5

We are trying to implement a jit compiler whose performance is supposed to be same as doing it with clang -o4. Is there a place where I could easily get the list of optimization passes invoked by clang with -o4 is specified?

Amit Prakash
  • 1,171
  • 10
  • 19

2 Answers2

9

As far as I know -O4 means same thing as -O3 + enabled LTO (Link Time Optimization). See the folloing code fragments:

Also see here:

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

For optimizations used with -O3 flag see this PassManagerBuilder.cpp file (look for OptLevel variable - it will have value 3).

Mārtiņš Možeiko
  • 12,733
  • 2
  • 45
  • 45
  • 4
    Note: This is no longer correct. The manual now explicitly states that " -O4 and higher: Currently equivalent to -O3". People who are using O4 and think they're doing LTO, aren't. – Chris Kitching Sep 14 '16 at 20:26
9

Note that as of LLVM version 5.1 -O4 no longer implies link time optimization. If you want that you need to pass -flto. See Xcode 5 Release Notes.

moobag
  • 199
  • 1
  • 1
  • 7
    Please use LLVM versions when talking about LLVM, not Apple LLVM versions - LLVM is currently at 3.5, so 5.1 is non-existent. – Griwes Sep 08 '14 at 11:20