Writing an LLVM Pass

Introduction --- What is a pass?

The LLVM pass framework is an important part of the LLVM system, because LLVM passes are where most of the interesting parts of the compiler exist. Passes perform the transformations and optimizations that make up the compiler, they build the analysis results that are used by these transformations, and they are, above all, a structuring technique for compiler code.

Unlike passes under the legacy pass manager where the pass interface is defined via inheritance, passes under the new pass manager rely on concept-based polymorphism, meaning there is no explicit interface (see comments in PassManager.h for more details). All LLVM passes inherit from the CRTP mix-in PassInfoMixin<PassT>. The pass should have a run() method which returns a PreservedAnalyses and takes in some unit of IR along with an analysis manager. For example, a function pass would have a PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); method.

We start by showing you how to construct a pass, from setting up the build, creating the pass, to executing and testing it. Looking at existing passes is always a great way to learn details.

Quick Start --- Writing hello world

Here we describe how to write the "hello world" of passes. The "HelloWorld" pass is designed to simply print out the name of non-external functions that exist in the program being compiled. It does not modify the program at all, it just inspects it.

The code below already exists; feel free to create a pass with a different name alongside the HelloWorld source files.

Setting up the build

First, configure and build LLVM as described in :doc:`GettingStarted`.

Next, we will reuse an existing directory (creating a new directory involves modifying more CMakeLists.txt``s and ``LLVMBuild.txt``s than we want). For this example, we'll use ``llvm/lib/Transforms/HelloNew/HelloWorld.cpp, which has already been created. If you'd like to create your own pass, add a new source file into llvm/lib/Transforms/HelloNew/CMakeLists.txt under HelloWorld.cpp:

add_llvm_component_library(LLVMHelloWorld
  HelloWorld.cpp

  DEPENDS
  intrinsics_gen
  )

Now that we have the build set up for a new pass, we need to write the code for the pass itself.

Basic code required

Now that the build is setup for a new pass, we just have to write it.

First we need to define the pass in a header file. We'll create llvm/include/llvm/Transforms/HelloNew/HelloWorld.h. The file should contain the following boilerplate:

#ifndef LLVM_TRANSFORMS_HELLONEW_HELLOWORLD_H
#define LLVM_TRANSFORMS_HELLONEW_HELLOWORLD_H

#include "llvm/IR/PassManager.h"

namespace llvm {

class HelloWorldPass : public PassInfoMixin<HelloWorldPass> {
public:
  PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
};

} // namespace llvm

#endif // LLVM_TRANSFORMS_HELLONEW_HELLOWORLD_H

This creates the class for the pass with a declaration of the run() method which actually runs the pass. Inheriting from PassInfoMixin<PassT> sets up some more boilerplate so that we don't have to write it ourselves.

Our class is in the llvm namespace so that we don't pollute the global namespace.

Next we'll create llvm/lib/Transforms/HelloNew/HelloWorld.cpp, starting with

#include "llvm/Transforms/HelloNew/HelloWorld.h"

... to include the header file we just created.

using namespace llvm;

... is required because the functions from the include files live in the llvm namespace. This should only be done in non-header files.

Next we have the pass's run() definition:

PreservedAnalyses HelloWorldPass::run(Function &F,
                                      FunctionAnalysisManager &AM) {
  errs() << F.getName() << "\n";
  return PreservedAnalyses::all();
}

... which simply prints out the name of the function to stderr. The pass manager will ensure that the pass will be run on every function in a module. The PreservedAnalyses return value says that all analyses (e.g. dominator tree) are still valid after this pass since we didn't modify any functions.

That's it for the pass itself. Now in order to "register" the pass, we need to add it to a couple places. Add the following to llvm\lib\Passes\PassRegistry.def in the FUNCTION_PASS section

FUNCTION_PASS("helloworld", HelloWorldPass())

... which adds the pass under the name "helloworld".

llvm\lib\Passes\PassRegistry.def is #include'd into llvm\lib\Passes\PassBuilder.cpp multiple times for various reasons. Since it constructs our pass, we need to also add the proper #include in llvm\lib\Passes\PassBuilder.cpp:

#include "llvm/Transforms/HelloNew/HelloWorld.h"

This should be all the code necessary for our pass, now it's time to compile and run it.

Running a pass with opt

Now that you have a brand new shiny pass, we can build :program:`opt` and use it to run some LLVM IR through the pass.

$ ninja -C build/ opt
# or whatever build system/build directory you are using

$ cat /tmp/a.ll
define i32 @foo() {
  %a = add i32 2, 3
  ret i32 %a
}

define void @bar() {
  ret void
}

$ build/bin/opt -disable-output /tmp/a.ll -passes=helloworld
foo
bar

Our pass ran and printed the names of functions as expected!

Testing a pass

Testing our pass is important to prevent future regressions. We'll add a lit test at llvm/test/Transforms/HelloNew/helloworld.ll. See :doc:`TestingGuide` for more information on testing.

$ cat llvm/test/Transforms/HelloNew/helloworld.ll
; RUN: opt -disable-output -passes=helloworld %s 2>&1 | FileCheck %s

; CHECK: {{^}}foo{{$}}
define i32 @foo() {
  %a = add i32 2, 3
  ret i32 %a
}

; CHECK-NEXT: {{^}}bar{{$}}
define void @bar() {
  ret void
}

$ ninja -C build check-llvm
# runs our new test alongside all other llvm lit tests

FAQs

Required passes

A pass that defines a static isRequired() method that returns true is a required pass. For example:

class HelloWorldPass : public PassInfoMixin<HelloWorldPass> {
public:
  PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);

  static bool isRequired() { return true; }
};

A required pass is a pass that may not be skipped. An example of a required pass is AlwaysInlinerPass, which must always be run to preserve alwaysinline semantics. Pass managers are required since they may contain other required passes.

An example of how a pass can be skipped is the optnone function attribute, which specifies that optimizations should not be run on the function. Required passes will still be run on optnone functions.

For more implementation details, see PassInstrumentation::runBeforePass().