How to Use the Command 'llvm-as' (with Examples)

The llvm-as command is a utility in the LLVM (Low-Level Virtual Machine) toolchain. It transforms human-readable LLVM Intermediate Representation (IR) files, typically with the .ll extension, into a more compact, machine-friendly format known as Bitcode, stored in files with the .bc extension. This transformation is crucial for various stages of compiling and linking in the LLVM compilation process, allowing tools to work with efficient, binary representations of the code.

Use case 1: Assemble an IR File

Code:

llvm-as -o path/to/out.bc path/to/source.ll

Motivation:

Assembling an IR file into Bitcode is a fundamental step in the LLVM compilation process. Converting the human-readable IR code to Bitcode speeds up subsequent compiler passes because it’s a compact binary format rather than plain text. By conducting this conversion early, developers can streamline their toolchain processes, saving both time and computing resources.

Explanation:

• llvm-as: The command itself, indicating the assembler for LLVM IR to Bitcode.
• -o path/to/out.bc: Specifies the output file path for the resulting Bitcode. The -o flag is crucial as it directs where the assembly output should be saved, allowing users to organize and manage their build artifacts efficiently.
• path/to/source.ll: Refers to the input file, which contains the LLVM IR. This is the source file in the compilation process, providing the input that llvm-as will transform into Bitcode.

Example Output:

The command will produce an output binary file named out.bc. While there isn’t a text output to the screen, the resulting Bitcode file is held in the specified directory, ready for further compilation stages, such as optimization with opt or linkage with llvm-link.

Use case 2: Assemble an IR File and Include a Module Hash

Code:

llvm-as --module-hash -o path/to/out.bc path/to/source.ll

Motivation:

Including a module hash is useful for ensuring integrity and verification purposes. This feature helps in debugging and identity management, verifying that the Bitcode has not been unexpectedly modified. It also acts as a fingerprint for the module, which can be used for versioning or cache invalidation in build systems.

Explanation:

• llvm-as: Again, the assembler command that converts IR to Bitcode.
• --module-hash: A flag indicating that a hash of the module should be included in the resulting Bitcode. This addition allows tracking and ensuring that the integrity of the module remains intact from the assembly stage onwards.
• -o path/to/out.bc: Directs the output to a specified file.
• path/to/source.ll: The input LLVM IR file that is to be assembled.

Example Output:

This command outputs a Bitcode file similar to the first example, but it includes metadata for a module hash. Developers won’t see this hash in a console printout, but they can verify it by inspecting the Bitcode metadata, usually with tools like llvm-dis for conversion back to IR for examination.

Use case 3: Read an IR File from stdin and Assemble It

Code:

cat path/to/source.ll | llvm-as -o path/to/out.bc

Motivation:

Reading from standard input is advantageous in environments where the IR file is dynamically generated or streamed from another process, such as in complex build systems or IDEs. This method allows seamless integration with scripts and systems that programmatically generate or alter IR code, facilitating automation and efficient handling of build processes.

Explanation:

• cat path/to/source.ll: This command reads the contents of source.ll and pipes it, suitable for integration with other processes or scripts.
• |: The pipe operator, which redirects the output of cat to the input of llvm-as. This flow allows the assembler to directly receive streamed data from any source.
• llvm-as -o path/to/out.bc: The llvm-as command with the output directory specified. Here, it acts on the input from the standard input stream rather than a file.

Example Output:

The assembly process creates out.bc, just as in previous examples. The difference lies in how the input is fed to llvm-as, showcasing the tool’s versatility in handling inputs, whether via files or dynamically through input streams.

Conclusion

These examples illustrate the flexibility and capability of the llvm-as command in the LLVM toolchain, showcasing its utility in various development scenarios. Whether working with static files, involving module verification through hashes, or integrating into automated build systems with dynamic input handling, llvm-as stands out as a vital component for developers working within the LLVM ecosystem.