On 03/07/2011 01:47 PM, Tom Gall wrote:
To quote the GCC manual:
-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-and-partition -fprefetch-loop-arrays -ftree-vect-loop-versionThat said (and unless there's other undocumented differences), it would seem to take some expertise to comment on the trade off between memory efforts (cache misses, TLB etc) vs instructional effects by having these optimizations off.
I am certainly not that expert but I suspect given the memory bus speeds that typical arm hardware has, we'd want -Os over -O2.
In my experience, -Os also turns off inlining of functions. The case I have looked at is swab32 function in u-boot. With -Os (and -march=armv7-a) I get calls to this function:
__fswab32: rev r0, r0 bx lr
With -O2, I get rev inlined.
Rob