enabling optimizations (-Og) with --enable-debug

[Michael Stahl]

On 01/15/2015 01:47 PM, Luke Deller wrote:
> Hi,
> 
> I saw that gcc-4.8 introduced a new optimization level -Og which enables 
> optimizations that do not interfere with debugging.
> 
> Yesterday I floated the idea on IRC of using this with --enable-debug 
> rather than -O0 if available, and some feedback was that it enables gcc 
> to report extra warnings which would be a good thing.

somebody had the same idea last week... and i've now discovered this old
thread...

> I tried it out and can see that it does indeed break the build if 
> --enable-werror is enabled, eg:
> 
> /home/deller/build/libo/vcl/unx/glxtest.cxx: In function ‘void glxtest()’:
> /home/deller/build/libo/vcl/unx/glxtest.cxx:236:44: error: ignoring 
> return value of ‘ssize_t write(int, const void*, size_t)’, declared with 
> attribute warn_unused_result [-Werror=unused-result]
>     write(write_end_of_the_pipe, buf, length);
> 
> So I guess this means we couldn't incorporate -Og until all these new 
> warnings are fixed right?

i've fixed all spurious warnings from GCC 6.2.1 -Og on master.

> Any other thoughts on whether this is a good idea to pursue?  One fear 
> was that it might slow down compilation, but it doesn't actually seem to 
> slow down "make clean && make" much at all for me.  I'll get some proper 
> timing measurements.

i've done a few timings, see the commit message of
https://gerrit.libreoffice.org/#/c/31334/

summary: from-scratch make check is a tiny bit slower (75m -O0, 76m -Og)
*but* incremental make check is 20% faster (12m vs. 15m) so for the
average developer it should be a win.

> (A draft patch is attached to enable -Og if available)

thanks, i've taken the configure part of this.

only open question is if it actually works as advertised, i.e. is gdb
confused by the optimized code or not.  anybody want to experiment?