Why is Arduino so fast?

Projects Programming
1

Hi,

first of all, I don't use Arduino a lot. I just like eclipse more than the Arduino IDE, and i always had my own "libs" which I copy/paste if needed.
However I am currently programming something computationally intensive (AHRS) and after finishing my first optimized version i tested the performace on it.
It took my Atmega about 9ms to calculate. (compiled and linked in eclipse, with the latest winavr)

I was curious about the performance of a other (Arduio-based, in my eyes not optimized at all) Code i found on the internet.
It took him 3ms to calculate basically the same. Both used floats.

So i wrote a little test programm. 30k runs: 2 float add, and 2 mulitply.
It took the in arduino compiled code 645ms
The one i compiled in eclipse needed 3933ms

both codes sum up integers at the same speed, so neither of them has timers in background that take too much processing power.

I tried the same compile arguments as arduino uses, but i couldn't get any improvements.

EDIT: funny enough the arduino AHRS-code ported to eclipse takes 17ms to calculate.

Can anyone explain this to me?
greets
Manio

2

Maybe you can post the code used ?
Did you compile under eclipse for the same platform?
Too many unclear ends ...

3

Both were compiled on Windows.
I don't remember the exact code. Something like:

uint16_t i = 0;
float a = 1.1234f;
float b = 1.3456f;
for( ; i < 30000; i++ )
{
    a += i * b;
    b -= a * i;
}
4

Are you using the same version of the compiler? And the same optimization configurations?

5

This sounds like optimization settings with the compiler.

6

Yes...

Can you post the size of the programs here as well? Also, how did you measure the time it took to complete the calculations? Did you write a pin to one and back to zero at the end of the calculations and measured with an oscilloscope? Used millis()? Serial Port?

How many times have you tested?

7

I measured with millis( ) but to clearify I let my uC send 2 chars wire the serial interface and measured the time on the computer.
Optimization settings where exactly the same.

I finally figured it out. Arduino links with the C linker (avr-gcc), Eclipse by default with c++ (avr-g++).
I haven't thought that the linker would make that much of a difference. Maybe the libs he links with do.

Thanks for you help anyways, i know I'm sometimes not a enjoyable questioner.

8

I finally figured it out. Arduino links with the C linker (avr-gcc), Eclipse by default with c++ (avr-g++).

gcc is not the linker. It's the compiler. The gcc compiler is capable of compiling either C or C++ code.

I measured with millis( )

That's akin to measuring time with a calendar.

9

afaik gcc links as well, why would it be in the "C++ Linker Settings" then?

if you measure 30k runs, and you get a difference of 6-7times (650ms vs 4000ms) then its precise enough to show the difference.

10

The difference in timing is quite large and I still suspect an optimization is taking 90% of the guts out of your main loop. I would be interested to see what happens if 'a' and 'b' have a 'volatile' storage class. I would predict the timings would be more in line with one another.

The underlying AVR-LIBC in the two cases is probably a different version, but not significantly different so as to contain a whole different float implementation.