dotnet/runtime · error · RuntimeError

Couldn't create git diff

Error message

Couldn't create git diff

What it means

Raised during asmdiffs when `--git_diff` is set (superpmi.py:2515-2527). After textual diffs are found, the script runs `git diff --output=<asm_diffs.diff> --no-index -- <base_dir> <diff_dir>`; git returns 0 for no differences and 1 for differences, both acceptable. Any other return code raises this error, meaning the git invocation itself failed.

Source

Thrown at src/coreclr/scripts/superpmi.py:2527

                                ran_jit_analyze = True

                        if not ran_jit_analyze:
                            logging.info("jit-analyze not found on PATH. Generate a diff analysis report by building jit-analyze from https://github.com/dotnet/jitutils and running:")
                            logging.info("    jit-analyze -r --base %s --diff %s", base_asm_location, diff_asm_location)

                        if self.coreclr_args.git_diff:
                            asm_diffs_location = os.path.join(asm_root_dir, "asm_diffs.diff")
                            git_diff_command = [ "git", "diff", "--output=" + asm_diffs_location, "--no-index", "--", base_asm_location, diff_asm_location ]
                            git_diff_time = time.time() * 1000
                            git_diff_proc = subprocess.Popen(git_diff_command, stdout=subprocess.PIPE)
                            git_diff_proc.communicate()
                            git_diff_elapsed_time = round((time.time() * 1000) - git_diff_time, 1)
                            git_diff_return_code = git_diff_proc.returncode
                            if git_diff_return_code == 0 or git_diff_return_code == 1: # 0 means no differences and 1 means differences
                                logging.info("Created git diff file at %s in %s ms", asm_diffs_location, git_diff_elapsed_time)
                                logging.info("-------")
                            else:
                                raise RuntimeError("Couldn't create git diff")

                    else:
                        logging.warning("No textual differences. Is this an issue with coredistools?")

                    # If we are not specifying custom metrics then print a summary here, otherwise leave the summarization up to jit-analyze.
                    if self.coreclr_args.metrics is None:
                        base_diff_sizes = [(int(r["Base ActualCodeBytes"]), int(r["Diff ActualCodeBytes"])) for r in diffs]
                        (num_size_improvements, num_size_regressions, num_size_same, byte_improvements, byte_regressions) = calculate_size_improvements_regressions(base_diff_sizes)

                        num_diffs_str = "{:,d} contexts with diffs".format(len(diffs))
                        logging.info("{} ({:,d} size improvements, {:,d} size regressions, {:,d} same size)".format(
                            num_diffs_str,
                            num_size_improvements,
                            num_size_regressions,
                            num_size_same,
                            byte_improvements,
                            byte_regressions))

View on GitHub (pinned to 290d5ab72c)

Solutions

  1. Verify `git` is installed and on PATH (`git --version`).
  2. Check that the asm output root directory is writable so `--output=` can create the diff file.
  3. Drop `--git_diff` if you do not need the `.diff` artifact (the asm diff analysis still runs without it).

Example fix

// before
superpmi.py asmdiffs --git_diff -mch_files foo.mch ...   # in a container without git
// after
superpmi.py asmdiffs -mch_files foo.mch ...   # omit --git_diff, or apt-get install git first
Defensive patterns

Strategy: try-catch

Validate before calling

import shutil
if args.git_diff and not shutil.which('git'):
    raise SystemExit('git not found on PATH; cannot honor --git_diff')

Try / catch

try:
    # ...the asmdiffs flow that may run git diff
except RuntimeError as e:
    if "Couldn't create git diff" in str(e):
        logging.error('git diff failed; install git or drop --git_diff')
        # degrade gracefully by continuing without the .diff artifact
    else:
        raise

Prevention

When it happens

Trigger: Running `superpmi.py asmdiffs --git_diff ...` where the `git diff --no-index` subprocess exits with a code other than 0 or 1. Common when git is not installed/on PATH, the output path is unwritable, or git itself errors out.

Common situations: Running in a minimal container/CI image without git installed; the asm_diffs.diff output location is on a read-only mount; the base/diff asm directories do not exist or are inaccessible to git.

Related errors

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/ddd5b2842ea1fc19. Report an issue: GitHub.