viability to turn on -mavx / -mavx2 by default

[Christian Krause]

Hello,

I have an "Intel(R) Core(TM) i5-4250U CPU @ 1.30GHz" (/proc/cpuinfo), 
which is haswell architecture and includes all of the following flags:

     sse sse2 ssse3 sse4_1 sse4_2 avx avx2


I'm usually compiling with these flags for optimization:

     CFLAGS="-march=native -mfpmath=sse -O2"
     CXXFLAGS="-march=native -mfpmath=sse -O2"


Now to my questions:

1.  Since I have avx and avx2, is it viable to add -mavx / -mavx2 to the 
above-mentioned flags to improve overall optimization? Does 
-march=native enable these automatically?

2.  Would adding these flags improve performance in all / most cases? 
Are there cases where adding -mavx / -mavx2 backfires in terms of e.g. 
performance?

3.  Are these flags worth something, even if the code does not 
explicitly use any avx/2 instructions?

4.  As of the 4.9.2 man page I quote

     > GCC depresses SSEx instructions when -mavx is used. Instead,
     > it generates new AVX instructions or AVX equivalence for all
     > SSEx instructions when needed.

     What happens with -mfpmath=sse when -mavx / -mavx2 is enabled? Or 
does this mean only the -msse* flags are depressed?

     Since -msse2avx is turned on by -mavx automatically are 
-mfpmath=sse instructions also encoded with VEX prefix? Would it not 
make sense to add an option to have -mpfmath=avx?

5.  If I have both avx and avx2, will -mavx2 switch on -mavx 
automatically? (this is not covered by the man page)


To answer some of my questions myself, I ran (see output attached):

     gcc -march=native -mfpmath=sse -O2 -Q --help=target -v

The output shows, that -mavx and -mavx2 are turned on by default with 
-march=native and my haswell architecture. However, -msse2avx is still 
disabled. Shouldn't this be enabled as well since -mavx is enabled? Is 
this a bug?

Best Regards
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