[CMake] default release build flags

[Burlen Loring]

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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