How to see which flags -march native will activate

Question

I m compiling my C   app using GCC 4 3  Instead of manually selecting the optimization flags I m using -march native  which in theory should add all optimization flags applicable to the hardware I m compiling on  But how can I check which flags is it actually using

User · Answer

If you want to find out how to set-up a non-native cross compile, I found this useful:

On the target machine,

% gcc -march=native -Q --help=target | grep march
-march=                               core-avx-i

Then use this on the build machine:

% gcc -march=core-avx-i ...

User · Answer

You can use the -Q --help target options   gcc -march native -Q --help target       The -v option may also be of use   You can see the documentation on the --help option here

User · Answer

It should be  -    is similar to -v    echo   gcc -    -E - -march native    To show the  real  native flags for gcc   You can make them appear more  clearly  with a command   gcc -    -E - -march native 2 gt  amp 1   sed -r   cc1  d s          -    g    and you can get rid of flags with -mno-  with   gcc -    -E - -march native 2 gt  amp 1   sed -r   cc1  d s          -     -mno-         g

User · Answer

I m going to throw my two cents into this question and suggest a slightly more verbose extension of elias s answer  As of gcc 4 6  running of gcc -march native -v -E -  lt   dev null emits an increasing amount of spam in the form of superfluous -mno-  flags  The following will strip these  gcc -march native -v -E -  lt   dev null 2 gt  amp 1   grep cc1   perl -pe  s  -mno- S   g  s     -   g    However  I have only verified the correctness of this on two different CPUs  an Intel Core2 and AMD Phenom   so I suggest also running the following script to be sure that all of these -mno-  flags can be safely stripped  2021 EDIT  There are indeed machines where -march native uses a particular -march value  but must disable some implied ISAs  Instruction Set Architecture  with -mno-      bin bash  gcc cmd  quot gcc quot     Optionally supply path to gcc as first argument if         then     gcc cmd  quot  1 quot  fi  with mno         quot   gcc cmd  quot  -march native -mtune native -v -E -  lt   dev null 2 gt  amp 1       grep cc1       perl -pe  s     -   g     without mno   echo  quot   with mno  quot    perl -pe  s  -mno- S   g      quot   gcc cmd  quot    with mno     -dM -E -  lt   dev null  gt   tmp gcctest a     quot   gcc cmd  quot    without mno  -dM -E -  lt   dev null  gt   tmp gcctest b     if diff -u  tmp gcctest  a b      then     echo  quot Safe to strip -mno-  options  quot  else     echo     echo  quot WARNING  Some -mno-  options are needed  quot      exit 1 fi  rm  tmp gcctest  a b      I haven t found a difference between gcc -march native -v -E -  lt   dev null and gcc -march native -    -E -  lt   dev null other than some parameters being quoted -- and parameters that contain no special characters  so I m not sure under what circumstances this makes any real difference  Finally  note that --march native was introduced in gcc 4 2  prior to which it is just an unrecognized argument

User · Answer

To see command-line flags  use   gcc -march native -E -v -  lt  dev null 2 gt  amp 1   grep cc1   If you want to see the compiler precompiler defines set by certain parameters  do this   echo   gcc -dM -E - -march native

[gcc] How to see which flags -march=native will activate?

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