Using two or more optimize option with GCC command

Question

I am trying to compile a opensource component from source code. I am compiling all C files in that component using gcc command.

When I pass options in order -O2 -Os, binary is in few KB's. But when I pass options in order -Os -O2 binary size is large.

I do know that order matter in case of including sub-directories or Linking libraries in gcc command. Why order matters for optimization arguments of gcc command ?

I am using gcc version 4.9.1.

Answer 1

Because it's just using the last¹ option it sees.

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^{1. From the man page: If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.}