FORTIFY_SOURCE for the kernel

[Warren Togami]

Josh Boyer wrote:
> 
> If it results in some form of improvement in performance, I'm all for it. 
> I don't think we don't want a degredation in performance just to gain some
> size benefits.  Though from past experience, it results in both size _and_
> performance improvements for most code.
> 
> I'm of the opinion that we should definitely try it.
> 
> Too bad the "Summer of Code" thing is over.  This would have made a great
> project for some eager coder to play with.
> 
> josh
> 


AFAIK, FC has long compiled kernel, firefox and thunderbird with -Os 
instead of -O2.  I personally did some benchmarking of firefox and 
thunderbird during FC1 in fedora.us.  The performance benefit then was 
small but real, so I added this to the %build sections.

export RPM_OPT_FLAGS=$(echo $RPM_OPT_FLAGS | sed 's/-O2/-Os/')

Recently I was helping my friend debug his genetic mutation simulation 
code.  He was building it with -O3 and was surprised gdb was failing.  I 
told him to try it at -O0 or -O1.  Not only was gdb able to debug it at 
the lower levels, his performance intensive code actually went much 
faster (maybe 40%) than -O3.  In the case of his code, it was both 
performance and very memory intensive (using 1-3GB of RAM).

Moral of a story like this is that there is no silver bullet of compiler 
optimization.  If you really want to tune for performance, you have to 
do benchmarking that simulates something like real-world usage.  Maybe 
even different parts of a larger application can be compiled with 
different flags based upon fine-grained profiling analysis.

The trouble however with this kind of analysis and fine-tuning with our 
distro is that things continue to change too fast.  We don't have all 
the features that we need, and everything is a moving target.  Also in 
most cases you can get greater long term efficiency and performance 
gains through profiling for the purpose of fixing/replacing algorithms. 
  Evolution IMAP for example is unusably bad and requires this kind of 
analysis and rewrite.  Compiler flags alone cannot fix bad design.

Warren Togami
wtogami at redhat.com