[clang] [clang][utils] Add auto mode to reduction script (PR #163282)

[Arthur Eubanks via cfe-commits]

================
@@ -424,11 +490,76 @@ def run_creduce(self):
             print("\n\nctrl-c detected, killed reduction tool")
             p.kill()
 
+    def run_llvm_reduce(self):
+        full_llvm_reduce_cmd = [
+            llvm_reduce_cmd,
+            f"--test={self.testfile}",
+            self.ir_file,
+        ]
+        print("\nRunning llvm-reduce tool...")
+        verbose_print(quote_cmd(full_llvm_reduce_cmd))
+        try:
+            p = subprocess.Popen(full_llvm_reduce_cmd)
+            p.communicate()
+        except KeyboardInterrupt:
+            # Hack to kill C-Reduce because it jumps into its own pgid
+            print("\n\nctrl-c detected, killed reduction tool")
+            p.kill()
+
+    def classify_crash(self) -> FailureType:
+        print("classifying crash ...")
+        if self.check_expected_output(args=self.clang_args + ["-fsyntax-only"]):
+            print("Found Frontend Crash")
+            return FailureType.FrontEnd
+
+        print("Found Middle/Backend failure")
+        args = self.clang_args + [
+            "-mllvm",
+            "--print-on-crash",
+            "-mllvm",
+            f"--print-on-crash-path={self.ir_file}",
+            "-mllvm",
+            "--print-module-scope",
+        ]
+
+        if not self.check_expected_output(args=args):
+            sys.exit("The interestingness test does not pass with '--print-on-crash'.")
+
+        # The output from --print-on-crash has an invalid first line (pass name).
+        remove_first_line(self.ir_file)
+
+        self.opt_level = extract_opt_level(self.clang_args) or "-O2"
+
+        if self.check_expected_output(
----------------
aeubanks wrote:

iiuc `opt` won't crash here because the IR we're passing it has already been through the optimization pipeline. we need to generate the IR with `-Xclang -disable-llvm-passes`

https://github.com/llvm/llvm-project/pull/163282