g++ O1 is not equal to O0 with all related optimization flags

Question

I know the title is a bit confusing. Let me clarify my problem with a little background:

My program behaves strangely when I compile it with -O1 flag vs

Answer 1

The optimizations flags applied by -O1 only apply when the optimizer is turned on. You need to specify -On with n > 0 in order for the optimization flags to actually do anything.

To put it another way, -O0 doesn't turn on the optimizer, so the optimization flags don't do anything.

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You can turn off optimzations flags by using the -fno form of the flag. For instance the

-fcompare-elim 

flag is turned on by -O1 and you can turn it back off using

-fno-compare-elim 

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Another thing to note, as pointed out by T.C., is that not all optimizations have a flag so there isn't any way to turn those particular optimizations off.