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Old 01-22-2011, 08:37 PM
"Jason L Tibbitts III"
 
Default About optflags

>>>>> "SB" == Sergio Belkin <sebelk@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<
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Old 01-22-2011, 09:28 PM
Sergio Belkin
 
Default About optflags

2011/1/22 Jason L Tibbitts III <tibbs@math.uh.edu>:
>>>>>> "SB" == Sergio Belkin <sebelk@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.

Let's say that I forget the "-O3" issue. Can I use that CXXFLAGS

"-ansi -Wall -Wno-deprecated" ?

Thanks in advance!




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Old 01-23-2011, 06:03 AM
Ralf Corsepius
 
Default About optflags

On 01/22/2011 11:28 PM, Sergio Belkin wrote:
> 2011/1/22 Jason L Tibbitts III<tibbs@math.uh.edu>:
>>>>>>> "SB" == Sergio Belkin<sebelk@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


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Old 01-23-2011, 04:22 PM
Ville Skyttä
 
Default About optflags

On Saturday 22 January 2011, Sergio Belkin wrote:

> Is it allowed to override "-O2" and use instead "-O3"? I see that
> those flags overrides those CXXFLAGS from Makefile sources.

http://fedoraproject.org/wiki/Packaging/Guidelines#Compiler_flags
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Old 01-23-2011, 04:38 PM
Ralf Corsepius
 
Default About optflags

On 01/23/2011 06:22 PM, Ville Skyttä wrote:
> On Saturday 22 January 2011, Sergio Belkin wrote:
>
>> Is it allowed to override "-O2" and use instead "-O3"? I see that
>> those flags overrides those CXXFLAGS from Makefile sources.
>
> http://fedoraproject.org/wiki/Packaging/Guidelines#Compiler_flags

IMO, we need to revisit this, because it leaves too much room and is too
unspecific (c.f. this thread)

Ralf


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