-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:
-fauto-inc-dec -fcprop-registers -fdce -fdefer-pop -fdelayed-branch -fdse -fguess-branch-probability
-fif-conversion2 -fif-conversion -finline-small-functions -fipa-pure-const -fipa-reference
-fmerge-constants -fsplit-wide-types -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-copyrename
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-fre -ftree-sra -ftree-ter -funit-at-a-time
-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.
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