How to use the command 'llvm-dis' (with examples)

The ’llvm-dis’ command is a powerful tool in the LLVM toolkit that allows you to convert LLVM bitcode files into human-readable LLVM Intermediate Representation (IR). This can be useful for better understanding and analyzing the inner workings of LLVM bitcode files.

Use case 1: Convert a bitcode file as LLVM IR and write the result to stdout

Code:

llvm-dis path/to/input.bc -o -

Motivation:

One possible motivation for using this example is when you want to quickly inspect the content of a bitcode file without creating a separate IR file. By specifying -o - as the output file, the LLVM Intermediate Representation will be printed directly to the console (stdout).

Explanation:

• llvm-dis: The command itself.
• path/to/input.bc: Specifies the path to the input bitcode file that you want to convert to LLVM IR.
• -o -: Specifies the output file. The - indicates that the output should be written to stdout.

Example output:

; ModuleID = 'path/to/input.bc'
source_filename = "path/to/input.bc"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: noinline nounwind uwtable
define i32 @main(i32, i8**) #0 {
  %3 = alloca [1000000 x i32]
  %s = alloca i32*
  %p = alloca [2 x i8**]
  [...]
}

Use case 2: Convert a bitcode file to an LLVM IR file with the same filename

Code:

llvm-dis path/to/file.bc

Motivation:

This use case is handy when you want to keep the original filename but still convert a bitcode file to LLVM IR. This can be useful for organization purposes or if you’re working with a large number of bitcode files.

Explanation:

• llvm-dis: The command itself.
• path/to/file.bc: Specifies the path to the input bitcode file that you want to convert to LLVM IR.

Example output:

This command does not directly produce any visible output. Instead, it creates a new file with the same name as the input file but with a ‘.ll’ extension, containing the LLVM Intermediate Representation.

Use case 3: Convert a bitcode file to LLVM IR, writing the result to the specified file

Code:

llvm-dis path/to/input.bc -o path/to/output.ll

Motivation:

This example is useful when you want to convert a bitcode file to LLVM IR and save the output in a specific location. The -o option specifies the output file path.

Explanation:

• llvm-dis: The command itself.
• path/to/input.bc: Specifies the path to the input bitcode file that you want to convert to LLVM IR.
• -o path/to/output.ll: Specifies the output file path (path/to/output.ll in this case) where the LLVM Intermediate Representation will be written.

Example output:

No visible output will be displayed on the console (stdout). However, a new file named ‘output.ll’ will be created at the specified path (‘path/to/output.ll’), containing the LLVM Intermediate Representation.