Re: [Fedora-packaging] About optflags
On 01/22/2011 11:28 PM, Sergio Belkin wrote:
2011/1/22 Jason L Tibbitts III<tibbs(a)math.uh.edu>:
>>>>>> "SB" == Sergio Belkin<sebelk(a)gmail.com> writes:
>
> SB> Is it allowed to override "-O2" and use instead "-O3"? I
see that
> SB> those flags overrides those CXXFLAGS from Makefile sources.
>
> Of course for your personal packages you can do whatever you like. In
> Fedora, though, the answer is that you should use the provided optflags
> unless you have specific justification for a change. In order to change
> -O2 to -O3 I'd want to see benchmarking and other such evidence that
> indicates that the change actually makes a difference. Otherwise it's
> just a case of "adding -O99 for uber mega speedz0rs!!!11!!1one".
>
> - J<
> --
> packaging mailing list
Thanks for the answer, could you tell me (I'm really no ironic) could
you tell me when I can find that such a evidence.
Well, I am not sure if I
understand correctly.
The problem with -O3 vs. -O2 is -O3 turning on, less-tested agressive
features, which
* may trip over bugs inside of the compiler (e.g. compiler ICEs).
* may cause mis-compiled/defective binaries.
* may break interaction with other tools (e.g. break debuginfos)
Also, "-O2 vs. -O3" benchmarks are of little significance, because
individual upstreams have little possibility to know about the
generality and significance of such benchmarks. I.e. though an upstream
may be able to prove "-O3 is 5% faster for application XXX on
Ubuntu-Y-x86_64", this figure doesn't tell much about the impact of "-O3
on fedora-X-powerpc".
Let's say that I forget the "-O3" issue. Can I use that
CXXFLAGS
"-ansi -Wall -Wno-deprecated" ?
-ansi normally doesn't affect code
generation, so it's mostly harmless.
-ansi however is somewhat problematic when it comes to portability,
because the impact of -ansi is not consistent across different OSes.
Adding further warning flags (-W*; not -Wno-* == suppressing warnings)
is harmless.
Ralf