问题
I have some legacy code that compiles with both -02 and -03 set. From the GCC man file I get the guarantee that:
-O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload and -ftree-vectorize options.
So, at first glance it would seem likely that turning both of these flags on would be the same as just -O3. However, that got me thinking is that the right thing to do in that case as -O2 is probably the "safer" option. Obviously, it is a simple matter compile some code with all of the permutations and see what happens in each case, but I was wondering if anyone knows if there is a specific policy that GCC has in regard to specifying multiple optimizations levels and if so what is the reasoning behind it?
回答1:
From the man page:
If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.
来源：https://stackoverflow.com/questions/5557261/gcc-multiple-optimization-flags