What causes "No .mc files generated." in dotnet/runtime?

Re-run collection with -log_level debug and inspect why no .mc files appear (check the shim path and per-assembly exit codes).

dotnet/runtime · error · RuntimeError

No .mc files generated.

Error message

No .mc files generated.

What it means

Raised by SuperPMICollect.__collect_mc_files__ in superpmi.py at the end of collection. It lists '*.mc' files in temp_location; if zero were produced, the collection produced no method contexts and the merge step would have nothing to work on, so it aborts.

Solutions

Re-run collection with -log_level debug and inspect why no .mc files appear (check the shim path and per-assembly exit codes).
Verify Core_Root contains the SuperPMI collector shim (superpmi-shim-collector.*) and the shim JIT, and that -core_root is correct.
Confirm -assemblies/-ilc_rsps actually resolve to compilable inputs and none are entirely excluded by -exclude.
For R2R images, add --disable_r2r so methods get JIT-compiled and collected.

Example fix

// before
python superpmi.py collect --pmi -assemblies bin/   # 'No .mc files generated.'
// after
python superpmi.py collect --pmi -assemblies bin/ --disable_r2r -log_level debug --core_root <valid Core_Root>

Defensive patterns

Strategy: validation

Validate before calling

import os, glob
mc_files = glob.glob(os.path.join(temp_location, "*.mc"))
if not mc_files:
    raise SystemExit("Collection produced no .mc files; verify the SuperPMI shim/JIT path and inputs")

Prevention

Always run collection with -log_level debug until it succeeds at least once.
Verify Core_Root has the collector shim and shim JIT, and that inputs are real compilable assemblies.

When it happens

Trigger: The collection command / PMI / crossgen2 / nativeaot run completed but the SuperPMI shim wrote no .mc files into the temp directory — e.g. the shim was not loaded (SuperPMIShimPath wrong), the target assemblies had no methods, or every collection invocation failed.

Common situations: Wrong SuperPMIShimPath or shim JIT missing from Core_Root; collection_command failed silently; -assemblies pointed at an empty/excluded directory; DOTNET_TieredCompilation/ReadyToRun settings prevented JIT compilation; disk/permission issue in temp_location.

Related errors

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/0c880c2771598baf. Report an issue: GitHub.

Appendix: source

Thrown at src/coreclr/scripts/superpmi.py:1350

                # Set environment variables.
                nativeaot_command_env = env_copy.copy()
                set_and_report_env(nativeaot_command_env, root_env)

                old_env = os.environ.copy()
                os.environ.update(nativeaot_command_env)

                ilc_rsps = list(filter(lambda ilc_rsp: ilc_rsp.endswith(".ilc.rsp"), ilc_rsps))
                helper = AsyncSubprocessHelper(ilc_rsps, verbose=True)
                helper.run_to_completion(run_nativeaot, self)

                os.environ.clear()
                os.environ.update(old_env)
            ################################################################################################ end of "self.coreclr_args.nativeaot is True"

        mc_files = [os.path.join(self.temp_location, item) for item in os.listdir(self.temp_location) if item.endswith(".mc")]
        if len(mc_files) == 0:
            raise RuntimeError("No .mc files generated.")

    def __merge_mc_files__(self):
        """ Merge the mc files that were generated

        Notes:
            mcs -merge <s_baseMchFile> <s_tempDir>\\*.mc -recursive -dedup -thin

        """

        logging.info("Merging MC files")

        pattern = os.path.join(self.temp_location, "*.mc")

        command = [self.mcs_path, "-merge", self.base_mch_file, pattern, "-recursive", "-dedup", "-thin"]
        run_and_log(command)

        if not os.path.isfile(self.base_mch_file):
            raise RuntimeError("MCH file failed to be generated at: %s" % self.base_mch_file)

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