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I am learning about gcc optimization. As far as I understand the compilers tries to improve/ reduce the code.

That is noticeable when I execute different files with different optimization levels. Adding -O3 during compiling I get the fastest result when I run the compiled program.

As it has been improved, I thought the assembly files must have less commands. However, when looking into that files I saw that it has even more than the default one.

  • Is it true that the assembly file will have more commands, nevertheless it is running faster than the other one? If yes, how is that possible? What has been improved?
  • What is the general influence on assembly / object files when using optimization flags in gcc?
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    The second question is interesting but too broad for Stackoverflow as it's not a simple answerable question that is suitable for the SO format. And the answer to the first question is yes, of course. In fact, trading space for speed is a common computing tradeoff. For starters, read the gcc manual optimsations section which summarises the optimisations options which will or will not affect code size. Commented Dec 22, 2019 at 21:13
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    @kaylum in fact, most of making a compiler generate good code is about finding the right space/speed tradeoff. Commented Dec 22, 2019 at 21:14

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Compiler optimizations may frequently cause the number of instructions to increase, in order for execution time to decrease. For example, if the compiler sees that a for-loop will always loop exactly N times, it may choose to generate N copies of the loop body, thereby avoiding jump/comparison instructions.

You can tell the compiler to optimize for size (e.g. -Os), in which case it will try really hard not to increase the number of instructions, sometimes at the expense of a slower program. This is very common in embedded applications where the amount of RAM is the most constraining factor.

Running e.g. gcc with -O2 will cause the compiler to try to balance the code to be as fast as possible, while not growing too much in size. Exact how this affects the assembler/object files is very complicated. If you want to know more, there are lots of online courses covering compiler construction and optimizations.

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Thank you @JesperE Great answer!!
Your first example was great... that's why I accepted your answer. I found exact what your described in my assembly, thanks!!

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