What optimization steps are done for -O4 in clang?

Question

We are trying to implement a jit compiler whose performance should be the same as with clang -o4. Is there a place where I could easily get a list of optimization options called by clang with -o4 specified?

Answer 1

As far as I know, -O4 means the same as -O3 + with LTO support (Link Time Optimization). See snippets of the following code:

• Tools.cpp // Manually translate -O to -O2 and -O4 to -O3;
• Driver.cpp // Check for -O4.

Also see here :

You can create bitcode files from clang using -emit-llvm or -flto, or the -O4 flag, which is synonymous with -O3 -flto.

For optimizations used with the -O3 flag, see this PassManagerBuilder.cpp file (find the OptLevel variable - it will have a value of 3).

Answer 2

Please note that with LLVM 5.1-O4, link time optimization is no longer required. If you want you to have to go -flto. See Xcode 5 Release Notes .