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which one will execute faster flag==0 or 0==flag and why?

P: 3
if of the following will execute faster
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  1. if ( flag == 0) 
  2. {
  3. }
  4.  
or
Expand|Select|Wrap|Line Numbers
  1. if ( 0 == flag) 
  2. {
  3. }
  4.  
Oct 20 '10 #1
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2 Replies


code green
Expert 100+
P: 1,726
insignificant I would say.
But I bet if(!flag) is faster still.
Oct 20 '10 #2

Oralloy
Expert 100+
P: 983
It all depends on what assembly code the compiler generates. Essentially, do "0 == flag" and "flag == 0" compile down to the same code, or not?

Then there are the questions of how the system executes the assembly language level comparison command. Various computers will behave differently, depending. On a PDP-11 or VAX, the entire test is boiled down to one memory instruction which may be 10 or 12 bytes long, depending on the "addressing mode" used for the operands. Who knows if loading an immediate "0" followed by a memory fetch to a stack location is faster than the other way around. Or perhaps it's one simple test instruction, depending on the command set available.

Lastly ... a really good optimizer will eliminate the whole code sequence, as there is nothing to do as the consequent of the test. In this case, execution time for the "if" statement is exactly zero.

Cheers! :)
Oralloy
Oct 20 '10 #3

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