Premature optimization is the root of all evil

/Ot Favor Fast Code.
/Os Favor Small Code.

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 indepen-
       dent: 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 state-
       ment 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 optimizations 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 compilation time.
 
           -O turns on the following optimization flags: -fdefer-pop -fmerge-constants -fthread-jumps -floop-optimize -fcrossjumping -fif-conversion -fif-conversion2 -fde-
           layed-branch -fguess-branch-probability -fcprop-registers
 
           -O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.
 
       -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 -fdelete-null-pointer-checks -fexpensive-optimiza-
           tions -fregmove -fschedule-insns  -fschedule-insns2 -fsched-interblock -fsched-spec -fcaller-saves -fpeephole2 -freorder-blocks  -freorder-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.
 
       -Os Optimize for size.  -Os enables all -O2 optimizations that do not typically increase code size.  It also performs further optimizations designed to reduce code
           size.
 
           -Os disables the following optimization flags: -falign-functions  -falign-jumps  -falign-loops -falign-labels  -freorder-blocks  -fprefetch-loop-arrays
 
           If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.