使用LLVM 5.0.1的AAResultsWrapperPass时出现段错误求助

Let's break down what's happening with your LLVM 5.0.1 AliasAnalysis setup and fix those errors step by step.

Your Setup Recap #

First, let's restate your code and error context for clarity:

Analysis Usage Definition #

void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequired<AAResultsWrapperPass>();
}

Retrieving AAResults #

AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();

Your code compiles without issues, but running this command triggers an error about an "Unnamed pass" followed by a segmentation fault:

clang -Xclang -load -Xclang /home/zhangjun/tools/llvm/llvm-build/lib/LLVMCPI.so -O0 -c test.c

Root Causes & Fixes #

1. Fix the "Unnamed pass" Error First #

LLVM requires every custom pass to have a human-readable name, returned by overriding the getPassName() method. The "Unnamed pass" error means you haven't implemented this yet. Add this to your pass class:

const char *getPassName() const override {
    return "CPI Alias Analysis Pass"; // Replace with your actual pass name
}

This will eliminate the unnamed pass warning and help LLVM track your pass correctly in the pipeline.

2. Resolve the Segmentation Fault (Linked to -O0) #

The segfault is almost certainly caused by LLVM disabling most analysis/optimization passes at -O0—including the underlying alias analysis implementations (like BasicAA, GlobalsAA) that AAResultsWrapperPass depends on. Without these, getAnalysis() might return an invalid pointer, and accessing getAAResults() will crash your program.

Option A: Switch to -O1 Instead of -O0

The simplest fix is to use -O1, which automatically enables all necessary analysis passes:

clang -Xclang -load -Xclang /home/zhangjun/tools/llvm/llvm-build/lib/LLVMCPI.so -O1 -c test.c

Option B: Force Required AA Passes at -O0

If you must stick with -O0, explicitly add the underlying AA passes to your analysis usage so LLVM knows to run them:

void getAnalysisUsage(AnalysisUsage &AU) const override {
    // Add the core AA passes that AAResultsWrapperPass relies on
    AU.addRequired<BasicAAWrapperPass>();
    AU.addRequired<GlobalsAAWrapperPass>();
    AU.addRequired<AAResultsWrapperPass>();
    AU.setPreservesAll(); // Add this if your pass doesn't modify IR
}

3. Add Safety Checks for Null Pointers #

To avoid silent crashes, add an assertion to verify you're getting a valid AAResults pointer:

auto &aaWrapper = getAnalysis<AAResultsWrapperPass>();
AA = &aaWrapper.getAAResults();
assert(AA && "Failed to retrieve valid AAResults!");

This will catch invalid pointers early and give you a clear error message instead of a segfault.

4. Double-Check Pass Registration #

Make sure your custom pass is properly registered with LLVM. For a function pass, your registration code should look like this (in your pass's .cpp file):

static RegisterPass<YourCustomPassClassName> 
X("cpi-pass", "CPI Analysis Pass Description", false, false);

The first string is the pass's command-line identifier, the second is a human-readable description.

内容的提问来源于stack exchange，提问作者Rambo