View LLVM optimizer pass diffs, inspired by John Regehr's blog post https://blog.regehr.org/archives/1603

passdiff.py

import sys
import subprocess
import tempfile

if len(sys.argv) < 2:
    print(f"Usage: {sys.argv[0]} <llvm args...>")
    sys.exit(1)

script = sys.argv[1:]
result = subprocess.run(
    ['opt', '-print-before-all', '-print-after-all'] + script,
    stdout=subprocess.PIPE, stderr=subprocess.PIPE,
)

def run_diff(name, before, after):
    before = b'\n'.join(b.rstrip() for b in before).strip()
    after = b'\n'.join(a.rstrip() for a in after).strip()
    with tempfile.NamedTemporaryFile() as b, tempfile.NamedTemporaryFile() as a:
        b.write(before)
        a.write(after)
        b.flush()
        a.flush()
        result = subprocess.run(['icdiff', '-W', b.name, a.name], stdout=subprocess.PIPE)
        if result.stdout:
            print('\n\n\n')
        print(f"Step: {name}")
        if result.stdout:
            sys.stdout.buffer.write(result.stdout)
            input("\n\nPress enter to continue:")

lines = result.stderr.split(b'\n')
name = None
before = []
after = []
mode = None
for line in lines:
    if line.startswith(b'***'):
        if mode is None or b'IR Dump After' in mode:
            if mode is not None:
                run_diff(name, before, after)
            name = line.decode('utf8').strip('* ')
            before = []
        else:
            after = []
        mode = line
        continue

    if b'IR Dump Before' in mode:
        before.append(line)
    else:
        after.append(line)

run_diff(name, before, after)