Re: [gentoo-user] CFLAGS changes

[Volker Armin Hemmann <volker.armin.hemmann@tu-clausthal.de>]

On Donnerstag, 21. Februar 2008, KH wrote:
> James wrote:
> > Hello,
> >
> >
> > Current  CFLAGS="-O2 -march=i686 -pipe"
> >
> >
> > I want to add "-fomit-frame-pointer" to my
> > CFLAGS on an existing system that has been running
> > for months.
> >
> >
> > Is this safe or do I have to rebuild everything with
> > somelike "emerge -e" ?
> >
> >
> > Should it be avoided completely?
> >
> > James
>
> Hi,
>
> I am not too good in English but I alway understood that:
> |-fomit-frame-pointer|
>
>     Don't keep the frame pointer in a register for functions that don't
>     need one. This avoids the instructions to save, set up and restore
>     frame pointers; it also makes an extra register available in many
>     functions. *It also makes debugging impossible on some machines.*
>
>     On some machines, such as the VAX, this flag has no effect, because
>     the standard calling sequence automatically handles the frame
>     pointer and nothing is saved by pretending it doesn't exist. The
>     machine-description macro |FRAME_POINTER_REQUIRED| controls whether
>     a target machine supports this flag. See Register Usage
>    
> <http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gccint/Registers.html#Registers>.
>
>
>     Enabled at levels -O, -O2, -O3, -Os.
>
>
> means, that fomit-frame-pointer is already active when -o ... is choose.
>
> see
> http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Optimize-Options.html#Optimize-
>Options

man gcc:
-O also turns on -fomit-frame-pointer on machines where doing so does not 
interfere with debugging.

It doesn't hurt to set it anyway. If it is enabled by OX, it will be ignored. 
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