Writing an LLVM Pass (legacy PM version) — LLVM 21.1.4 documentation

# Writing an LLVM Pass (legacy PM version)

## Introduction — What is a pass?

Warning

This document deals with the legacy pass manager. LLVM uses the new pass manager for the optimization pipeline (the codegen pipeline still uses the legacy pass manager), which has its own way of defining passes. For more details, see Writing an LLVM Pass and Using the New Pass Manager.

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.

All LLVM passes are subclasses of the Pass class, which implement functionality by overriding virtual methods inherited from `Pass`. Depending on how your pass works, you should inherit from the ModulePass , CallGraphSCCPass, FunctionPass , or LoopPass, or RegionPass classes, which gives the system more information about what your pass does, and how it can be combined with other passes. One of the main features of the LLVM Pass Framework is that it schedules passes to run in an efficient way based on the constraints that your pass meets (which are indicated by which class they derive from).

## Pass classes and requirements

One of the first things that you should do when designing a new pass is to decide what class you should subclass for your pass. Here we talk about the classes available, from the most general to the most specific.

When choosing a superclass for your `Pass`, you should choose the most specific class possible, while still being able to meet the requirements listed. This gives the LLVM Pass Infrastructure information necessary to optimize how passes are run, so that the resultant compiler isn’t unnecessarily slow.

### The `ImmutablePass` class

The most plain and boring type of pass is the “ImmutablePass” class. This pass type is used for passes that do not have to be run, do not change state, and never need to be updated. This is not a normal type of transformation or analysis, but can provide information about the current compiler configuration.

Although this pass class is very infrequently used, it is important for providing information about the current target machine being compiled for, and other static information that can affect the various transformations.

`ImmutablePass`es never invalidate other transformations, are never invalidated, and are never “run”.

### The `ModulePass` class

The ModulePass class is the most general of all superclasses that you can use. Deriving from `ModulePass` indicates that your pass uses the entire program as a unit, referring to function bodies in no predictable order, or adding and removing functions. Because nothing is known about the behavior of `ModulePass` subclasses, no optimization can be done for their execution.

A module pass can use function level passes (e.g. dominators) using the `getAnalysis` interface `getAnalysis<DominatorTree>(llvm::Function *)` to provide the function to retrieve analysis result for, if the function pass does not require any module or immutable passes. Note that this can only be done for functions for which the analysis ran, e.g. in the case of dominators you should only ask for the `DominatorTree` for function definitions, not declarations.

To write a correct `ModulePass` subclass, derive from `ModulePass` and override the `runOnModule` method with the following signature:

#### The `runOnModule` method

    virtual bool runOnModule(Module &M) = 0;

The `runOnModule` method performs the interesting work of the pass. It should return `true` if the module was modified by the transformation and `false` otherwise.

### The `CallGraphSCCPass` class

The CallGraphSCCPass is used by passes that need to traverse the program bottom-up on the call graph (callees before callers). Deriving from `CallGraphSCCPass` provides some mechanics for building and traversing the `CallGraph`, but also allows the system to optimize execution of `CallGraphSCCPass`es. If your pass meets the requirements outlined below, and doesn’t meet the requirements of a FunctionPass, you should derive from `CallGraphSCCPass`.

`TODO`: explain briefly what SCC, Tarjan’s algo, and B-U mean.

To be explicit, CallGraphSCCPass subclasses are:

1. … not allowed to inspect or modify any `Function`s other than those in the current SCC and the direct callers and direct callees of the SCC.

2. … required to preserve the current `CallGraph` object, updating it to reflect any changes made to the program.

3. … not allowed to add or remove SCC’s from the current Module, though they may change the contents of an SCC.

4. … allowed to add or remove global variables from the current Module.

5. … allowed to maintain state across invocations of runOnSCC (including global data).

Implementing a `CallGraphSCCPass` is slightly tricky in some cases because it has to handle SCCs with more than one node in it. All of the virtual methods described below should return `true` if they modified the program, or `false` if they didn’t.

#### The `doInitialization(CallGraph &)` method

    virtual bool doInitialization(CallGraph &CG);

The `doInitialization` method is allowed to do most of the things that `CallGraphSCCPass`es are not allowed to do. They can add and remove functions, get pointers to functions, etc. The `doInitialization` method is designed to do simple initialization type of stuff that does not depend on the SCCs being processed. The `doInitialization` method call is not scheduled to overlap with any other pass executions (thus it should be very fast).

#### The `runOnSCC` method

    virtual bool runOnSCC(CallGraphSCC &SCC) = 0;

The `runOnSCC` method performs the interesting work of the pass, and should return `true` if the module was modified by the transformation, `false` otherwise.

#### The `doFinalization(CallGraph &)` method

    virtual bool doFinalization(CallGraph &CG);

The `doFinalization` method is an infrequently used method that is called when the pass framework has finished calling runOnSCC for every SCC in the program being compiled.

### The `FunctionPass` class

In contrast to `ModulePass` subclasses, FunctionPass subclasses do have a predictable, local behavior that can be expected by the system. All `FunctionPass` execute on each function in the program independent of all of the other functions in the program. `FunctionPass`es do not require that they are executed in a particular order, and `FunctionPass`es do not modify external functions.

To be explicit, `FunctionPass` subclasses are not allowed to:

1. Inspect or modify a `Function` other than the one currently being processed.

2. Add or remove `Function`s from the current `Module`.

3. Add or remove global variables from the current `Module`.

4. Maintain state across invocations of runOnFunction (including global data).

Implementing a `FunctionPass` is usually straightforward. `FunctionPass`es may override three virtual methods to do their work. All of these methods should return `true` if they modified the program, or `false` if they didn’t.

#### The `doInitialization(Module &)` method

    virtual bool doInitialization(Module &M);

The `doInitialization` method is allowed to do most of the things that `FunctionPass`es are not allowed to do. They can add and remove functions, get pointers to functions, etc. The `doInitialization` method is designed to do simple initialization type of stuff that does not depend on the functions being processed. The `doInitialization` method call is not scheduled to overlap with any other pass executions (thus it should be very fast).

A good example of how this method should be used is the LowerAllocations pass. This pass converts `malloc` and `free` instructions into platform dependent `malloc()` and `free()` function calls. It uses the `doInitialization` method to get a reference to the `malloc` and `free` functions that it needs, adding prototypes to the module if necessary.

#### The `runOnFunction` method

    virtual bool runOnFunction(Function &F) = 0;

The `runOnFunction` method must be implemented by your subclass to do the transformation or analysis work of your pass. As usual, a `true` value should be returned if the function is modified.

#### The `doFinalization(Module &)` method

    virtual bool doFinalization(Module &M);

The `doFinalization` method is an infrequently used method that is called when the pass framework has finished calling runOnFunction for every function in the program being compiled.

### The `LoopPass` class

All `LoopPass` execute on each loop in the function independent of all of the other loops in the function. `LoopPass` processes loops in loop nest order such that outer most loop is processed last.

`LoopPass` subclasses are allowed to update loop nest using `LPPassManager` interface. Implementing a loop pass is usually straightforward. `LoopPass`es may override three virtual methods to do their work. All these methods should return `true` if they modified the program, or `false` if they didn’t.

A `LoopPass` subclass which is intended to run as part of the main loop pass pipeline needs to preserve all of the same function analyses that the other loop passes in its pipeline require. To make that easier, a `getLoopAnalysisUsage` function is provided by `LoopUtils.h`. It can be called within the subclass’s `getAnalysisUsage` override to get consistent and correct behavior. Analogously, `INITIALIZE_PASS_DEPENDENCY(LoopPass)` will initialize this set of function analyses.

#### The `doInitialization(Loop *, LPPassManager &)` method

    virtual bool doInitialization(Loop *, LPPassManager &LPM);

The `doInitialization` method is designed to do simple initialization type of stuff that does not depend on the functions being processed. The `doInitialization` method call is not scheduled to overlap with any other pass executions (thus it should be very fast). `LPPassManager` interface should be used to access `Function` or `Module` level analysis information.

#### The `runOnLoop` method

    virtual bool runOnLoop(Loop *, LPPassManager &LPM) = 0;

The `runOnLoop` method must be implemented by your subclass to do the transformation or analysis work of your pass. As usual, a `true` value should be returned if the function is modified. `LPPassManager` interface should be used to update loop nest.

#### The `doFinalization()` method

    virtual bool doFinalization();

The `doFinalization` method is an infrequently used method that is called when the pass framework has finished calling runOnLoop for every loop in the program being compiled.

### The `RegionPass` class

`RegionPass` is similar to LoopPass, but executes on each single entry single exit region in the function. `RegionPass` processes regions in nested order such that the outer most region is processed last.

`RegionPass` subclasses are allowed to update the region tree by using the `RGPassManager` interface. You may override three virtual methods of `RegionPass` to implement your own region pass. All these methods should return `true` if they modified the program, or `false` if they did not.

#### The `doInitialization(Region *, RGPassManager &)` method

    virtual bool doInitialization(Region *, RGPassManager &RGM);

The `doInitialization` method is designed to do simple initialization type of stuff that does not depend on the functions being processed. The `doInitialization` method call is not scheduled to overlap with any other pass executions (thus it should be very fast). `RPPassManager` interface should be used to access `Function` or `Module` level analysis information.

#### The `runOnRegion` method

    virtual bool runOnRegion(Region *, RGPassManager &RGM) = 0;

The `runOnRegion` method must be implemented by your subclass to do the transformation or analysis work of your pass. As usual, a true value should be returned if the region is modified. `RGPassManager` interface should be used to update region tree.

#### The `doFinalization()` method

    virtual bool doFinalization();

The `doFinalization` method is an infrequently used method that is called when the pass framework has finished calling runOnRegion for every region in the program being compiled.

### The `MachineFunctionPass` class

A `MachineFunctionPass` is a part of the LLVM code generator that executes on the machine-dependent representation of each LLVM function in the program.

Code generator passes are registered and initialized specially by `TargetMachine::addPassesToEmitFile` and similar routines, so they cannot generally be run from the opt or bugpoint commands.

A `MachineFunctionPass` is also a `FunctionPass`, so all the restrictions that apply to a `FunctionPass` also apply to it. `MachineFunctionPass`es also have additional restrictions. In particular, `MachineFunctionPass`es are not allowed to do any of the following:

1. Modify or create any LLVM IR `Instruction`s, `BasicBlock`s, `Argument`s, `Function`s, `GlobalVariable`s, `GlobalAlias`es, or `Module`s.

2. Modify a `MachineFunction` other than the one currently being processed.

3. Maintain state across invocations of runOnMachineFunction (including global data).

#### The `runOnMachineFunction(MachineFunction &MF)` method

    virtual bool runOnMachineFunction(MachineFunction &MF) = 0;

`runOnMachineFunction` can be considered the main entry point of a `MachineFunctionPass`; that is, you should override this method to do the work of your `MachineFunctionPass`.

The `runOnMachineFunction` method is called on every `MachineFunction` in a `Module`, so that the `MachineFunctionPass` may perform optimizations on the machine-dependent representation of the function. If you want to get at the LLVM `Function` for the `MachineFunction` you’re working on, use `MachineFunction`’s `getFunction()` accessor method — but remember, you may not modify the LLVM `Function` or its contents from a `MachineFunctionPass`.

### Pass registration

Passes are registered with the `RegisterPass` template. The template parameter is the name of the pass that is to be used on the command line to specify that the pass should be added to a program. The first argument is the name of the pass, which is to be used for the `-help` output of programs, as well as for debug output generated by the –debug-pass option.

If you want your pass to be easily dumpable, you should implement the virtual print method:

#### The `print` method

    virtual void print(llvm::raw_ostream &O, const Module *M) const;

The `print` method must be implemented by “analyses” in order to print a human readable version of the analysis results. This is useful for debugging an analysis itself, as well as for other people to figure out how an analysis works. Use the opt `-analyze` argument to invoke this method.

The `llvm::raw_ostream` parameter specifies the stream to write the results on, and the `Module` parameter gives a pointer to the top level module of the program that has been analyzed. Note however that this pointer may be `NULL` in certain circumstances (such as calling the `Pass::dump()` from a debugger), so it should only be used to enhance debug output, it should not be depended on.

#### Scheduling a MachineFunctionPass

Backends create a `TargetPassConfig` and use `addPass` to schedule `MachineFunctionPass`es. External plugins can register a callback to modify and insert additional passes:

    RegisterTargetPassConfigCallback X{[](auto &TM, auto &PM, auto *TPC) {
      TPC->insertPass(/* ... */);
      TPC->substitutePass(/* ... */);
    }};

Note that passes still have to be registered:

    __attribute__((constructor)) static void initCodeGenPlugin() {
      initializeExamplePass(*PassRegistry::getPassRegistry());
    }

### Specifying interactions between passes

One of the main responsibilities of the `PassManager` is to make sure that passes interact with each other correctly. Because `PassManager` tries to optimize the execution of passes it must know how the passes interact with each other and what dependencies exist between the various passes. To track this, each pass can declare the set of passes that are required to be executed before the current pass, and the passes which are invalidated by the current pass.

Typically this functionality is used to require that analysis results are computed before your pass is run. Running arbitrary transformation passes can invalidate the computed analysis results, which is what the invalidation set specifies. If a pass does not implement the getAnalysisUsage method, it defaults to not having any prerequisite passes, and invalidating all other passes.

#### The `getAnalysisUsage` method

    virtual void getAnalysisUsage(AnalysisUsage &Info) const;

By implementing the `getAnalysisUsage` method, the required and invalidated sets may be specified for your transformation. The implementation should fill in the AnalysisUsage object with information about which passes are required and not invalidated. To do this, a pass may call any of the following methods on the `AnalysisUsage` object:

#### The `AnalysisUsage::addRequired<>` and `AnalysisUsage::addRequiredTransitive<>` methods

If your pass requires a previous pass to be executed (an analysis for example), it can use one of these methods to arrange for it to be run before your pass. LLVM has many different types of analyses and passes that can be required, spanning the range from `DominatorSet` to `BreakCriticalEdges`. Requiring `BreakCriticalEdges`, for example, guarantees that there will be no critical edges in the CFG when your pass has been run.

Some analyses chain to other analyses to do their job. For example, an AliasAnalysis implementation is required to chain to other alias analysis passes. In cases where analyses chain, the `addRequiredTransitive` method should be used instead of the `addRequired` method. This informs the `PassManager` that the transitively required pass should be alive as long as the requiring pass is.

#### The `AnalysisUsage::addPreserved<>` method

One of the jobs of the `PassManager` is to optimize how and when analyses are run. In particular, it attempts to avoid recomputing data unless it needs to. For this reason, passes are allowed to declare that they preserve (i.e., they don’t invalidate) an existing analysis if it’s available. For example, a simple constant folding pass would not modify the CFG, so it can’t possibly affect the results of dominator analysis. By default, all passes are assumed to invalidate all others.

The `AnalysisUsage` class provides several methods which are useful in certain circumstances that are related to `addPreserved`. In particular, the `setPreservesAll` method can be called to indicate that the pass does not modify the LLVM program at all (which is true for analyses), and the `setPreservesCFG` method can be used by transformations that change instructions in the program but do not modify the CFG or terminator instructions.

`addPreserved` is particularly useful for transformations like `BreakCriticalEdges`. This pass knows how to update a small set of loop and dominator related analyses if they exist, so it can preserve them, despite the fact that it hacks on the CFG.

#### Example implementations of `getAnalysisUsage`

    // This example modifies the program, but does not modify the CFG
    void LICM::getAnalysisUsage(AnalysisUsage &AU) const {
      AU.setPreservesCFG();
      AU.addRequired<LoopInfoWrapperPass>();
    }

#### The `getAnalysis<>` and `getAnalysisIfAvailable<>` methods

The `Pass::getAnalysis<>` method is automatically inherited by your class, providing you with access to the passes that you declared that you required with the getAnalysisUsage method. It takes a single template argument that specifies which pass class you want, and returns a reference to that pass. For example:

    bool LICM::runOnFunction(Function &F) {
      LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
      //...
    }

This method call returns a reference to the pass desired. You may get a runtime assertion failure if you attempt to get an analysis that you did not declare as required in your getAnalysisUsage implementation. This method can be called by your `run*` method implementation, or by any other local method invoked by your `run*` method.

A module level pass can use function level analysis info using this interface. For example:

    bool ModuleLevelPass::runOnModule(Module &M) {
      //...
      DominatorTree &DT = getAnalysis<DominatorTree>(Func);
      //...
    }

In above example, `runOnFunction` for `DominatorTree` is called by pass manager before returning a reference to the desired pass.

If your pass is capable of updating analyses if they exist (e.g., `BreakCriticalEdges`, as described above), you can use the `getAnalysisIfAvailable` method, which returns a pointer to the analysis if it is active. For example:

    if (DominatorSet *DS = getAnalysisIfAvailable<DominatorSet>()) {
      // A DominatorSet is active.  This code will update it.
    }

## Pass Statistics

The Statistic class is designed to be an easy way to expose various success metrics from passes. These statistics are printed at the end of a run, when the `-stats` command line option is enabled on the command line. See the Statistics section in the Programmer’s Manual for details.

### What PassManager does

The PassManager class takes a list of passes, ensures their prerequisites are set up correctly, and then schedules passes to run efficiently. All of the LLVM tools that run passes use the PassManager for execution of these passes.

The PassManager does two main things to try to reduce the execution time of a series of passes:

1. Share analysis results. The `PassManager` attempts to avoid recomputing analysis results as much as possible. This means keeping track of which analyses are available already, which analyses get invalidated, and which analyses are needed to be run for a pass. An important part of work is that the `PassManager` tracks the exact lifetime of all analysis results, allowing it to free memory allocated to holding analysis results as soon as they are no longer needed.

2. Pipeline the execution of passes on the program. The `PassManager` attempts to get better cache and memory usage behavior out of a series of passes by pipelining the passes together. This means that, given a series of consecutive FunctionPass, it will execute all of the FunctionPass on the first function, then all of the FunctionPasses on the second function, etc… until the entire program has been run through the passes.

   This improves the cache behavior of the compiler, because it is only touching the LLVM program representation for a single function at a time, instead of traversing the entire program. It reduces the memory consumption of compiler, because, for example, only one DominatorSet needs to be calculated at a time.

The effectiveness of the `PassManager` is influenced directly by how much information it has about the behaviors of the passes it is scheduling. For example, the “preserved” set is intentionally conservative in the face of an unimplemented getAnalysisUsage method. Not implementing when it should be implemented will have the effect of not allowing any analysis results to live across the execution of your pass.

The `PassManager` class exposes a `--debug-pass` command line options that is useful for debugging pass execution, seeing how things work, and diagnosing when you should be preserving more analyses than you currently are. (To get information about all of the variants of the `--debug-pass` option, just type “`llc -help-hidden`”).

By using the –debug-pass=Structure option, for example, we can see inspect the default optimization pipelines, e.g. (the output has been trimmed):

    $ llc -mtriple=arm64-- -O3 -debug-pass=Structure file.ll > /dev/null
    (...)
    ModulePass Manager
    Pre-ISel Intrinsic Lowering
    FunctionPass Manager
      Expand large div/rem
      Expand fp
      Expand Atomic instructions
    SVE intrinsics optimizations
      FunctionPass Manager
        Dominator Tree Construction
    FunctionPass Manager
      Simplify the CFG
      Dominator Tree Construction
      Natural Loop Information
      Canonicalize natural loops
    (...)

#### The `releaseMemory` method

    virtual void releaseMemory();

The `PassManager` automatically determines when to compute analysis results, and how long to keep them around for. Because the lifetime of the pass object itself is effectively the entire duration of the compilation process, we need some way to free analysis results when they are no longer useful. The `releaseMemory` virtual method is the way to do this.

If you are writing an analysis or any other pass that retains a significant amount of state (for use by another pass which “requires” your pass and uses the getAnalysis method) you should implement `releaseMemory` to, well, release the memory allocated to maintain this internal state. This method is called after the `run*` method for the class, before the next call of `run*` in your pass.

## Registering dynamically loaded passes

Size matters when constructing production quality tools using LLVM, both for the purposes of distribution, and for regulating the resident code size when running on the target system. Therefore, it becomes desirable to selectively use some passes, while omitting others and maintain the flexibility to change configurations later on. You want to be able to do all this, and, provide feedback to the user. This is where pass registration comes into play.

The fundamental mechanisms for pass registration are the `MachinePassRegistry` class and subclasses of `MachinePassRegistryNode`.

An instance of `MachinePassRegistry` is used to maintain a list of `MachinePassRegistryNode` objects. This instance maintains the list and communicates additions and deletions to the command line interface.

An instance of `MachinePassRegistryNode` subclass is used to maintain information provided about a particular pass. This information includes the command line name, the command help string and the address of the function used to create an instance of the pass. A global static constructor of one of these instances registers with a corresponding `MachinePassRegistry`, the static destructor unregisters. Thus a pass that is statically linked in the tool will be registered at start up. A dynamically loaded pass will register on load and unregister at unload.

### Using existing registries

There are predefined registries to track instruction scheduling (`RegisterScheduler`) and register allocation (`RegisterRegAlloc`) machine passes. Here we will describe how to register a register allocator machine pass.

Implement your register allocator machine pass. In your register allocator `.cpp` file add the following include:

    #include "llvm/CodeGen/RegAllocRegistry.h"

Also in your register allocator `.cpp` file, define a creator function in the form:

    FunctionPass *createMyRegisterAllocator() {
      return new MyRegisterAllocator();
    }

Note that the signature of this function should match the type of `RegisterRegAlloc::FunctionPassCtor`. In the same file add the “installing” declaration, in the form:

    static RegisterRegAlloc myRegAlloc("myregalloc",
                                       "my register allocator help string",
                                       createMyRegisterAllocator);

Note the two spaces prior to the help string produces a tidy result on the `-help` query.

    $ llc -help
      ...
      -regalloc                    - Register allocator to use (default=linearscan)
        =linearscan                -   linear scan register allocator
        =local                     -   local register allocator
        =simple                    -   simple register allocator
        =myregalloc                -   my register allocator help string
      ...

And that’s it. The user is now free to use `-regalloc=myregalloc` as an option. Registering instruction schedulers is similar except use the `RegisterScheduler` class. Note that the `RegisterScheduler::FunctionPassCtor` is significantly different from `RegisterRegAlloc::FunctionPassCtor`.

To force the load/linking of your register allocator into the llc/lli tools, add your creator function’s global declaration to `Passes.h` and add a “pseudo” call line to `llvm/Codegen/LinkAllCodegenComponents.h`.

### Creating new registries

The easiest way to get started is to clone one of the existing registries; we recommend `llvm/CodeGen/RegAllocRegistry.h`. The key things to modify are the class name and the `FunctionPassCtor` type.

Then you need to declare the registry. Example: if your pass registry is `RegisterMyPasses` then define:

    MachinePassRegistry<RegisterMyPasses::FunctionPassCtor> RegisterMyPasses::Registry;

And finally, declare the command line option for your passes. Example:

    cl::opt<RegisterMyPasses::FunctionPassCtor, false,
            RegisterPassParser<RegisterMyPasses> >
    MyPassOpt("mypass",
              cl::init(&createDefaultMyPass),
              cl::desc("my pass option help"));

Here the command option is “`mypass`”, with `createDefaultMyPass` as the default creator.

### Using GDB with dynamically loaded passes

Unfortunately, using GDB with dynamically loaded passes is not as easy as it should be. First of all, you can’t set a breakpoint in a shared object that has not been loaded yet, and second of all there are problems with inlined functions in shared objects. Here are some suggestions to debugging your pass with GDB.

For sake of discussion, I’m going to assume that you are debugging a transformation invoked by opt, although nothing described here depends on that.

#### Setting a breakpoint in your pass

First thing you do is start gdb on the opt process:

    $ gdb opt
    GNU gdb 5.0
    Copyright 2000 Free Software Foundation, Inc.
    GDB is free software, covered by the GNU General Public License, and you are
    welcome to change it and/or distribute copies of it under certain conditions.
    Type "show copying" to see the conditions.
    There is absolutely no warranty for GDB.  Type "show warranty" for details.
    This GDB was configured as "sparc-sun-solaris2.6"...
    (gdb)

Note that opt has a lot of debugging information in it, so it takes time to load. Be patient. Since we cannot set a breakpoint in our pass yet (the shared object isn’t loaded until runtime), we must execute the process, and have it stop before it invokes our pass, but after it has loaded the shared object. The most foolproof way of doing this is to set a breakpoint in `PassManager::run` and then run the process with the arguments you want:

    $ (gdb) break llvm::PassManager::run
    Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70.
    (gdb) run test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption]
    Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption]
    Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70
    70      bool PassManager::run(Module &M) { return PM->run(M); }
    (gdb)

Once the opt stops in the `PassManager::run` method you are now free to set breakpoints in your pass so that you can trace through execution or do other standard debugging stuff.

#### Miscellaneous Problems

Once you have the basics down, there are a couple of problems that GDB has, some with solutions, some without.

- Inline functions have bogus stack information. In general, GDB does a pretty good job getting stack traces and stepping through inline functions. When a pass is dynamically loaded however, it somehow completely loses this capability. The only solution I know of is to de-inline a function (move it from the body of a class to a `.cpp` file).

- Restarting the program breaks breakpoints. After following the information above, you have succeeded in getting some breakpoints planted in your pass. Next thing you know, you restart the program (i.e., you type “`run`” again), and you start getting errors about breakpoints being unsettable. The only way I have found to “fix” this problem is to delete the breakpoints that are already set in your pass, run the program, and re-set the breakpoints once execution stops in `PassManager::run`.

Hopefully these tips will help with common case debugging situations. If you’d like to contribute some tips of your own, just contact Chris.