GCC Optimizations

Hi :)

Been playing around with optimizations when compiling LFS using JHALFS.

Sharing my findings to those who might find it useful, and looking for 
feedback on what could be improved :)


So far I've come up with this:

CFLAGS="-g0 -O3 -pipe -m64 -mfxsr -march=nehalem"

CXXFLAGS=$CFLAGS

LDFLAGS="-Wl,-O3 -Wl,-s"

And using 'make -j6'  (JHALFS makes exceptions for packages which 
doesn't play well with parallel builds).


Also been playing around with "-ffast-math" and "-funroll-loops" - but 
according to the interwebs, they can cause slower/bigger code, so 
omitting them for now.

Not sure if I gain much from the instruction flag "-mfxsr", but it's the 
only instruction set supported by my CPU that isn't enabled by 
"-march=nehalem".

Nor if I gain anything from specifying "-m64" since it's compiled on a 
pure x86_64 system.


Explanations:

               - This is the number 0 (Zero)
              /
             /  - This is the uppercase letter "O" as in "Open"
            /  /
CFLAGS="-g0 -O3 -pipe -m64 -mfxsr -march=nehalem"

-g0    =  Prevents GCC from adding Debug Information to Binaries and Libs.
-O3    =  The highest recommended level of Optimization done by GCC.
-pipe  =  Makes GCC use pipes instead of slow files when parsing information from one process to another.
-m64   =  Sets|'int'|  to 32 bits and|'long'|  and pointer types to 64 bits.
-mfxsr =  An instruction set - but haven't been able to find any useful information.
-march=nehalem = Tells GCC it can use the instruction sets found in Nehalem CPU's: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT instruction set support.

                - This is the lowercase letter L as in "Love"
               /
              /  - The uppercase letter O as in "Okay"
             /  /
LDFLAGS="-Wl,-O3 -Wl,-s"

-Wl,flag = Tells GCC to pass the following flag to the Library Linker.
-Wl,-O3  = Tells GCC to pass the optimization -O3 flag to the Linker.
-Wl,-s   = Tells GCC to pass the -s flag the the linker so it removes all symbol tables and relocation information's from the executable. (Unsafe?)


The -Wl works with comma-delimited flags, so LDFLAGS could also be written as:

LDFLAGS="-Wl,-O3,-s"


Since the -Wl flag instructs GCC to pass the attached flag (eg. -O3 or -s) to its linker, the flags could be embedded (anywhere) in the CFLAGS like this:

CFLAGS="-g0 -O3 -pipe -m64 -Wl,-O3,s -mfxsr -march=nehalem"

Hope someone finds it useful :)

Best regards
    Michael

----

Some useful links to the GCC Documentation:

https://gcc.gnu.org/onlinedocs/gcc/index.html

https://gcc.gnu.org/onlinedocs/gcc/Option-Summary.html

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

https://gcc.gnu.org/onlinedocs/gcc/Link-Options.html

https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html

https://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html


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