Clang Crash In CodeGen: `-std=c89 -O1` Debugging

by Alex Johnson 49 views

Encountering a compiler crash can be a frustrating experience, especially when it occurs with seemingly standard compilation flags. In this article, we'll dissect a specific instance of a Clang crash in CodeGen when using the -std=c89 -O1 flags. We'll explore the context, analyze the provided code snippet and backtrace, and discuss potential causes and debugging strategies. If you're grappling with a similar Clang crash, or if you're simply curious about compiler internals, this article is for you.

Understanding the Crash Context: Clang, CodeGen, and Optimization Levels

Before we dive into the specifics of this crash, let's establish some context. Clang is a powerful and widely-used C, C++, Objective-C, and Objective-C++ compiler. It's a crucial part of the LLVM project and is known for its standards compliance, performance, and helpful diagnostic messages. CodeGen, short for code generation, is a critical phase in the compilation process where the compiler translates the intermediate representation (IR) of the code into machine code that can be executed by the target processor. This phase involves complex optimizations and transformations, making it a potential area for bugs.

The compilation flags -std=c89 and -O1 play significant roles in this crash scenario. -std=c89 instructs Clang to compile the code according to the C89 standard, an older version of the C language standard. This standard has certain limitations and differences compared to more recent standards like C99 or C11. -O1 specifies an optimization level. Compiler optimizations are transformations applied to the code to improve its performance, such as reducing execution time or memory usage. -O1 represents a moderate level of optimization, balancing performance gains with compilation time. Higher optimization levels (like -O2 or -O3) perform more aggressive optimizations, which can sometimes expose latent bugs in the code or the compiler itself. When a crash occurs with specific optimization levels, it often points to a subtle interaction between the code and the compiler's optimization algorithms.

Analyzing the Reproducer Code: A Simple Loop with a Potential Trigger

To effectively troubleshoot a compiler crash, a reproducer – a minimal code snippet that triggers the crash – is invaluable. In this case, a reproducer is provided, allowing us to isolate the problem and examine the code closely:

void *bar (int);

void *foo (void)
{
 char *c = "abc";
 for (int a = 1;; a = 0)
 {
 for (char *s = c; *s; ++s)
 {
 }
 if (!a) break;
 }
}

This C code defines a function foo that contains nested loops. The outer loop is an infinite loop (;;) that initializes an integer a to 1 and then sets it to 0 in each iteration. The inner loop iterates through the characters of the string "abc" using a character pointer s. The if (!a) break; statement provides a condition to exit the outer loop when a becomes 0. At first glance, this code might appear simple, but it contains elements that can potentially trigger unexpected behavior when combined with specific compiler optimizations.

The key areas to focus on in this code are the infinite outer loop, the character pointer manipulation in the inner loop, and the conditional break statement. The compiler's optimization passes might attempt to simplify or eliminate these loops, and if done incorrectly, could lead to a crash. Specifically, the interaction between the infinite loop, the loop condition, and the string literal might be a source of issues.

Deciphering the Backtrace: A Glimpse into the Compiler's Internal State

When a program crashes, a backtrace, also known as a stack trace, provides a snapshot of the call stack at the point of the crash. It's like a series of breadcrumbs leading back to the origin of the problem. In the provided backtrace, we see a sequence of function calls within the Clang compiler that culminated in the crash:

Stack dump:
0. Program arguments: /opt/compiler-explorer/clang-trunk/bin/clang++ -g -o /app/output.s -mllvm --x86-asm-syntax=intel -fno-verbose-asm -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -x c -std=c89 -O1 <source>
1. <eof> parser at end of file
2. <source>:3:7: LLVM IR generation of declaration 'foo'
3. <source>:3:7: Generating code for declaration 'foo'
#0 0x0000000003cdab88 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3cdab88)
#1 0x0000000003cd855c llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3cd855c)
#2 0x0000000003c1e468 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#3 0x00007d7d5fe42520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#4 0x0000000003525664 llvm::BasicBlock::getContext() const (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3525664)
#5 0x000000000361789f llvm::BranchInst::BranchInst(llvm::BasicBlock*, llvm::User::AllocInfo, llvm::InsertPosition) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x361789f)
#6 0x00000000044dc98c clang::CodeGen::CodeGenFunction::PopCleanupBlock(bool, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x44dc98c)
#7 0x00000000044dcba9 clang::CodeGen::CodeGenFunction::PopCleanupBlocks(clang::CodeGen::EHScopeStack::stable_iterator, std::initializer_list<llvm::Value**>) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x44dcba9)
#8 0x00000000041d69c5 clang::CodeGen::CodeGenFunction::FinishFunction(clang::SourceLocation) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x41d69c5)
#9 0x00000000041e80ec clang::CodeGen::CodeGenFunction::GenerateCode(clang::GlobalDecl, llvm::Function*, clang::CodeGen::CGFunctionInfo const&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x41e80ec)
#10 0x0000000004245c5b clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x4245c5b)
#11 0x0000000004240f04 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x4240f04)
#12 0x0000000004241863 clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x4241863)
#13 0x000000000424bb2f clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) (.part.0) CodeGenModule.cpp:0:0
#14 0x000000000459b4e1 (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) ModuleBuilder.cpp:0:0
#15 0x00000000045863e9 clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x45863e9)
#16 0x00000000061c5c34 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x61c5c34)
#17 0x0000000004599785 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-trunk/bin/clang+++0x4599785)
#18 0x000000000489ae2a clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-trunk/bin/clang+++0x489ae2a)
#19 0x0000000004819d0b clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x4819d0b)
#20 0x000000000498f96b clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x498f96b)
#21 0x0000000000dcda95 cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0xdcda95)
#22 0x0000000000dc594b ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>) driver.cpp:0:0
#23 0x0000000000dc59ed int llvm::function_ref<int (llvm::SmallVectorImpl<char const*>&)>::callback_fn<clang_main(int, char**, llvm::ToolContext const&)::'lambda'(llvm::SmallVectorImpl<char const*>&)>(long, llvm::SmallVectorImpl<char const*>&) driver.cpp:0:0
#24 0x0000000004606ab9 void llvm::function_ref<void ()>::callback_fn<clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#25 0x0000000003c1e883 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3c1e883)
#26 0x0000000004606cd9 clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const (.part.0) Job.cpp:0:0
#27 0x00000000045c9662 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-trunk/bin/clang+++0x45c9662)
#28 0x00000000045ca541 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&, bool) const (/opt/compiler-explorer/clang-trunk/bin/clang+++0x45ca541)
#29 0x00000000045d312c clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x45d312c)
#30 0x0000000000dca419 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0xdca419)
#31 0x0000000000c74c74 main (/opt/compiler-explorer/clang-trunk/bin/clang+++0xc74c74)
#32 0x00007d7d5fe29d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#33 0x00007d7d5fe29e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#34 0x0000000000dc53e5 _start (/opt/compiler-explorer/clang-trunk/bin/clang+++0xdc53e5)

Let's break down some key frames in the backtrace:

  • Frames 2 and 3: Indicate that the crash occurred during LLVM IR generation for the foo function, specifically while generating code for it. This suggests the issue arises in the CodeGen phase.
  • Frames 4 and 5: Point to llvm::BasicBlock::getContext() and llvm::BranchInst::BranchInst. These functions are related to LLVM's intermediate representation and control flow. A BasicBlock is a fundamental unit of execution in LLVM IR, and BranchInst represents a branch instruction that transfers control to another basic block. This hints that the crash might involve incorrect construction or manipulation of basic blocks and branch instructions.
  • Frames 6, 7, and 8: Involve clang::CodeGen::CodeGenFunction::PopCleanupBlock, clang::CodeGen::CodeGenFunction::PopCleanupBlocks, and clang::CodeGen::CodeGenFunction::FinishFunction. These functions are part of Clang's code generation process, specifically related to handling cleanup blocks and finalizing function code generation. Cleanup blocks are used for exception handling and ensuring resources are properly released. The fact that these functions are in the backtrace suggests a potential issue with how cleanup blocks are being managed or how the function's code generation is being finalized.
  • Frames 9 through 15: Show the progression of code generation from the function level (CodeGenFunction) to the module level (CodeGenModule) and further up to handling top-level declarations. This provides a broader context of where the crash fits within the overall compilation process.

By analyzing the backtrace, we can narrow down the potential cause of the crash to issues within the CodeGen phase, particularly related to LLVM IR generation, basic block manipulation, and cleanup block handling.

Potential Causes and Debugging Strategies

Based on the code, compilation flags, and backtrace, here are some potential causes for the Clang crash:

  1. Incorrect Loop Optimization: The compiler's optimization passes might be incorrectly transforming the loops in the foo function. For example, the infinite outer loop combined with the conditional break might lead to unexpected behavior if the compiler attempts to optimize it aggressively. The -O1 flag enables optimizations, so this is a plausible cause.
  2. String Literal Handling: The way the compiler handles the string literal "abc" might be contributing to the issue. C89 has specific rules about string literals, and if the compiler isn't handling them correctly in conjunction with the optimizations, it could lead to a crash.
  3. Basic Block Construction Errors: The backtrace suggests problems with basic block manipulation. If the compiler is creating or linking basic blocks incorrectly during code generation, it could lead to a crash. This might be related to how the loops and conditional branches are being translated into LLVM IR.
  4. Cleanup Block Management: The presence of PopCleanupBlock and related functions in the backtrace suggests a potential issue with exception handling or resource cleanup. While the code doesn't explicitly use exceptions, the compiler might be generating cleanup blocks implicitly, and if these blocks are not handled correctly, it could lead to a crash.

To debug this issue, several strategies can be employed:

  1. Reduce Optimization Level: Try compiling the code without optimizations (remove the -O1 flag) or with a lower optimization level (e.g., -O0). If the crash disappears, it strengthens the hypothesis that an optimization pass is the culprit.
  2. Experiment with C Standard: Try compiling with a more recent C standard (e.g., -std=c99 or -std=c11). If the crash is specific to C89, it might indicate a bug related to C89-specific semantics.
  3. Simplify the Code: Further simplify the reproducer by removing parts of the code to see if the crash still occurs. For example, try removing the inner loop or the conditional break. This can help pinpoint the exact code construct that's triggering the crash.
  4. Inspect LLVM IR: Use Clang's option to emit LLVM IR (-emit-llvm) to examine the intermediate representation generated by the compiler. This can provide valuable insights into how the code is being translated and where the optimization passes might be going wrong.
  5. Compiler Bug Reporting: If you've narrowed down the issue and believe it's a compiler bug, report it to the LLVM project (as suggested in the original crash message). Provide the reproducer, compilation flags, and backtrace to help the developers diagnose and fix the problem.

Conclusion: A Journey into Compiler Internals

This Clang crash, triggered by a seemingly simple code snippet and specific compilation flags, illustrates the complexity of modern compilers and the importance of understanding compiler internals for effective debugging. By analyzing the code, backtrace, and potential causes, we've gained a deeper appreciation for the intricate processes involved in code generation and optimization. While compiler crashes can be frustrating, they also offer valuable learning opportunities. By systematically investigating and debugging these issues, we can contribute to the robustness and reliability of our development tools.

For more in-depth information about Clang and LLVM, you can visit the official LLVM Project Website. This website provides comprehensive documentation, tutorials, and community resources for those interested in learning more about these powerful compiler technologies.