Re: What is the correct way to disable -O2?

[Eric Blake]

On 1/23/19 12:10 PM, Peng Yu wrote:
> Hi,
> 
> I had to use the following command to disable -O2 in the default of
> configure of bash.
> 
> http://git.savannah.gnu.org/cgit/bash.git/tree/configure
> 
> CFLAGS='' -g -O0" ./configure

That's not the command you used.  Maybe you meant:

CFLAGS="-g -O0" ./configure

but in that case, the GNU Coding Standards recommends that you spell it:

./configure CFLAGS="-g -O0"

(although the difference between two forms doesn't really matter to
configure scripts generated by Autoconf, it CAN matter to other
configure scripts that are still compliant with GCS).

> 
> But the following command does not disable -O2. I think that -O2
> should be applicable to both C and C++ code, so it should have been
> set to CPPFLAGS as  CPPFLAGS is for both C and C++ but CFLAGS is only
> for C.

Rather, CPPFLAGS is for the preprocessor (and affects C and C++ in that
both use the pre-processor), but gets processed prior to CFLAGS on the
command line, and compilers go with the last version set (so if CFLAGS
contains -O2, that overrides any -O0 in CPPFLAGS).

You are correct that setting CFLAGS="-g -O2" at configure time is the
preferred way to disable optimization; but whether it works or not also
depends on whether a particular project properly followed
Autoconf/Automake conventions.  Your command line works on other
projects that use both of the autotools; but bash is notorious for using
Autoconf but not Automake; so it could very well be a bug specific to
bash's hand-rolled Makefile.in files for not obeying the GNU Coding
Standards in the same way that Automake would do for you automatically.
But that puts it as a question for automake and/or bash lists, rather
than the autoconf list.

> 
> CPPFLAGS='' -g -O0" ./configure
> 
> Could anybody let me know what is the standard way to disable -O2? I'd
> like a way that works for many autoconf-based packages besides just
> bash.
> 
> Also, the configure file has this. Both -g and -O2 are compile-time
> options. Why are they used for linking? Is this usage correct?

They are more than just compile-time options - the linker needs to know
whether to include debug sections in the resulting binary.

> 
> AUTO_LDFLAGS="-g ${GCC+-O2}"
> 

The identifier AUTO_LDFLAGS is not present in Autoconf sources; at this
point, any question about its intent should be asked to the bash project
as the place that declared that variable.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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