[MC][MIPS]`Clang` Crashes When Assembling Invalid MIPS `beql` Instructions with `--arch=mips`

[venkyqz]

Description

When using the latest trunk version Clang with the --arch=mips option to assemble MIPS assembly code containing invalid beql instructions (e.g., using floating-point registers, memory references, or invalid syntax as operands), the assembler crashes with a SIGSEGV due to an assertion failure in MipsAsmParser::expandBranchImm. This issue was discovered in Compiler Explorer using x86-64 clang (assertions trunk).

Interesting Facts

• Clang Versions Before 4.0.1: In Clang versions before 4.0.1, Clang does not crash and correctly reports invalid MIPS instructions with errors like "invalid operand for instruction". This suggests the crash is a regression or issue introduced in later versions.

• Crash Triggered by --arch=mips: The crash only occurs when the --arch=mips flag is explicitly set. The MIPS backend seems to fail to handle invalid operands, leading to a SIGSEGV.

• No Crash When Architecture Is Inferred: Without the --arch=mips flag, Clang infers the architecture itself and does not crash, instead reporting errors like "invalid instruction mnemonic 'beql'".

Related Issues

This issue may be related to #22763, which addressed incomplete support for immediate branch pseudo-instructions in the MIPS integrated assembler.

Steps to Reproduce

Here is the comprehensive reproduce link in Compiler Explorer: https://godbolt.org/z/xf4WavzxE

[llvmbot]

@llvm/issue-subscribers-backend-mips

Author: Venky Qi Zhang (venkyqz)

# Description When using the latest trunk version `Clang` with the `--arch=mips` option to assemble MIPS assembly code containing invalid `beql` instructions (e.g., using floating-point registers, memory references, or invalid syntax as operands), the assembler crashes with a `SIGSEGV` due to an assertion failure in `MipsAsmParser::expandBranchImm`. This issue was discovered in Compiler Explorer using `x86-64 clang (assertions trunk)`.

Interesting Facts

• Clang Versions Before 4.0.1: In Clang versions before 4.0.1, Clang does not crash and correctly reports invalid MIPS instructions with errors like "invalid operand for instruction". This suggests the crash is a regression or issue introduced in later versions.

• Crash Triggered by --arch=mips: The crash only occurs when the --arch=mips flag is explicitly set. The MIPS backend seems to fail to handle invalid operands, leading to a SIGSEGV.

• No Crash When Architecture Is Inferred: Without the --arch=mips flag, Clang infers the architecture itself and does not crash, instead reporting errors like "invalid instruction mnemonic 'beql'".

Related Issues

This issue may be related to #22763, which addressed incomplete support for immediate branch pseudo-instructions in the MIPS integrated assembler.

Steps to Reproduce

Here is the comprehensive reproduce link in compiler explorer: https://godbolt.org/z/xf4WavzxE

<img width="3000" height="1577" alt="Image" src="https://github.com/user-attachments/assets/136a169e-b0ef-43ee-98d6-1871dfe22121" />

[EugeneZelenko]

@venkyqz: You don't need to add GodBolt screenshots - link is enough.

[brad0]

cc @yingopq @MaskRay