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

The opt command is a tool that takes LLVM source files and runs specified optimizations and/or analyses on them. It provides various options to optimize or analyze LLVM bitcode files. This article will illustrate three common use cases of the opt command.

Use case 1: Run an optimization or analysis on a bitcode file

Code:

opt -passname path/to/file.bc -S -o file_opt.bc

Motivation: This use case is useful when you want to apply a specific optimization or analysis pass on an LLVM bitcode file. The -passname argument specifies the desired pass to be executed.

Explanation:

• opt is the command itself.
• -passname is the argument that specifies a particular optimization or analysis pass to be run on the bitcode file.
• path/to/file.bc is the path to the input LLVM bitcode file.
• -S instructs opt to generate human-readable LLVM assembly as output.
• -o file_opt.bc specifies the output file name with the applied optimization or analysis.

Example output: After running the command, file_opt.bc will contain the optimized or analyzed LLVM bitcode file.

Use case 2: Output the Control Flow Graph of a function to a .dot file

Code:

opt -dot-cfg -S path/to/file.bc -disable-output

Motivation: This use case is valuable when you need to visualize the Control Flow Graph (CFG) of a function in an LLVM bitcode file. The -dot-cfg argument generates a .dot file representing the CFG.

Explanation:

• opt is the command.
• -dot-cfg instructs opt to generate a Control Flow Graph in .dot format.
• -S generates human-readable LLVM assembly as output.
• path/to/file.bc is the path to the input LLVM bitcode file.
• -disable-output disables the production of any additional output.

Example output: The command will produce a .dot file representing the Control Flow Graph of the function. This file can be visualized using graph visualization tools such as Graphviz.

Use case 3: Optimize the program at level 2 and output the result to another file

Code:

opt -O2 path/to/file.bc -S -o path/to/output_file.bc

Motivation: This use case is beneficial when you want to apply a specific optimization level to an LLVM bitcode file. In this case, the -O2 argument enables optimization at level 2.

Explanation:

• opt is the command.
• -O2 specifies the optimization level. Level 2 enables a higher level of optimization.
• path/to/file.bc is the path to the input LLVM bitcode file.
• -S generates human-readable LLVM assembly as output.
• -o path/to/output_file.bc specifies the output file name with the applied optimization.

Example output: After executing the command, path/to/output_file.bc will contain the optimized LLVM bitcode file at level 2.

Conclusion:

The opt command provides a variety of options to optimize and analyze LLVM bitcode files. It can be used for running specific optimization or analysis passes, generating Control Flow Graphs, and applying different optimization levels. Understanding these use cases allows developers to optimize their LLVM bitcode programs effectively.