llvm-cat

Concatenate Bitcode files

$ llvm-cat [path/to/file1.bc] [path/to/file2.bc] -o [path/to/out.bc]

llvm-cat [options] <input_bitcode_file...> [-o <output_bitcode_file>]

-o <file>
    Specify the output bitcode file. If omitted, output would conceptually go to standard output.

-h, --help
    Display a help message and exit.

-v, --version
    Display version information and exit.

The llvm-cat command, while conceptually similar to the standard cat utility, is designed for operations on LLVM bitcode files. LLVM bitcode is a low-level, platform-independent binary representation of code generated by LLVM compilers, serving as the intermediate representation (IR) within the LLVM compilation pipeline. Conceptually, llvm-cat would aim to concatenate multiple bitcode files into a single output bitcode file.

However, unlike plain text files, LLVM bitcode files represent structured modules containing functions, global variables, types, and other definitions. Simple byte-level concatenation of two arbitrary bitcode modules is generally incorrect and will likely result in an invalid or malformed combined module. This is because bitcode modules often have conflicting definitions (e.g., multiple main functions, duplicate global variables or types) that require careful resolution during merging.

For this reason, llvm-cat is not a standard or commonly available tool in modern LLVM distributions. Its functionality for merging LLVM bitcode modules has been comprehensively superseded by the llvm-link utility. llvm-link performs a semantic merge, handling symbol resolution, type merging, and global variable initialization, ensuring that the resulting combined bitcode module is valid and semantically correct. Therefore, while llvm-cat conceptually describes a concatenation operation, llvm-link is the practically used and robust command for combining LLVM bitcode.

The llvm-cat command is not a standard or widely distributed tool in modern LLVM installations. Its direct byte-level concatenation approach is typically insufficient for correctly merging LLVM bitcode modules due to the complex internal structure and interdependencies of bitcode. Users should almost always prefer llvm-link for combining LLVM modules.

The fundamental difference lies in their approach: llvm-cat (conceptually) performs a simple byte-for-byte concatenation, akin to appending one file's content to another. This is fine for plain text or unstructured binary data. In contrast, llvm-link understands the internal structure of LLVM bitcode modules. It performs a semantic merge, resolving duplicate definitions, linking cross-module references, and ensuring the resulting bitcode is a valid, single compilation unit. This distinction is crucial for maintaining the integrity and correctness of LLVM IR.

llvm-cat was likely an early or experimental tool within the LLVM project's development. Given the semantic complexities of combining LLVM bitcode modules (which are structured binary programs, not raw byte streams), it was quickly superseded by llvm-link, a more sophisticated tool designed to perform a proper, semantically-aware merge of bitcode modules. Its existence highlights the distinct challenges of working with intermediate representations compared to plain text files.

cat(1), llvm-link(1), llvm-as(1), llvm-dis(1)