If you want to check what optimizations your current gcc settings give you, you need to use:
echo | gcc -dM -E - -march=native
You can then grep the output to check stuff like MMX, SSE or 3dNOW.
To test a specific setting, like “-march=x86-64 -msse3” :
echo | gcc -dM -E - -march=x86-64 -msse3
To see what gcc recommends for your platform:
echo | gcc -dM -E - -march=native
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