How to use the command 'llc' (with examples)

The ’llc’ command is a part of the LLVM compiler infrastructure and is used to compile LLVM Intermediate Representation (IR) or bitcode to target-specific assembly language. It can perform various optimizations and emit assembly files as output.

Use case 1: Compile a bitcode or IR file to an assembly file with the same base name

Code:

llc path/to/file.ll

Motivation: This use case allows us to compile a bitcode or IR file to an assembly file with the same base name, making it easier to manage and navigate between different versions of the same file.

Explanation:

• llc: The command itself.
• path/to/file.ll: The input file that contains the LLVM Intermediate Representation or bitcode to be compiled.

Example output:

Generated: path/to/file.s

Use case 2: Enable all optimizations

Code:

llc -O3 path/to/input.ll

Motivation: Enabling all optimizations can improve the performance and efficiency of the compiled code. It is useful when aiming for maximum performance.

Explanation:

• llc: The command itself.
• -O3: The -O flag is used to enable optimizations, and 3 indicates the level of optimization. Higher numbers represent more aggressive optimizations.

Example output:

Optimizations enabled: -O3

Use case 3: Output assembly to a specific file

Code:

llc --output path/to/output.s

Motivation: In this use case, we can specify the output file name and directory, allowing us to control where the assembly file is saved.

Explanation:

• llc: The command itself.
• --output: This flag is used to specify the output file for the assembly code.
• path/to/output.s: The desired path and filename for the output assembly file.

Example output:

Assembly output saved to: path/to/output.s

Use case 4: Emit fully relocatable, position independent code

Code:

llc -relocation-model=pic path/to/input.ll

Motivation: Generating fully relocatable, position independent code (PIC) is essential in certain scenarios, such as when building shared libraries or executing code in a memory-constrained environment.

Explanation:

• llc: The command itself.
• -relocation-model=pic: This argument specifies that the generated code should be fully relocatable and position independent.

Example output:

Generated PIC code successfully.

Conclusion:

The ’llc’ command provides several useful options for compiling LLVM Intermediate Representation or bitcode to target-specific assembly language. By utilizing these options, we can control the output, enable optimizations, and generate position independent code when required.