Gentoo linux
 

 Tips:

To change default scheduler append elevator={cfq|deadline|anticipatory} to your grub/lilo conf

CFLAGS

-O2
Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. The compiler does not perform loop unrolling or function inlining when you specify -O2. As compared to -O, this option increases both compilation time and the performance of the generated code.

-O2 turns on all optimization flags specified by -O. It also turns on the following optimization flags:

          -fforce-mem 
          -foptimize-sibling-calls 
          -fstrength-reduce 
          -fcse-follow-jumps  -fcse-skip-blocks 
          -frerun-cse-after-loop  -frerun-loop-opt 
          -fgcse  -fgcse-lm  -fgcse-sm  -fgcse-las 
          -fdelete-null-pointer-checks 
          -fexpensive-optimizations 
          -fregmove 
          -fschedule-insns  -fschedule-insns2 
          -fsched-interblock  -fsched-spec 
          -fcaller-saves 
          -fpeephole2 
          -freorder-blocks  -freorder-functions 
          -fstrict-aliasing 
          -funit-at-a-time 
          -falign-functions  -falign-jumps 
          -falign-loops  -falign-labels 
          -fcrossjumping
     
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging impossible on some machines.

On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag. See Register Usage.

Enabled at levels -O, -O2, -O3, -Os.


 

-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization will most benefit processors with lots of registers. It can, however, make debugging impossible, since variables will no longer stay in a “home register”.
-fno-ident
Ignore the `#ident' directive.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions.
-march=cpu-type
Generate instructions for the machine type cpu-type. The choices for cpu-type are the same as for -mtune. Moreover, specifying -march=cpu-type implies -mtune=cpu-type.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions. The option -fomit-frame-pointer removes the frame pointer for all functions which might make debugging harder.

LDFLAGS

--sort-common
This option tells ld to sort the common symbols by size when it places them in the appropriate output sections. First come all the one byte symbols, then all the two bytes, then all the four bytes, and then everything else. This is to prevent gaps between symbols due to alignment constraints.

-Wl

(it's L not 1) enables passing LDFLAGS to linker when called indirectly from gcc.

- O1

                 turns on ld optimization

-s  , --strip-all
Omit all symbol information from the output file.
--enable-new-dtags
--disable-new-dtags
This linker can create the new dynamic tags in ELF. But the older ELF systems may not understand them. If you specify --enable-new-dtags, the dynamic tags will be created as needed. If you specify --disable-new-dtags, no new dynamic tags will be created. By default, the new dynamic tags are not created. Note that those options are only available for ELF systems.

Set the default size of the linker's hash tables to a prime number close to number. Increasing this value can reduce the length of time it takes the linker to perform its tasks, at the expense of increasing the linker's memory requirements. Similarly reducing this value can reduce the memory requirements at the expense of speed.

--as-needed
--no-as-needed
This option affects ELF DT_NEEDED tags for dynamic libraries mentioned on the command line after the --as-needed option. Normally, the linker will add a DT_NEEDED tag for each dynamic library mentioned on the command line, regardless of whether the library is actually needed. --as-needed causes DT_NEEDED tags to only be emitted for libraries that satisfy some symbol reference from regular objects which is undefined at the point that the library was linked. --no-as-needed restores the default behaviour.