cc optimize mode

[WG]

Greetings,


when compiling a program with option -O (gcc on Linux) alle unused
variables are stripped.

However, I would like to keep a specific variable.
I use it for version checking in object files with
'strings file.o | grep Id:'.

I do not want to enter a dummy instruction using this variable to
prevent the optimizer from stripping the variable away.

Is this even possible when using the option -O?


Thanks for hints
WG

 

[Jorgen Grahn]

Greetings,


when compiling a program with option -O (gcc on Linux) alle unused
variables are stripped.

-O2 is much more useful than -O.

However, I would like to keep a specific variable.
I use it for version checking in object files with
'strings file.o | grep Id:'.

Better, use ident(1) from the RCS package.

I do not want to enter a dummy instruction using this variable to
prevent the optimizer from stripping the variable away.

Why not? It doesn't have to be something that's actually *called*.

const char* id(void) { return "$Id$"; }

Is this even possible when using the option -O?

Read the gcc manual to see if there are any gcc-specific extensions
for doing this. For functions, there's the 'used' attribute; don't
know about variables.

/Jorgen

 

[Ike Naar]

when compiling a program with option -O (gcc on Linux) alle unused
variables are stripped.

However, I would like to keep a specific variable.
I use it for version checking in object files with
'strings file.o | grep Id:'.

I do not want to enter a dummy instruction using this variable to
prevent the optimizer from stripping the variable away.

Is this even possible when using the option -O?

How is the variable defined?

If the variable has file scope and external linkage,
as below, gcc 4.1.2 -O will keep it:

/* begin id.c */

char const * const version_id = "Id:"; /* never used */

int main(void)
{
return 0;
}

/* end id.c */

 

[WG]

Thanks for your tipps, helped me a lot!

WG