I finally decided to use the power of emacs to automate a task that I perform at least a dozen times per day: converting a snippet of C source code into LLVM IR assembly. The command to do this is simple:

clang -emit-llvm -c -o - file.c | opt -S

The only annoying part about using the command was finding a spare terminal and finding the input file on the file system (and then remembering where I put that terminal when I wanted to refer to it later). Enter emacs. It took a while to navigate the emacs documentation to figure out how to write what I wanted and learn the relevant terminology. The function I came up with pipes the current buffer (or region, if it is active) to clang and opt, puts the output in a new buffer, activates the llvm major mode, and switches to the new buffer.

(defun llvmize (&optional start end)
  "Convert the current buffer or region (containing C code) to LLVM assembly via clang and opt"
  (let ((start (if mark-active (region-beginning) (point-min)))
        (end (if mark-active (region-end) (point-max)))
        (default-major-mode 'llvm-mode)
        (buf (generate-new-buffer "*llvm-asm*")))
    (set-buffer-major-mode buf)
    (shell-command-on-region start end "clang -emit-llvm -x c -c -o - - | opt -S -mem2reg -basicaa -gvn" buf)
    (set-buffer buf)
    (setq buffer-read-only t)
    (switch-to-buffer-other-window buf)))

This has a few nice benefits over just throwing LLVM IR assembly dumps into random terminals:

  • Nice syntax highlighting
  • Standard emacs buffer management applies
  • Cuts down on terminal proliferation (a serious problem)

I feel like this style of function is generally useful and I should probably make a few more to automate my life a bit. This function still needs a few improvements:

  • I want to look at the file extension (if there is one) of the active buffer so that I can use -x c++ instead of always just using -x c for clang.
  • I also want to find the best clang and opt binaries on the system; debian installs opt as opt-$version, so this function doesn't work as-is.
  • I want to add some keymaps to the freshly-created buffer to easily quit and close the window when I am done with it.

Back to reading the emacs documentation.