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.

DigitalRoss · Accepted Answer · 2016-01-07T05:22:22.490

1

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

^{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.}

edited Jan 07 '16 at 05:22

answered Jan 07 '16 at 05:20

DigitalRoss

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score 1 · Answer 2 · answered Jan 07 '16 at 05:21

From the GCC man page:

If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.

You can't combine -O2 and -Os on the command line.

But here's the description of -Os:

Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size.

Looks like -Os is already doing what you want.

Using two or more optimize option with GCC command

2 Answers2