Constant time evaluation











up vote
3
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I can't get this line to run in constant time on my micro-controller:



int zz,yy; //some binary variables  



zz = (yy) ? 0 : (1 & zz);


I tried to change it to



zz = (yy) ? (0 & zz) : (1 & zz);


because & should force an evaluation of the right side even though the left side defines the result as far as I know. But it did not help.



Can anyone suggest me a solution how to make this line constant time?






c microcontroller







share|improve this question
























  • Can you clarify what you mean by 'constant time'? I would consider this O(1), so do you mean something else, or ...?
    – Austin Hastings
    Nov 22 at 16:00










  • by constant time, I mean that it always should take x cycles running on my microcontroller, x should be equal for both cases 0 and 1&zz
    – jonnyx
    Nov 22 at 16:01






  • 5




    It entirely depends on your compiler and its settings. Maybe (!yy) * (1 & zz) helps?
    – Quentin
    Nov 22 at 16:03






  • 1




    it is not about optimization, it is about being constant time (which often implies slower code)
    – jonnyx
    Nov 24 at 19:35








  • 1




    Take a look at Hacker's Delight for more ideas. Or search for "bit twiddling".
    – starblue
    Nov 25 at 10:53

















up vote
3
down vote

favorite












I can't get this line to run in constant time on my micro-controller:



int zz,yy; //some binary variables  



zz = (yy) ? 0 : (1 & zz);


I tried to change it to



zz = (yy) ? (0 & zz) : (1 & zz);


because & should force an evaluation of the right side even though the left side defines the result as far as I know. But it did not help.



Can anyone suggest me a solution how to make this line constant time?






c microcontroller







share|improve this question
























  • Can you clarify what you mean by 'constant time'? I would consider this O(1), so do you mean something else, or ...?
    – Austin Hastings
    Nov 22 at 16:00










  • by constant time, I mean that it always should take x cycles running on my microcontroller, x should be equal for both cases 0 and 1&zz
    – jonnyx
    Nov 22 at 16:01






  • 5




    It entirely depends on your compiler and its settings. Maybe (!yy) * (1 & zz) helps?
    – Quentin
    Nov 22 at 16:03






  • 1




    it is not about optimization, it is about being constant time (which often implies slower code)
    – jonnyx
    Nov 24 at 19:35








  • 1




    Take a look at Hacker's Delight for more ideas. Or search for "bit twiddling".
    – starblue
    Nov 25 at 10:53















up vote
3
down vote

favorite









up vote
3
down vote

favorite











I can't get this line to run in constant time on my micro-controller:



int zz,yy; //some binary variables  



zz = (yy) ? 0 : (1 & zz);


I tried to change it to



zz = (yy) ? (0 & zz) : (1 & zz);


because & should force an evaluation of the right side even though the left side defines the result as far as I know. But it did not help.



Can anyone suggest me a solution how to make this line constant time?






c microcontroller







share|improve this question















I can't get this line to run in constant time on my micro-controller:



int zz,yy; //some binary variables  



zz = (yy) ? 0 : (1 & zz);


I tried to change it to



zz = (yy) ? (0 & zz) : (1 & zz);


because & should force an evaluation of the right side even though the left side defines the result as far as I know. But it did not help.



Can anyone suggest me a solution how to make this line constant time?






c microcontroller




c microcontroller






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Nov 26 at 15:06









Stoogy

552422




552422










asked Nov 22 at 15:56









jonnyx

1059




1059












  • Can you clarify what you mean by 'constant time'? I would consider this O(1), so do you mean something else, or ...?
    – Austin Hastings
    Nov 22 at 16:00










  • by constant time, I mean that it always should take x cycles running on my microcontroller, x should be equal for both cases 0 and 1&zz
    – jonnyx
    Nov 22 at 16:01






  • 5




    It entirely depends on your compiler and its settings. Maybe (!yy) * (1 & zz) helps?
    – Quentin
    Nov 22 at 16:03






  • 1




    it is not about optimization, it is about being constant time (which often implies slower code)
    – jonnyx
    Nov 24 at 19:35








  • 1




    Take a look at Hacker's Delight for more ideas. Or search for "bit twiddling".
    – starblue
    Nov 25 at 10:53




















  • Can you clarify what you mean by 'constant time'? I would consider this O(1), so do you mean something else, or ...?
    – Austin Hastings
    Nov 22 at 16:00










  • by constant time, I mean that it always should take x cycles running on my microcontroller, x should be equal for both cases 0 and 1&zz
    – jonnyx
    Nov 22 at 16:01






  • 5




    It entirely depends on your compiler and its settings. Maybe (!yy) * (1 & zz) helps?
    – Quentin
    Nov 22 at 16:03






  • 1




    it is not about optimization, it is about being constant time (which often implies slower code)
    – jonnyx
    Nov 24 at 19:35








  • 1




    Take a look at Hacker's Delight for more ideas. Or search for "bit twiddling".
    – starblue
    Nov 25 at 10:53


















Can you clarify what you mean by 'constant time'? I would consider this O(1), so do you mean something else, or ...?
– Austin Hastings
Nov 22 at 16:00




Can you clarify what you mean by 'constant time'? I would consider this O(1), so do you mean something else, or ...?
– Austin Hastings
Nov 22 at 16:00












by constant time, I mean that it always should take x cycles running on my microcontroller, x should be equal for both cases 0 and 1&zz
– jonnyx
Nov 22 at 16:01




by constant time, I mean that it always should take x cycles running on my microcontroller, x should be equal for both cases 0 and 1&zz
– jonnyx
Nov 22 at 16:01




5




5




It entirely depends on your compiler and its settings. Maybe (!yy) * (1 & zz) helps?
– Quentin
Nov 22 at 16:03




It entirely depends on your compiler and its settings. Maybe (!yy) * (1 & zz) helps?
– Quentin
Nov 22 at 16:03




1




1




it is not about optimization, it is about being constant time (which often implies slower code)
– jonnyx
Nov 24 at 19:35






it is not about optimization, it is about being constant time (which often implies slower code)
– jonnyx
Nov 24 at 19:35






1




1




Take a look at Hacker's Delight for more ideas. Or search for "bit twiddling".
– starblue
Nov 25 at 10:53






Take a look at Hacker's Delight for more ideas. Or search for "bit twiddling".
– starblue
Nov 25 at 10:53



















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