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I recently discovered that the newer GCC versions now support using -march=native which is pretty cool, because GCC will then find the CFLAGS which suit your system. Here is how I found out which CFLAGS, GCC finds best for my system:


echo 'int main(){return 0;}' > test.c && gcc -v -Q -march=native -O2 test.c -o test && rm test.c test


This command should be executed from the terminal, and make sure you are using GCC >= 4.2.0:


echo 'int main(){return 0;}' > test.c && gcc -v -Q -march=native -O2 test.c -o test && rm test.c test
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: /var/tmp/portage/sys-devel/gcc-4.3.1/work/gcc-4.3.1/configure --prefix=/usr --bindir=/usr/i686-pc-linux-gnu/gcc-bin/4.3.1 --includedir=/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/include --datadir=/usr/share/gcc-data/i686-pc-linux-gnu/4.3.1 --mandir=/usr/share/gcc-data/i686-pc-linux-gnu/4.3.1/man --infodir=/usr/share/gcc-data/i686-pc-linux-gnu/4.3.1/info --with-gxx-include-dir=/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/include/g++-v4 --host=i686-pc-linux-gnu --build=i686-pc-linux-gnu --disable-altivec --enable-nls --without-included-gettext --with-system-zlib --disable-checking --disable-werror --enable-secureplt --disable-multilib --enable-libmudflap --disable-libssp --enable-cld --disable-libgcj --with-arch=i686 --enable-languages=c,c++,treelang,fortran --enable-shared --enable-threads=posix --enable-__cxa_atexit --enable-clocale=gnu --with-bugurl=http://bugs.gentoo.org/ --with-pkgversion='Gentoo 4.3.1 p1.0'
Thread model: posix
gcc version 4.3.1 (Gentoo 4.3.1 p1.0)
COLLECT_GCC_OPTIONS='-v' '-Q' '-O2' '-o' 'test'
/usr/libexec/gcc/i686-pc-linux-gnu/4.3.1/cc1 -v test.c -march=prescott --param l1-cache-size=32 --param l1-cache-line-size=64 -mtune=generic -dumpbase test.c -auxbase test -O2 -version -o /tmp/cc8FolQd.s
ignoring nonexistent directory "/usr/local/include"
ignoring nonexistent directory "/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../../i686-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/include
/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/include-fixed
/usr/include
End of search list.
GNU C (Gentoo 4.3.1 p1.0) version 4.3.1 (i686-pc-linux-gnu)
compiled by GNU C version 4.3.1, GMP version 4.2.2, MPFR version 2.3.1.
GGC heuristics: --param ggc-min-expand=98 --param ggc-min-heapsize=127766
options passed: -v test.c -march=prescott --param l1-cache-size=32
--param l1-cache-line-size=64 -mtune=generic -O2
options enabled: -falign-labels -falign-loops -fargument-alias
-fauto-inc-dec -fbranch-count-reg -fcaller-saves -fcommon
-fcprop-registers -fcrossjumping -fcse-follow-jumps -fdefer-pop
-fdelete-null-pointer-checks -fearly-inlining
-feliminate-unused-debug-types -fexpensive-optimizations
-fforward-propagate -ffunction-cse -fgcse -fgcse-lm
-fguess-branch-probability -fident -fif-conversion -fif-conversion2
-finline-functions-called-once -finline-small-functions -fipa-pure-const
-fipa-reference -fivopts -fkeep-static-consts -fleading-underscore
-fmath-errno -fmerge-constants -fmerge-debug-strings
-fmove-loop-invariants -foptimize-register-move -foptimize-sibling-calls
-fpcc-struct-return -fpeephole -fpeephole2 -fregmove -freorder-blocks
-freorder-functions -frerun-cse-after-loop -fsched-interblock -fsched-spec
-fsched-stalled-insns-dep -fschedule-insns2 -fsigned-zeros
-fsplit-ivs-in-unroller -fsplit-wide-types -fstrict-aliasing
-fstrict-overflow -fthread-jumps -ftoplevel-reorder -ftrapping-math
-ftree-ccp -ftree-ch -ftree-copy-prop -ftree-copyrename -ftree-cselim
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-fre -ftree-loop-im
-ftree-loop-ivcanon -ftree-loop-optimize -ftree-parallelize-loops=
-ftree-pre -ftree-reassoc -ftree-salias -ftree-scev-cprop -ftree-sink
-ftree-sra -ftree-store-ccp -ftree-ter -ftree-vect-loop-version -ftree-vrp
-funit-at-a-time -fvar-tracking -fvect-cost-model
-fzero-initialized-in-bss -m32 -m80387 -m96bit-long-double
-maccumulate-outgoing-args -malign-stringops -mcld -mfancy-math-387
-mfp-ret-in-387 -mfused-madd -mglibc -mieee-fp -mmmx -mno-red-zone
-mno-sse4 -mpush-args -msahf -msse -msse2 -msse3 -mtls-direct-seg-refs
Compiler executable checksum: 11cc1d92ac4cc0eab595215165f2ebd2
main
Analyzing compilation unit
Performing interprocedural optimizations
Assembling functions:
main
Execution times (seconds)
expand : 0.01 (100%) usr 0.00 ( 0%) sys 0.01 ( 1%) wall 3 kB ( 1%) ggc
scheduling 2 : 0.00 ( 0%) usr 0.00 ( 0%) sys 0.74 (87%) wall 0 kB ( 0%) ggc
TOTAL : 0.01 0.00 0.85 584 kB
Internal checks disabled; compiler is not suited for release.
Configure with --enable-checking=release to enable checks.
COLLECT_GCC_OPTIONS='-v' '-Q' '-O2' '-o' 'test'
/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../../i686-pc-linux-gnu/bin/as -V -Qy -o /tmp/cc4KdoRg.o /tmp/cc8FolQd.s
GNU assembler version 2.18 (i686-pc-linux-gnu) using BFD version (GNU Binutils) 2.18
COMPILER_PATH=/usr/libexec/gcc/i686-pc-linux-gnu/4.3.1/:/usr/libexec/gcc/i686-pc-linux-gnu/4.3.1/:/usr/libexec/gcc/i686-pc-linux-gnu/:/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/:/usr/lib/gcc/i686-pc-linux-gnu/:/usr/libexec/gcc/i686-pc-linux-gnu/4.3.1/:/usr/libexec/gcc/i686-pc-linux-gnu/:/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/:/usr/lib/gcc/i686-pc-linux-gnu/:/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../../i686-pc-linux-gnu/bin/
LIBRARY_PATH=/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/:/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/:/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../../i686-pc-linux-gnu/lib/:/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-Q' '-O2' '-o' 'test'
/usr/libexec/gcc/i686-pc-linux-gnu/4.3.1/collect2 --eh-frame-hdr -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 -o test /usr/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../crt1.o /usr/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../crti.o /usr/lib/gcc/i686-pc-linux-gnu/4.3.1/crtbegin.o -L/usr/lib/gcc/i686-pc-linux-gnu/4.3.1 -L/usr/lib/gcc/i686-pc-linux-gnu/4.3.1 -L/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../../i686-pc-linux-gnu/lib -L/usr/lib/gcc/i686-pc-linux-gnu/4.3.1/../../.. /tmp/cc4KdoRg.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/i686-pc-linux-gnu/4.3.1/crtend.o /usr/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../crtn.o


The options that passed for my system are:


options passed: -v test.c -march=prescott --param l1-cache-size=32 --param l1-cache-line-size=64 -mtune=generic -O2


Crickey!