For loop performance difference, and compiler optimization

Question

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I chose David\'s answer because he was the only one to present a solution to the difference in the for-loops with no optimization flags. The other answers demonstrate what

Answer 1

Checking the assembly output is really the only way to illuminate such things.

Compiler optimisations will do a great deal of things, including things that are not strictly "standard compliant" (although, that is not the case with -O1 and -O2, to my knowledge) - for instance check, -Ofast switch.

I have found this helpful: http://gcc.godbolt.org/, and with your demo code here