Clang Assertion Failure: Asm Goto With Callbr Landing Pad

by Alex Johnson 58 views

This comprehensive article addresses a critical issue encountered in the Clang compiler: an assertion failure within the FollowCopyChain function during SelectionDAG construction. This failure specifically arises when utilizing asm goto statements in conjunction with call branch (callbr) landing pads. We will delve into the technical details of this issue, providing a clear explanation of the root cause, its implications, and potential solutions. This analysis is crucial for developers and compiler enthusiasts aiming to understand and resolve this Clang bug. By the end of this article, you will gain a solid understanding of the error and how it manifests during the compilation process.

Understanding the Issue: Clang and asm goto

The core problem lies within Clang's code generation process, specifically when handling inline assembly (asm) statements that include goto labels. These asm goto statements allow developers to insert custom assembly code into their C/C++ programs and then jump to specific labels within the assembly block. While this feature offers flexibility, it can introduce complexities for the compiler's optimization and code generation phases. The issue is further compounded when combined with callbr landing pads, which are used in exception handling scenarios. Understanding this interaction is key to grasping the nature of the bug. Let's break down the key components:

  • asm goto Statements: These statements enable embedding assembly code directly within C/C++ code, providing fine-grained control over hardware and low-level operations. The goto part allows jumps to labels defined within the asm block, making the control flow less predictable for the compiler.
  • SelectionDAG: This is an intermediate representation (IR) used by LLVM (the compiler infrastructure upon which Clang is built) to represent code in a way that facilitates optimization and code generation. It's a directed acyclic graph where nodes represent operations, and edges represent data dependencies.
  • FollowCopyChain Function: This function, located in SelectionDAGBuilder.cpp, is part of the SelectionDAG construction process. It's responsible for traversing chains of copy instructions to identify the ultimate source of a value. This is crucial for optimization, as the compiler needs to know where data originates to perform transformations.
  • callbr Landing Pads: These are blocks of code that are executed when an exception is thrown during a function call. They are essential for implementing exception handling in C++ and other languages.

The assertion failure occurs because the FollowCopyChain function encounters an unexpected situation when processing asm goto statements with output operands in the presence of callbr landing pads. The compiler's internal logic makes assumptions about the structure of copy chains, and these assumptions are violated in this specific scenario. This violation triggers the assertion, causing the compilation process to halt. In essence, the bug stems from a mismatch between the compiler's expectations and the actual control flow introduced by the combination of asm goto and callbr.

The Technical Deep Dive: Reproduction and Stack Dump Analysis

To illustrate the issue, consider the minimal example provided in the original bug report:

float f;

void test_asm_goto() {
  asm goto("" : "=r" (f) : : : lab);
lab:
  f = 0;
}

This seemingly simple code snippet is enough to trigger the Clang crash under certain conditions. The asm goto statement attempts to write a value to the floating-point variable f, and the goto jumps to the lab label. When compiled with optimization flags (which are often enabled by default), Clang's code generation process encounters the bug.

Analyzing the Stack Dump

The stack dump provides invaluable clues about the location and context of the crash. Here's a breakdown of the key parts of the provided stack dump:

clang-21: /workspace/install/llvm/src/llvm-project/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp:12719: llvm::Register FollowCopyChain(llvm::MachineRegisterInfo&, llvm::Register): Assertion `MI->getOpcode() == TargetOpcode::COPY &&