Constant time evaluation

up vote
3
down vote

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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?

edited Nov 26 at 15:06

Stoogy

552422

asked Nov 22 at 15:56

jonnyx

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

|
show 5 more comments

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?

edited Nov 26 at 15:06

Stoogy

552422

asked Nov 22 at 15:56

jonnyx

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

|
show 5 more comments

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?

edited Nov 26 at 15:06

Stoogy

552422

asked Nov 22 at 15:56

jonnyx

1059

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

edited Nov 26 at 15:06

Stoogy

552422

asked Nov 22 at 15:56

jonnyx

1059

edited Nov 26 at 15:06

Stoogy

552422

asked Nov 22 at 15:56

jonnyx

1059

edited Nov 26 at 15:06

Stoogy

552422

edited Nov 26 at 15:06

Stoogy

552422

edited Nov 26 at 15:06

Stoogy

552422

asked Nov 22 at 15:56

jonnyx

1059

asked Nov 22 at 15:56

jonnyx

1059

asked Nov 22 at 15:56

jonnyx

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

|
show 5 more comments

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

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

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

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

|
show 5 more comments

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