Alias Analysis Infrastructure in LLVM

1. Introduction
2. AliasAnalysis Overview
   - Representation of Pointers
   - Must, May, and No Alias Responses
   - The `getModRefInfo` methods
3. Writing a new AliasAnalysis Implementation
   - Different Pass styles
   - Required initialization calls
   - Interfaces which may be specified
   - The AliasAnalysis chaining behavior
   - Efficiency Issues
4. Using AliasAnalysis results
   - Using the `-load-vn` Pass
   - Using the `AliasSetTracker` class
   - Using the AliasAnalysis interface directly
5. Helpful alias analysis related tools
   - The `-no-aa` pass
   - The `-print-alias-sets` pass
   - The `-count-aa` pass
   - The `-aa-eval` pass

Written by Chris Lattner

Introduction

Alias Analysis (or Pointer Analysis) is a technique which attempts to determine whether or not two pointers ever can point to the same object in memory. Traditionally, Alias Analyses respond to a query with either a Must, May, or No alias response, indicating that two pointers do point to the same object, might point to the same object, or are known not to point to the same object.

The AliasAnalysis class is the centerpiece of the LLVM Alias Analysis related infrastructure. This class is the common interface between clients of alias analysis information and the implementations providing it. In addition to simple alias analysis information, this class exposes Mod/Ref information from those implementations which can provide it, allowing for powerful analyses and transformations to work well together.

This document contains information necessary to successfully implement this interface, use it, and to test both sides. It also explains some of the finer points about what exactly results mean. If you feel that something is unclear or should be added, please let me know.

AliasAnalysis Overview

The AliasAnalysis class defines the interface that Alias Analysis implementations should support. This class exports two important enums: `AliasResult` and `ModRefResult` which represent the result of an alias query or a mod/ref query, respectively.

The AliasAnalysis interface exposes information about memory, represented in several different ways. In particular, memory objects are represented as a starting address and size, and function calls are represented as the actual `call` or `invoke` instructions that performs the call. The AliasAnalysis interface also exposes some helper methods which allow you to get mod/ref information for arbitrary instructions.

Representation of Pointers

Most importantly, the AliasAnalysis class provides several methods which are used to query whether or not pointers alias, whether function calls can modify or read memory, etc.

Representing memory objects as a starting address and a size is critically important for precise Alias Analyses. For example, consider this (silly) C code:

      int i;
      char C[2];
      char A[10];
      /* ... */
      for (i = 0; i != 10; ++i) {
        C[0] = A[i];          /* One byte store */
        C[1] = A[9-i];        /* One byte store */
      }

In this case, the `basicaa` pass will disambiguate the stores to `C[0]` and `C[1]` because they are accesses to two distinct locations one byte apart, and the accesses are each one byte. In this case, the LICM pass can use store motion to remove the stores from the loop. In constrast, the following code:

      int i;
      char C[2];
      char A[10];
      /* ... */
      for (i = 0; i != 10; ++i) {
        ((short*)C)[0] = A[i];  /* Two byte store! */
        C[1] = A[9-i];          /* One byte store */
      }

In this case, the two stores to C do alias each other, because the access to the `&C[0]` element is a two byte access. If size information wasn't available in the query, even the first case would have to conservatively assume that the accesses alias.

Must, May, and No Alias Responses

An Alias Analysis implementation can return one of three responses: MustAlias, MayAlias, and NoAlias. The No and May alias results are obvious: if the two pointers may never equal each other, return NoAlias, if they might, return MayAlias.

The Must Alias response is trickier though. In LLVM, the Must Alias response may only be returned if the two memory objects are guaranteed to always start at exactly the same location. If two memory objects overlap, but do not start at the same location, MayAlias must be returned.

The `getModRefInfo` methods

The `getModRefInfo` methods return information about whether the execution of an instruction can read or modify a memory location. Mod/Ref information is always conservative: if an action may read a location, Ref is returned.

Writing a new AliasAnalysis Implementation

Writing a new alias analysis implementation for LLVM is quite straight-forward. There are already several implementations that you can use for examples, and the following information should help fill in any details. For a minimal example, take a look at the `no-aa` implementation.

Different Pass styles

The first step to determining what type of LLVM pass you need to use for your Alias Analysis. As is the case with most other analyses and transformations, the answer should be fairly obvious from what type of problem you are trying to solve:

1. If you require interprocedural analysis, it should be a `Pass`.
2. If you are a global analysis, subclass `FunctionPass`.
3. If you are a local pass, subclass `BasicBlockPass`.
4. If you don't need to look at the program at all, subclass `ImmutablePass`.

In addition to the pass that you subclass, you should also inherit from the `AliasAnalysis` interface, of course, and use the `RegisterAnalysisGroup` template to register as an implementation of `AliasAnalysis`.

Required initialization calls

Your subclass of AliasAnalysis is required to invoke two methods on the AliasAnalysis base class: `getAnalysisUsage` and `InitializeAliasAnalysis`. In particular, your implementation of `getAnalysisUsage` should explicitly call into the `AliasAnalysis::getAnalysisUsage` method in addition to doing any declaring any pass dependencies your pass has. Thus you should have something like this:

        void getAnalysisUsage(AnalysisUsage &AU) const {
          AliasAnalysis::getAnalysisUsage(AU);
          // declare your dependencies here.
        }

Additionally, your must invoke the `InitializeAliasAnalysis` method from your analysis run method (`run` for a `Pass`, `runOnFunction` for a `FunctionPass`, `runOnBasicBlock` for a `BasicBlockPass`, or `InitializeAliasAnalysis` for an `ImmutablePass`). For example (as part of a `Pass`):

        bool run(Module &M) {
          InitializeAliasAnalysis(this);
          // Perform analysis here...
          return false;
        }

Interfaces which may be specified

All of the AliasAnalysis virtual methods default to providing conservatively correct information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries respectively). Depending on the capabilities of the analysis you are implementing, you just override the interfaces you can improve.

The AliasAnalysis chaining behavior

With only two special exceptions (the `basicaa` and `no-aa` passes) every alias analysis pass should chain to another alias analysis implementation (for example, you could specify "`-basic-aa -ds-aa -andersens-aa -licm`" to get the maximum benefit from the three alias analyses). To do this, simply "Require" AliasAnalysis in your `getAnalysisUsage` method, and if you need to return a conservative MayAlias or Mod/Ref result, simply chain to a lower analysis.

Efficiency Issues

From the LLVM perspective, the only thing you need to do to provide an efficient alias analysis is to make sure that alias analysis queries are serviced quickly. The actual calculation of the alias analysis results (the "run" method) is only performed once, but many (perhaps duplicate) queries may be performed. Because of this, try to move as much computation to the run method as possible (within reason).

Using AliasAnalysis results

There are several different ways to use alias analysis results. In order of preference, these are...

Using the `-load-vn` Pass

The `load-vn` pass uses alias analysis to provide value numbering information for `load` instructions. If your analysis or transformation can be modelled in a form that uses value numbering information, you don't have to do anything special to handle load instructions: just use the `load-vn` pass, which uses alias analysis.

Using the `AliasSetTracker` class

Many transformations need information about alias sets that are active in some scope, rather than information about pairwise aliasing. The `AliasSetTracker` class is used to efficiently build these Alias Sets from the pairwise alias analysis information provided by the AliasAnalysis interface.

First you initialize the AliasSetTracker by use the "`add`" methods to add information about various potentially aliasing instructions in the scope you are interested in. Once all of the alias sets are completed, your pass should simply iterate through the constructed alias sets, using the AliasSetTracker `begin()`/`end()` methods.

The `AliasSet`s formed by the `AliasSetTracker` are guaranteed to be disjoint, calculate mod/ref information and volatility for the set, and keep track of whether or not all of the pointers in the set are Must aliases. The AliasSetTracker also makes sure that sets are properly folded due to call instructions, and can provide a list of pointers in each set.

As an example user of this, the Loop Invariant Code Motion pass uses AliasSetTrackers to build alias information about each loop nest. If an AliasSet in a loop is not modified, then all load instructions from that set may be hoisted out of the loop. If any alias sets are stored and are must alias sets, then the stores may be sunk to outside of the loop, promoting the memory location to a register for the duration of the loop nest. Both of these transformations obviously only apply if the pointer argument is loop-invariant.

The AliasSetTracker implementation

The AliasSetTracker class is implemented to be as efficient as possible. It uses the union-find algorithm to efficiently merge AliasSets when a pointer is inserted into the AliasSetTracker that aliases multiple sets. The primary data structure is a hash table mapping pointers to the AliasSet they are in.

The AliasSetTracker class must maintain a list of all of the LLVM Value*'s that are in each AliasSet. Since the hash table already has entries for each LLVM Value* of interest, the AliasesSets thread the linked list through these hash-table nodes to avoid having to allocate memory unnecessarily, and to make merging alias sets extremely efficient (the linked list merge is constant time).

You shouldn't need to understand these details if you are just a client of the AliasSetTracker, but if you look at the code, hopefully this brief description will help make sense of why things are designed the way they are.

Using the AliasAnalysis interface directly

As a last resort, your pass could use the AliasAnalysis interface directly to service your pass. If you find the need to do this, please let me know so I can see if something new needs to be added to LLVM.

Helpful alias-analysis-related tools

If you're going to be working with the AliasAnalysis infrastructure, there are several nice tools that may be useful for you and are worth knowing about...

The `-no-aa` pass

The `-no-aa` analysis is just like what it sounds: an alias analysis that never returns any useful information. This pass can be useful if you think that alias analysis is doing something wrong and are trying to narrow down a problem. If you don't specify an alias analysis, the default will be to use the `basicaa` pass which does quite a bit of disambiguation on its own.

The `-print-alias-sets` pass

The `-print-alias-sets` pass is exposed as part of the `analyze` tool to print out the Alias Sets formed by the `AliasSetTracker` class. This is useful if you're using the `AliasSetTracker`.

The `-count-aa` pass

The `-count-aa` pass is useful to see how many queries a particular pass is making and what kinds of responses are returned by the alias analysis. An example usage is:

      $ opt -basicaa -count-aa -ds-aa -count-aa -licm

Which will print out how many queries (and what responses are returned) by the `-licm` pass (of the `-ds-aa` pass) and how many queries are made of the `-basicaa` pass by the `-ds-aa` pass. This can be useful when evaluating an alias analysis for precision.

The `-aa-eval` pass

The `-aa-eval` pass simply iterates through all pairs of pointers in a function and asks an alias analysis whether or not the pointers alias. This gives an indication of the precision of the alias analysis. Statistics are printed.