CODEWITHSUNDEEP
linuxgccgcc4.7
Krishna M
Krishna M

Reputation: 1195

Using two or more optimize option with GCC command

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.

Upvotes: 1

Views: 169

Answers (2)

DigitalRoss
DigitalRoss

Reputation: 146211

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.

Upvotes: 1

Mat
Mat

Reputation: 206861

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.

Upvotes: 1

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