-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 
              -fdelayed-branch 
              -fguess-branch-probability 
              -fcprop-registers 
              -floop-optimize 
              -fif-conversion 
              -fif-conversion2 
              -ftree-ccp 
              -ftree-dce 
              -ftree-dominator-opts 
              -ftree-dse 
              -ftree-ter 
              -ftree-lrs 
              -ftree-sra 
              -ftree-copyrename 
              -ftree-fre 
              -ftree-ch 
              -fmerge-constants
         

    -O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.

    -O doesn't turn on -ftree-sra for the Ada compiler. This option must be explicitly specified on the command line to be enabled for the Ada compiler.