[CMake] default release build flags

[Robert Maynard]

I think that changing the Release mode to generate non portable code
is very nasty side-effect that would bite numerous projects down the
road. I think a better avenue would be to think about adding another
build configuration called ReleaseNative which would produce highly
optimized non portable code. ( Name would need iteration ).

Are you encountering this issue mainly when building locally or for
HPC machines? I ask because maybe expanding the cross compilation
infrastructure to set native flags would be a better avenue.

On Thu, Apr 28, 2016 at 2:21 PM, Burlen Loring <burlen.loring at gmail.com> wrote:
> Hi Guys,
>
> I'd like to discuss changing the defaults of CMAKE_C/CXX_FLAGS_RELEASE on
> gcc, and potentially gcc like compilers such as clang and intel.
>
> Currently the default is "-O3 -DNDEBUG". I would like to discuss changing
> this to "-O3 -march=native -mtune=native -DNDEBUG". This change will enable
> numerous optimizations and produce faster code targeted for the cpu in use
> during the compilation.
>
> The majority of people want cpu specific optimization enabled when compiling
> code for use on their system. however many folks (specifically users of
> scientific codes who are often not computer scientists) don't realize
> requirement of passing march and mtune flags to enable cpu specific
> instruction sets. Of course there are a small number of developers making
> binaries for distribution who do not want to use cpu specific instructions,
> but the number of developers make binaries for distribution is small
> compared to folks who just need to compile code for their own use, and the
> folks making binaries are well aware of the issues and can be expected to
> know to disable such features.
>
> on my system here is the difference
>
> -03:
>
> $gcc -O3 -Q --help=target | grep enabled
>   -m64                                [enabled]
>   -m80387                             [enabled]
>   -m96bit-long-double                 [enabled]
>   -malign-stringops                   [enabled]
>   -mfancy-math-387                    [enabled]
>   -mfentry                            [enabled]
>   -mfp-ret-in-387                     [enabled]
>   -mglibc                             [enabled]
>   -mhard-float                        [enabled]
>   -mieee-fp                           [enabled]
>   -mlong-double-80                    [enabled]
>   -mno-sse4                           [enabled]
>   -mpush-args                         [enabled]
>   -mred-zone                          [enabled]
>   -mstackrealign                      [enabled]
>   -mtls-direct-seg-refs               [enabled]
>
> -O3 -march=native -mtune=native
>
> $gcc -O3 -march=native -mtune=native -Q --help=target | grep enabled
>   -m64                                [enabled]
>   -m80387                             [enabled]
>   -m96bit-long-double                 [enabled]
>   -maes                               [enabled]
>   -malign-stringops                   [enabled]
>   -mavx                               [enabled]
>   -mcx16                              [enabled]
>   -mf16c                              [enabled]
>   -mfancy-math-387                    [enabled]
>   -mfentry                            [enabled]
>   -mfp-ret-in-387                     [enabled]
>   -mfsgsbase                          [enabled]
>   -mfxsr                              [enabled]
>   -mglibc                             [enabled]
>   -mhard-float                        [enabled]
>   -mieee-fp                           [enabled]
>   -mlong-double-80                    [enabled]
>   -mmmx                               [enabled]
>   -mpclmul                            [enabled]
>   -mpopcnt                            [enabled]
>   -mpush-args                         [enabled]
>   -mrdrnd                             [enabled]
>   -mred-zone                          [enabled]
>   -msahf                              [enabled]
>   -msse                               [enabled]
>   -msse2                              [enabled]
>   -msse3                              [enabled]
>   -msse4                              [enabled]
>   -msse4.1                            [enabled]
>   -msse4.2                            [enabled]
>   -mssse3                             [enabled]
>   -mstackrealign                      [enabled]
>   -mtls-direct-seg-refs               [enabled]
>   -mxsave                             [enabled]
>   -mxsaveopt                          [enabled]
>
> notice specifically the lack of sse and avx in -O3! these instruction sets
> play a major role in providing computational performance in modern cpus, and
> that's why I think they should be enable by default in cmake Release builds.
> clang and intel also support these flags, although there may be better
> alternatives.
>
> What do you think?
>
> Burlen
>
> --
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