Guida scritta e curata da Sabayonio, vice admin di Sabayon Mania.

Questa che vi presentiamo è una guida per Gentoo che può essere adattata però anche a Sabayon.

Richiede una certa dimestichezza con l’uso di Portage. Quindi se non sapete cosa state facendo chiedete prima sul forum.

Gentoo offre molti tipi di personalizzazioni in fase di compilazione. Anche chi non utilizza Gentoo avrà (a lungo andare) sentito parlare di make.conf.
make.conf è il file di configurazione che informa portage su cosa e come passare alcuni parametri di ottimizzazione al compilatore.

Tra i vari (tanti) parametri di ottimizzazione che il compilatore offre spicca tra tutti -march= , che nella riga del make.conf è contenuta in CFLAGS=”-O2 -march=native -pipe .

Non mi avventuro nella spiegazione di sudetto parametro , ma vorrei soffermarmi su -march il quale indica al compilatore per quale tipo di cpu ottimizzare un determinato pacchetto.

Esempio : -march=pentium ottimizzerà il codice per un pentium di prima generazione
-march = pentiumpro ottimizzerà il codice per un pentium pro

e così via

compilazioni generiche sono : x86_64 , i686 , i586 etc per codice comune a quelle architetture non eccessivamente ottimizzato.

Dalla versione 4.2 di gcc è possibile passare native a -march ; questo setterà automaticamente le opzioni della propria cpu nel caso si abbia dei dubbi su cosa passare al compilatore.

Ma “native” quale impostazione utilizzerà ? Quale “architettura” passerà al compilatore ? Scopriamolo con questo test in “C” :

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

l’output che si otterrà sarà qualcosa di simile a :

sing built-in specs.
COLLECT_GCC=/usr/x86_64-pc-linux-gnu/gcc-bin/4.6.2/gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/4.6.2/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /var/tmp/portage/sys-devel/gcc-4.6.2/work/gcc-4.6.2/configure –prefix=/usr –bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/4.6.2 –includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/include –datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.6.2 –mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.6.2/man –infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.6.2/info –with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/include/g++-v4 –host=x86_64-pc-linux-gnu –build=x86_64-pc-linux-gnu –disable-altivec –disable-fixed-point –with-ppl –with-cloog –disable-ppl-version-check –with-cloog-include=/usr/include/cloog-ppl –enable-lto –enable-nls –without-included-gettext –with-system-zlib –disable-werror –enable-secureplt –enable-multilib –enable-libmudflap –disable-libssp –enable-libgomp –with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/4.6.2/python –enable-checking=release –enable-objc-gc –enable-languages=c,c++,java,objc,obj-c++,fortran –enable-shared –enable-threads=posix –enable-__cxa_atexit –enable-clocale=gnu –enable-targets=all –with-bugurl=http://bugs.gentoo.org/ –with-pkgversion=’Gentoo 4.6.2 p1.0, pie-0.4.5′
Thread model: posix
gcc version 4.6.2 (Gentoo 4.6.2 p1.0, pie-0.4.5)
COLLECT_GCC_OPTIONS=’-v’ ‘-Q’ ‘-march=native’ ‘-O2′ ‘-o’ ‘test’
/usr/libexec/gcc/x86_64-pc-linux-gnu/4.6.2/cc1 -v test.c -D_FORTIFY_SOURCE=2 -march=amdfam10 -mcx16 -msahf -mno-movbe -mno-aes -mno-pclmul -mpopcnt -mabm -mno-lwp -mno-fma -mno-fma4 -mno-xop -mno-bmi -mno-tbm -mno-avx -mno-sse4.2 -mno-sse4.1 –param l1-cache-size=64 –param l1-cache-line-size=64 –param l2-cache-size=512 -mtune=amdfam10 -dumpbase test.c -auxbase test -O2 -version -o /tmp/ccC2Ast2.s
GNU C (Gentoo 4.6.2 p1.0, pie-0.4.5) version 4.6.2 (x86_64-pc-linux-gnu)
compiled by GNU C version 4.6.2, GMP version 5.0.2, MPFR version 3.1.0, MPC version 0.9
GGC heuristics: –param ggc-min-expand=100 –param ggc-min-heapsize=131072
ignoring nonexistent directory “/usr/local/include”
ignoring nonexistent directory “/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/../../../../x86_64-pc-linux-gnu/include”
#include “…” search starts here:
#include search starts here:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/include
/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/include-fixed
/usr/include
End of search list.
GNU C (Gentoo 4.6.2 p1.0, pie-0.4.5) version 4.6.2 (x86_64-pc-linux-gnu)
compiled by GNU C version 4.6.2, GMP version 5.0.2, MPFR version 3.1.0, MPC version 0.9
GGC heuristics: –param ggc-min-expand=100 –param ggc-min-heapsize=131072
options passed: -v test.c -D_FORTIFY_SOURCE=2 -march=amdfam10 -mcx16
-msahf -mno-movbe -mno-aes -mno-pclmul -mpopcnt -mabm -mno-lwp -mno-fma
-mno-fma4 -mno-xop -mno-bmi -mno-tbm -mno-avx -mno-sse4.2 -mno-sse4.1
–param l1-cache-size=64 –param l1-cache-line-size=64
–param l2-cache-size=512 -mtune=amdfam10 -O2
options enabled: -falign-labels -fasynchronous-unwind-tables
-fauto-inc-dec -fbranch-count-reg -fcaller-saves
-fcombine-stack-adjustments -fcommon -fcompare-elim -fcprop-registers
-fcrossjumping -fcse-follow-jumps -fdefer-pop -fdelete-null-pointer-checks
-fdevirtualize -fdwarf2-cfi-asm -fearly-inlining
-feliminate-unused-debug-types -fexpensive-optimizations
-fforward-propagate -ffunction-cse -fgcse -fgcse-lm
-fguess-branch-probability -fident -fif-conversion -fif-conversion2
-findirect-inlining -finline -finline-functions-called-once
-finline-small-functions -fipa-cp -fipa-profile -fipa-pure-const
-fipa-reference -fipa-sra -fira-share-save-slots -fira-share-spill-slots
-fivopts -fkeep-static-consts -fleading-underscore -fmath-errno
-fmerge-constants -fmerge-debug-strings -fmove-loop-invariants
-fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls
-fpartial-inlining -fpeephole -fpeephole2 -fprefetch-loop-arrays
-freg-struct-return -fregmove -freorder-blocks -freorder-functions
-frerun-cse-after-loop -fsched-critical-path-heuristic
-fsched-dep-count-heuristic -fsched-group-heuristic -fsched-interblock
-fsched-last-insn-heuristic -fsched-rank-heuristic -fsched-spec
-fsched-spec-insn-heuristic -fsched-stalled-insns-dep -fschedule-insns2
-fshow-column -fsigned-zeros -fsplit-ivs-in-unroller -fsplit-wide-types
-fstrict-aliasing -fstrict-overflow -fstrict-volatile-bitfields
-fthread-jumps -ftoplevel-reorder -ftrapping-math -ftree-bit-ccp
-ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-copy-prop
-ftree-copyrename -ftree-cselim -ftree-dce -ftree-dominator-opts
-ftree-dse -ftree-forwprop -ftree-fre -ftree-loop-if-convert
-ftree-loop-im -ftree-loop-ivcanon -ftree-loop-optimize
-ftree-parallelize-loops= -ftree-phiprop -ftree-pre -ftree-pta
-ftree-reassoc -ftree-scev-cprop -ftree-sink -ftree-slp-vectorize
-ftree-sra -ftree-switch-conversion -ftree-ter -ftree-vect-loop-version
-ftree-vrp -funit-at-a-time -funwind-tables -fvar-tracking
-fvar-tracking-assignments -fvect-cost-model -fzee
-fzero-initialized-in-bss -m128bit-long-double -m3dnow -m64 -m80387 -mabm
-maccumulate-outgoing-args -malign-stringops -mcx16 -mfancy-math-387
-mfp-ret-in-387 -mglibc -mieee-fp -mmmx -mno-sse4 -mpopcnt -mpush-args
-mred-zone -msahf -msse -msse2 -msse3 -msse4a -mtls-direct-seg-refs
Compiler executable checksum: 28388eca5d3f9ace699e0dfb3707ef35
main
Analyzing compilation unit
Performing interprocedural optimizations
Assembling functions:
main
Execution times (seconds)
integrated RA : 0.00 ( 0%) usr 0.00 ( 0%) sys 0.01 (50%) wall 2 kB ( 0%) ggc
TOTAL : 0.00 0.00 0.02 1508 kB
COLLECT_GCC_OPTIONS=’-v’ ‘-Q’ ‘-march=native’ ‘-O2′ ‘-o’ ‘test’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/../../../../x86_64-pc-linux-gnu/bin/as –64 -o /tmp/ccVrCph3.o /tmp/ccC2Ast2.s
COMPILER_PATH=/usr/libexec/gcc/x86_64-pc-linux-gnu/4.6.2/:/usr/libexec/gcc/x86_64-pc-linux-gnu/4.6.2/:/usr/libexec/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/../../../../x86_64-pc-linux-gnu/bin/
LIBRARY_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/:/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/../../../../x86_64-pc-linux-gnu/lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS=’-v’ ‘-Q’ ‘-march=native’ ‘-O2′ ‘-o’ ‘test’
/usr/libexec/gcc/x86_64-pc-linux-gnu/4.6.2/collect2 –eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o test /usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/../../../../lib64/crt1.o /usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/../../../../lib64/crti.o /usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/crtbegin.o -L/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2 -L/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/../../../../lib64 -L/lib/../lib64 -L/usr/lib/../lib64 -L/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/../../../../x86_64-pc-linux-gnu/lib -L/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/../../.. /tmp/ccVrCph3.o -lgcc –as-needed -lgcc_s –no-as-needed -lc -lgcc –as-needed -lgcc_s –no-as-needed /usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/crtend.o /usr/lib/gcc/x86_64-pc-linux-gnu/4.6.2/../../../../lib64/crtn.o

la parte che interessa a noi sarà quella iniziale circa :

gcc version 4.6.2 (Gentoo 4.6.2 p1.0, pie-0.4.5)
COLLECT_GCC_OPTIONS='-v' '-Q' '-march=native' '-O2' '-o' 'test'
/usr/libexec/gcc/x86_64-pc-linux-gnu/4.6.2/cc1 -v test.c -D_FORTIFY_SOURCE=2 -march=amdfam10 -mcx16 -msahf -mno-movbe -mno-aes -mno-pclmul -mpopcnt -mabm -mno-lwp -mno-fma -mno-fma4 -mno-xop -mno-bmi -mno-tbm -mno-avx -mno-sse4.2 -mno-sse4.1 --param l1-cache-size=64 --param l1-cache-line-size=64 --param l2-cache-size=512 -mtune=amdfam10 -dumpbase test.c -auxbase test -O2 -version -o /tmp/ccC2Ast2.s
GNU C (Gentoo 4.6.2 p1.0, pie-0.4.5) version 4.6.2 (x86_64-pc-linux-gnu)
compiled by GNU C version 4.6.2, GMP version 5.0.2, MPFR version 3.1.0, MPC version 0.9

la parte evidenziata in grassetto sarà l’ottimizzazione effettiva della propria cpu (il parametro -march varierà a seconda della vostra cpu.)

Nel mio caso impostando -march=native nel make.conf , Gcc passerà l’ottimizzazione per una CPU AMD : amdfam10

Buon Test CFLAGS :-*