Options That Control Optimization
 
       These options control various sorts of optimizations.
 
       Without any optimization option, the compiler's goal is to reduce the
       cost of compilation and to make debugging produce the expected results.
       Statements are independent: if you stop the program with a breakpoint
       between statements, you can then assign a new value to any variable or
       change the program counter to any other statement in the function and
       get exactly the results you would expect from the source code.
 
       Turning on optimization flags makes the compiler attempt to improve the
       performance and/or code size at the expense of compilation time and
       possibly the ability to debug the program.
 
       Not all optimizations are controlled directly by a flag.         Only opti-
       mizations that have a flag are listed.
 
       -O
       -O1 Optimize.  Optimizing compilation takes somewhat more time, and a
           lot more memory for a large function.
 
           With -O, the compiler tries to reduce code size and execution time,
           without performing any optimizations that take a great deal of com-
           pilation time.
 
           In Apple's version of GCC, -fstrict-aliasing, -freorder-blocks, and
           -fsched-interblock are disabled by default when optimizing.
 
       -O2 Optimize even more.        GCC performs nearly all supported optimiza-
           tions that do not involve a space-speed tradeoff.  The compiler
           does not perform loop unrolling or function inlining when you spec-
           ify -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-sib-
           ling-calls -fstrength-reduce -fcse-follow-jumps  -fcse-skip-blocks
           -frerun-cse-after-loop  -frerun-loop-opt -fgcse   -fgcse-lm
           -fgcse-sm -fdelete-null-pointer-checks -fexpensive-optimizations
           -fregmove -fschedule-insns  -fschedule-insns2 -fsched-interblock
           -fsched-spec -fcaller-saves -fpeephole2 -freorder-blocks  -fre-
           order-functions -fstrict-aliasing -falign-functions        -falign-jumps
           -falign-loops  -falign-labels
 
           Please note the warning under -fgcse about invoking -O2 on programs
           that use computed gotos.
 
       -O3 Optimize yet more.  -O3 turns on all optimizations specified by -O2
           and also turns on the -finline-functions and -frename-registers
           options.
 
       -O0 Do not optimize.  This is the default.