-fif-conversion -fif-conversion2 -fforward-propagate -fmerge-constants -finline-small-functions -findirect-inlining -fpartial-inlining -fdevirtualize -floop-interchange -ftree-ccp
-O

          -fauto-inc-dec 
          -fbranch-count-reg 
          -fcombine-stack-adjustments 
          -fcompare-elim 
          -fcprop-registers 
          -fdce 
          -fdefer-pop 
          -fdelayed-branch 
          -fdse 
          -fforward-propagate 
          -fguess-branch-probability 
          -fif-conversion2 
          -fif-conversion 
          -finline-functions-called-once 
          -fipa-pure-const 
          -fipa-profile 
          -fipa-reference 
          -fmerge-constants 
          -fmove-loop-invariants 
          -freorder-blocks 
          -fshrink-wrap 
          -fsplit-wide-types 
          -fssa-backprop 
          -fssa-phiopt 
          -ftree-bit-ccp 
          -ftree-ccp 
          -ftree-ch 
          -ftree-coalesce-vars 
          -ftree-copy-prop 
          -ftree-dce 
          -ftree-dominator-opts 
          -ftree-dse 
          -ftree-forwprop 
          -ftree-fre 
          -ftree-phiprop 
          -ftree-sink 
          -ftree-slsr 
          -ftree-sra 
          -ftree-pta 
          -ftree-ter 
          -funit-at-a-time

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

          -fthread-jumps 
          -falign-functions  -falign-jumps 
          -falign-loops  -falign-labels 
          -fcaller-saves 
          -fcrossjumping 
          -fcse-follow-jumps  -fcse-skip-blocks 
          -fdelete-null-pointer-checks 
          -fdevirtualize -fdevirtualize-speculatively 
          -fexpensive-optimizations 
          -fgcse  -fgcse-lm  
          -fhoist-adjacent-loads 
          -finline-small-functions 
          -findirect-inlining 
          -fipa-cp 
          -fipa-cp-alignment 
          -fipa-sra 
          -fipa-icf 
          -fisolate-erroneous-paths-dereference 
          -flra-remat 
          -foptimize-sibling-calls 
          -foptimize-strlen 
          -fpartial-inlining 
          -fpeephole2 
          -freorder-blocks-algorithm=stc 
          -freorder-blocks-and-partition -freorder-functions 
          -frerun-cse-after-loop  
          -fsched-interblock  -fsched-spec 
          -fschedule-insns  -fschedule-insns2 
          -fstrict-aliasing -fstrict-overflow 
          -ftree-builtin-call-dce 
          -ftree-switch-conversion -ftree-tail-merge 
          -ftree-pre 
          -ftree-vrp 
          -fipa-ra
-O3
Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the 
      -finline-functions, 
      -funswitch-loops, 
      -fpredictive-commoning, 
      -fgcse-after-reload, 
      -ftree-loop-vectorize, 
      -ftree-loop-distribute-patterns, 
      -fsplit-paths 
      -ftree-slp-vectorize, 
      -fvect-cost-model, 
      -ftree-partial-pre 
      -fipa-cp-clone 
-O0
Reduce compilation time and make debugging produce the expected results. 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  -freorder-blocks-algorithm=stc 
          -freorder-blocks-and-partition  -fprefetch-loop-arrays

-Ofast
Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math and the Fortran-specific -fno-protect-parens and -fstack-arrays. 
-Og
Optimize debugging experience. -Og enables optimizations that do not interfere with debugging. It should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience.