How fast are min, max and constrain macros ?

Projects Programming
1

Hello,

I would like to know what of these two is faster?

This:

x = min(x, maxValue);

Or this:

if (x > maxValue) x = maxValue;

Knowing that the min macro is ((a)<(b)?(a):(b)) shouldn't the second version be even a little faster when x is not greater than maxValue because it doesn't assign a new value to x ?

And what about the constrain macro ?

This:

x = constrain(x, minValue, maxValue);

Or this:

if (x < minValue) x = minValue;
else if (x > maxValue) x = maxValue;
2

I think the compiler will generate the same code.

3

When I compiled it it showed 2 bytes more when using the min macro.
Any ideas why ?

4

objdump.exe -D or -S (?) can show the generated assembly ...
it is in the arduino distribution.

5

Arduino IDE 1.5.7 BETA.
Arduino Uno

x = min(x, maxValue);
x = volatile int 50, maxValue = volatile int 51 : 1.3 us
x = volatile int 50, maxValue = volatile int 49 : 1.2 us (code size is about 6 bytes longer)
x = volatile int 50, maxValue = const int 51 : 1.1 us
x = volatile int 50, maxValue = const int 49 : 0.88 us (code size is about of 9 bytes longer)

if (x > maxValue) x = maxValue;
x = volatile int 50, maxValue = volatile int 51 : 0.7 us
x = volatile int 50, maxValue = volatile int 49 : 0.7 us
x = volatile int 50, maxValue = const int 51 : 0.5 us
x = volatile int 50, maxValue = const int 49 : 0.5 us

When using two variables that are located somewhere in memory, the second version (if...) is almost twice as fast. The difference is less big when a constant value is used for the 'maxValue'.
I have a large increase of the code size with the min() macro. That is because I use many code lines that are all the same, to improve the timing measurement. The extra bytes are per code line. The compiler optimizes those many code lines in a different way.

6

Peter_n:
Arduino IDE 1.5.7 BETA.
Arduino Uno

x = min(x, maxValue);
x = volatile int 50, maxValue = volatile int 51 : 1.3 us
x = volatile int 50, maxValue = volatile int 49 : 1.2 us (code size is about 6 bytes longer)
x = volatile int 50, maxValue = const int 51 : 1.1 us
x = volatile int 50, maxValue = const int 49 : 0.88 us (code size is about of 9 bytes longer)

if (x > maxValue) x = maxValue;
x = volatile int 50, maxValue = volatile int 51 : 0.7 us
x = volatile int 50, maxValue = volatile int 49 : 0.7 us
x = volatile int 50, maxValue = const int 51 : 0.5 us
x = volatile int 50, maxValue = const int 49 : 0.5 us

When using two variables that are located somewhere in memory, the second version (if...) is almost twice as fast. The difference is less big when a constant value is used for the 'maxValue'.
I have a large increase of the code size with the min() macro. That is because I use many code lines that are all the same, to improve the timing measurement. The extra bytes are per code line. The compiler optimizes those many code lines in a different way.

Thank you very much, I really appreciate your help :smiley:
So to summarize it: Using the second version is faster (even if it's just 0.1 uS) and even uses less bytes (if I got the second part right)

7

Yes, but it is very arbitrary. Normally the 'x' value is already in a register, and the compiler optimization uses that. So in the end the difference might be less.