LLUBI(1)

LLUBI(1) 💅✨

This is the LLVM manual page for llubi, the UB-aware interpreter, and we’re doing it in group-chat voice because you deserve to actually get it. 🫶

🌸 NAME 🌸

llubi — LLVM UB-aware Interpreter.

✨ SYNOPSIS ✨

llubi [options] [filename] [program args]

🦋 DESCRIPTION 🦋

Okay so llubi directly executes programs in LLVM bitcode format and tracks values in LLVM IR semantics. Unlike lli, llubi is designed to be aware of undefined behaviors during execution. It detects immediate undefined behaviors such as integer division by zero, and it respects poison generating flags like nsw and nuw. Because it captures most of the guardable undefined behaviors, it is highly suitable for constructing an interesting-ness test for miscompilation bugs. No cap, that’s the whole point. 🔥

If filename is not specified, then llubi reads the LLVM bitcode for the program from standard input. 🫶

The optional args specified on the command line are passed to the program as arguments. Periodt. 💅

💖 GENERAL OPTIONS 💖

• -fake-argv0=executable — Override the argv[0] value passed into the executing program. 🎀
• -entry-function=function — Specify the name of the function to execute as the program’s entry point. By default, llubi uses the function named main. 👑
• -help — Print a summary of command line options. 🌸
• -verbose — Print results for each instruction executed. 🔍
• -version — Print out the version of llubi and exit without doing anything else. ✨

🧚 INTERPRETER OPTIONS 🧚

• -max-mem=N — Limit the amount of memory (in bytes) that can be allocated by the program, including stack, heap, and global variables. If the limit is exceeded, execution will be terminated. By default, there is no limit (N = 0). 💾
• -max-stack-depth=N — Limit the maximum stack depth to N. If the limit is exceeded, execution will be terminated. The default limit is 256. Set N to 0 to disable the limit. 🥞
• -max-steps=N — Limit the number of instructions executed to N. If the limit is reached, execution will be terminated. By default, there is no limit (N = 0). 👟
• -vscale=N — Set the value of llvm.vscale to N. The default value is 4. 📐
• -seed=N — Set the seed for random number generator to N. By default, the seed is 0. 🎲
• -undef-behavior=mode — Set the behavior for undefined values (e.g., load from uninitialized memory or freeze a poison value). The options for mode are:
  • nondet — Each load from the same uninitialized byte yields a freshly random value. This is the default behavior. 🌀
  • zero — Uninitialized values are treated as zero. 0️⃣
• -nan-behavior=mode — Set the behavior for payload preserving behavior of floating-point NaN values. The options for mode are:
  • nondet — The actual behavior is randomly chosen from the modes below. This is the default behavior. 🎰
  • preferred — The quiet bit is set and the payload is all-zero. 🧊
  • quieting — The quiet bit is set and the payload is copied from any input operand that is a NaN. 🤫
  • unchanged — The quiet bit and payload are copied from any input operand that is a NaN. 🪞
  • target-specific — The quiet bit is set and the payload is picked from a known target-specific set of extra possible NaN payloads. 🎯
• -deterministic — Disable interpreter-introduced non-determinism (off by default). This option implies -undef-behavior=zero and -nan-behavior=preferred. 🔒
• -fuse-fmuladd — Treat llvm.fmuladd.* as llvm.fma.*. It is the default behavior. Otherwise, it is expanded into a * b + c. 🧮
• -disable-verify — Disable the validation of the input LLVM IR. The user is responsible for the validity of inputs. The verifier is executed by default.

Bestie, -disable-verify is a “you break it, you own it” situation: the user is responsible for the validity of inputs. The verifier is executed by default. ⚠️

🌈 EXIT STATUS 🌈

If llubi fails to load the program, or an error occurs during execution (e.g., an immediate undefined behavior is triggered), it will exit with an exit code of 1. If the return type of entry function is not an integer type, it will return 0. Otherwise, it will return the exit code of the program. 💀✨

👑 Author 👑

Maintained by the LLVM Team (https://llvm.org/). 🫶

🎀 Copyright 🎀

2003-2026, LLVM Project 💜

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LLVM 23 · 2026-10-20 · LLUBI(1) — that’s the page, bestie; go run some bitcode 💅✨