[Qemu-devel] Should configure --enable-debug add -Og to CFLAGS?

[Qemu-devel] Should configure --enable-debug add -Og to CFLAGS?

[Markus Armbruster]

The fine manual advises:

'-O0'
     Reduce compilation time and make debugging produce the expected
     results.  This is the default.
[...]
'-Og'
     Optimize debugging experience.  '-Og' should be the optimization
     level of choice for the standard edit-compile-debug cycle, offering
     a reasonable level of optimization while maintaining fast
     compilation and a good debugging experience.  It is a better choice
     than '-O0' for producing debuggable code because some compiler
     passes that collect debug information are disabled at '-O0'.

Our configure --enable-debug effectively picks -O0.  Should it pick -Og
instead?

Re: [Qemu-devel] Should configure --enable-debug add -Og to CFLAGS?

[Paolo Bonzini]

On 27/06/19 17:43, Markus Armbruster wrote:
> The fine manual advises:
> 
> '-O0'
>      Reduce compilation time and make debugging produce the expected
>      results.  This is the default.
> [...]
> '-Og'
>      Optimize debugging experience.  '-Og' should be the optimization
>      level of choice for the standard edit-compile-debug cycle, offering
>      a reasonable level of optimization while maintaining fast
>      compilation and a good debugging experience.  It is a better choice
>      than '-O0' for producing debuggable code because some compiler
>      passes that collect debug information are disabled at '-O0'.
> 
> Our configure --enable-debug effectively picks -O0.  Should it pick -Og
> instead?
> 

    commit 48e56d503e18bd1e8a75463fd7cc1580bf7e7650
    Author: Paolo Bonzini <pbonzini@redhat.com>
    Date:   Tue Mar 6 11:32:44 2018 +0100

    Revert "build-sys: compile with -Og or -O1 when --enable-debug"

    This reverts commit 906548689e37ab6cca1e93b3f8d9327a4e17e8af.
    Even with -Og, the debug experience is noticeably worse
    because gdb shows a lot more "<optimised out>" variables and
    function arguments.

    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

:(

Paolo

Re: [Qemu-devel] Should configure --enable-debug add -Og to CFLAGS?

[Markus Armbruster]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 27/06/19 17:43, Markus Armbruster wrote:
>> The fine manual advises:
>> 
>> '-O0'
>>      Reduce compilation time and make debugging produce the expected
>>      results.  This is the default.
>> [...]
>> '-Og'
>>      Optimize debugging experience.  '-Og' should be the optimization
>>      level of choice for the standard edit-compile-debug cycle, offering
>>      a reasonable level of optimization while maintaining fast
>>      compilation and a good debugging experience.  It is a better choice
>>      than '-O0' for producing debuggable code because some compiler
>>      passes that collect debug information are disabled at '-O0'.
>> 
>> Our configure --enable-debug effectively picks -O0.  Should it pick -Og
>> instead?
>> 
>
>     commit 48e56d503e18bd1e8a75463fd7cc1580bf7e7650
>     Author: Paolo Bonzini <pbonzini@redhat.com>
>     Date:   Tue Mar 6 11:32:44 2018 +0100
>
>     Revert "build-sys: compile with -Og or -O1 when --enable-debug"
>
>     This reverts commit 906548689e37ab6cca1e93b3f8d9327a4e17e8af.
>     Even with -Og, the debug experience is noticeably worse
>     because gdb shows a lot more "<optimised out>" variables and
>     function arguments.
>
>     Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>
> :(

Aha, GCC's documentation is more aspirational than factual here.
Nevermind!

Re: [Qemu-devel] Should configure --enable-debug add -Og to CFLAGS?

[Daniel P. Berrangé]

On Thu, Jun 27, 2019 at 05:43:30PM +0200, Markus Armbruster wrote:
> The fine manual advises:
> 
> '-O0'
>      Reduce compilation time and make debugging produce the expected
>      results.  This is the default.
> [...]
> '-Og'
>      Optimize debugging experience.  '-Og' should be the optimization
>      level of choice for the standard edit-compile-debug cycle, offering
>      a reasonable level of optimization while maintaining fast
>      compilation and a good debugging experience.  It is a better choice
>      than '-O0' for producing debuggable code because some compiler
>      passes that collect debug information are disabled at '-O0'.
> 
> Our configure --enable-debug effectively picks -O0.  Should it pick -Og
> instead?

NB, it is the --enable-debug-info option to configure that actually
adds the "-g" flag for debug info in objects, and turns off -O2.

--enable-debug turns on TCG and Mutex debugging & disables fortify
source, but doesn't affect optimization level.

Adding -Og to --enable-debug-info would be problematic for Fedora/RHEL
distro builds, as we want debugging symbols, but also still want -O2
to be used.

Yes, the naming of these options is a trapdoor over a pit of crocodiles :-)

Regards,
Daniel
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