Sunday, February 19, 2012

LLVM pass on Windows: integrating with opt

I've been using LLVM for a few weeks now and my primary interaction with it has been writing custom passes. LLVM has a very nice system for handling user defined passes: you create an independent plug-in and load it dynamically using the opt tool. A full explanation can be found in the LLVM pass documentation.

Unfortunately, plug-ins are dynamically linked in LLVM which excludes this approach on Windows (without relying on Cygwin). For all my searching I was not able to find instructions on integrating a pass directly with the opt tool but I needed to support custom passes on Windows without using Cygwin. Mostly as a note for myself, I am listing here the steps I used to create my own custom pass as part of the opt tool; hopefully they will be useful for others as well.

I've followed as best I could the format used by existing infrastructure within LLVM. This does not mean that I have not missed something critical or what I have provided adheres to LLVM policy and/or standards.

N.B. These examples assume you are:
  •  already familiar with writing a custom pass as described by the LLVM documentation
  •  capable of understanding Makefiles and how to modify them to suit your needs
  •  using llvm-3.0 (that is the version used in these examples)
These examples will build a pass named mypass which will simply print out all function names. This pass will be part of the Custom library (also created here).

The first step is to create a new directory named Custom under llvm/lib/Transforms. This is where all passes for the new library will live. Create a very simple function pass and place it in this new directory - I used the following (MyPass.cpp)
#define DEBUG_TYPE "mypass"
#include "llvm/Function.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Custom.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;

namespace {
  struct MyPass : public FunctionPass {
    static char ID;
    MyPass() : FunctionPass(ID) {

     * Just print the function name 
    bool runOnFunction(Function &F) {
      bool Changed = false;
      errs().write_escaped(F.getName()) << "\n";
      return Changed;

char MyPass::ID = 0;
INITIALIZE_PASS(MyPass, "mypass", "Print all function names",
                false, false)

FunctionPass *llvm::createMyPassPass() {
  return new MyPass();
To load this, and multiple other custom passes, it is necessary to create an initialization routine that iterates over all passes in the library. In this example the only call in that routine is the call to the pass above. Still in the Custom directory, create the following as Custom.cpp:
#include "llvm/InitializePasses.h"
#include "llvm-c/Initialization.h"

using namespace llvm;

/// initializeCustom - Initialize all passes in the Custom library
void llvm::initializeCustom(PassRegistry &Registry) {

/// LLVMInitializeCustom - C binding for initializeCustom.
void LLVMInitializeCustom(LLVMPassRegistryRef R) {
You will also need to support either build system with a Makefile and CMakeLists.txt.
LEVEL = ../../..

include $(LEVEL)/Makefile.common

That is the entirety of writing a new pass library for LLVM. The remainder of the work is integrating that new code into the larger system.

First, you need to define a way for LLVM to create an instance of your pass. Save the following as Custom.h in llvm/include/llvm/Transforms/

namespace llvm {

FunctionPass *createMyPassPass();


Then you need to provide the prototypes for the two initialization routines. Add the following two lines to llvm/include/llvm/InitializePasses.h - make sure you do so inside the llvm namespace.
void initializeCustom(PassRegistry&);
void initializeMyPassPass(PassRegistry&);
Next, you need to set any passes to be linked by adding entries into llvm/include/llvm/LinkAllPasses.h
#include "llvm/Transforms/Custom.h"

// This part must reside in the constructor of struct ForcePassLinking
(void) llvm::createMyPassPass();
Updates are required to all relevant makefiles in the source tree. You will need to adjust the following to account for the additions discussed above:
  • llvm/lib/Transforms/Makefile
  • llvm/lib/Transforms/CMakeLists.txt
  • llvm/tools/opt/Makefile (add link dependency for 'custom')
  • llvm/tools/opt/CMakeLists.txt (add link dependency for 'custom')
  • llvm/tools/bugpoint/Makefile (add link dependency for 'custom')
  • llvm/tools/bugpoint/CMakeLists.txt (add link dependency for 'custom')
Finally, add the code to load the custom library in opt. Add the initialization call in the appropriate place in opt.cpp
The new pass should now be visible (on all platforms) without having to load a library with opt.
    -mergereturn                               - Unify function exit nodes
    -module-debuginfo                          - Decodes module-level debug info
    -mypass                                    - Print all function names
    -no-aa                                     - No Alias Analysis (always returns 'may' alias)
    -no-path-profile                           - No Path Profile Information


  1. This seems to be right on the money, Kudos!

  2. Whops, seemed to have encountered a problem:

    LINK : fatal error LNK1104: cannot open file '..\..\lib\Debug\LLVMCustom.lib'

    Help is much appreciated

  3. You may want to check your project configuration to ensure that lib is added to the linker dependencies. As far as I remember this worked for me by simply running build clean and then a full rebuild. If your make files are properly filled out there should not be a problem.

  4. I tried to follow your tutorial, but I failed, I got the following message after compiling everything:

    Error 1 error LNK2019: unresolved external symbol "class llvm::FunctionPass * __cdecl llvm::createMyPassPass(void)" (?createMyPassPass@llvm@@YAPAVFunctionPass@1@XZ) referenced in function "public: __thiscall `anonymous namespace'::ForcePassLinking::ForcePassLinking(void)" (??0ForcePassLinking@?A0x8aef65a6@@QAE@XZ) cc1_main.obj clang

    As you can see, I have clang on my LLVM.sln, the error comes from this project. Any hint?

  5. It looks as if the linker is having trouble finding your definition of createMyPassPass. In the examples above I have that definition in MyPass.cpp (in llvm/lib/Transforms). Have you checked that that definition exists? Perhaps there is a spelling error?

  6. I still get llvm-config error unknown component name