gcc, clang, and Intel all both support this feature, calling it "Link-Time Optimization".[1,2,3] This allows them to inline virtual functions they can resolve,

gcc, clang, and Intel all both support this feature, calling it "Link-Time Optimization".[1,2,3] This allows them to inline virtual functions they can resolve, non-inline functions which are not visible to external units, as well as perform combinatorial optimizations by being able to "see" more of the code together at once. In addition to binary size (which I've never worried about too much, as I mostly write code for scientific computing), it has often improved runtime significantly. The only other specific optimization besides -flto that I have had very real speedups from (not counting -O[23] or -Ofast) has been -funroll-loops. I use it on nearly every project. (And, to be honest, I haven't seen a big slowdown in compile time over just -O3, though I think a lot of my costs there are from templates.) [1] https://gcc.gnu.org/wiki/LinkTimeOptimization https://gcc.gnu.org/wiki/LinkTimeOptimization [2] https://llvm.org/docs/LinkTimeOptimization.html https://llvm.org/docs/LinkTimeOptimization.html [3] https://software.intel.com/en-us/node/522667 https://software.intel.com/en-us/node/522667