I am using the Instruments to profile a very simple program. Here is the result:
My question is: why the heaviest back trace are annotated on a very simple line of code? I think that line only needs some ALU instructions to implement. Is this a bug or I missed something?
I see nothing unusual in the screenshot you linked to. The 99x line in your screenshot has nothing to do with the heaviness of the backtrace. It is the line of code where Instruments recorded the most samples. The 99x line is inside a loop. Code inside loops is going to execute more often and have more samples.