed before substraction, as shown in the manual.

03 Feb 2009, 22:23

IceStudent

Joined: 19 Dec 2003
Posts: 60
Location: Ukraine

IceStudent 03 Feb 2009, 23:05

In the manual a higher priority haves largest value?

03 Feb 2009, 23:05

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20401
Location: In your JS exploiting you and your system

revolution 04 Feb 2009, 00:53

IceStudent wrote:

In the manual a higher priority haves largest value?

Just look at + and * priority. That will tell you whether you are going up or down in priority.

04 Feb 2009, 00:53

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