Brandon Holt

LLVM Command line options

30 Jan 2014

Command-line options

Okay, command-line flags are not the most exiting topic ever. But, potentially pretty useful when you're making a complicated pass and want to experiment with a bunch of different options. And I just have to say, this is some absolutely stunning C++ magic. I thought how LLVM smoothly handles strings with StringRef/Twine was impressive, but this is on a whole new level. But we'll start with the mundanes of actually using it in the simplest way possible.

Even when using the previously mentioned trick to run your pass with Clang, you can register your own command-line options to affect how it runs. For detailed instructions, refer to LLVM's excellent CommandLine Manual. The short version:

#include <llvm/Support/CommandLine.h>

// declare a dead-simple command line flag
static cl::opt<bool> BeAwesome("awesome-mode", cl::desc("Enable super awesome extra feature."));

// later, in your code...
virtual void runOnFunction(Function &F) {
  if (BeAwesome) { /* ... */ }
enum OptLevel {
  Debug, O1, O2, O3

cl::opt<OptLevel> OptimizationLevel(cl::desc("Choose optimization level:"),
   clEnumValN(Debug, "g", "No optimizations, enable debugging"),
    clEnumVal(O1        , "Enable trivial optimizations"),
    clEnumVal(O2        , "Enable default optimizations"),
    clEnumVal(O3        , "Enable expensive optimizations"),

Hats off to the some of the best C++ code around.