Index: llvm/test/tools/llvm-reduce/command-line-behavior.test
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-reduce/command-line-behavior.test
@@ -0,0 +1,12 @@
+# Print the help output if no arguments are specified
+# RUN: llvm-reduce --help | grep "LLVM automatic testcase reducer"
+# RUN: llvm-reduce | grep "LLVM automatic testcase reducer"
+
+# Don't require any other arguments for --print-delta-passes
+# RUN: llvm-reduce --print-delta-passes  | grep "Delta passes (pass to \`--delta-passes=\` as a comma separated list)"
+
+# Missing test input
+# RUN: not llvm-reduce --test FileCheck 2>&1 | grep "error: reduction testcase positional argument must be specified"
+
+# Missing test script
+# RUN: not llvm-reduce some-input 2>&1 | grep "error: --test option must be specified"
Index: llvm/tools/llvm-reduce/llvm-reduce.cpp
===================================================================
--- llvm/tools/llvm-reduce/llvm-reduce.cpp
+++ llvm/tools/llvm-reduce/llvm-reduce.cpp
@@ -45,18 +45,19 @@
              "debugging (crash reports, llvm-symbolizer and core dumps)"),
     cl::cat(LLVMReduceOptions));
 
+// FIXME: should be able to do this without other arguments, also add in help.
 static cl::opt<bool>
     PrintDeltaPasses("print-delta-passes",
                      cl::desc("Print list of delta passes, passable to "
                               "--delta-passes as a comma separated list"),
                      cl::cat(LLVMReduceOptions));
 
-static cl::opt<std::string> InputFilename(cl::Positional, cl::Required,
+static cl::opt<std::string> InputFilename(cl::Positional,
                                           cl::desc("<input llvm ll/bc file>"),
                                           cl::cat(LLVMReduceOptions));
 
 static cl::opt<std::string>
-    TestFilename("test", cl::Required,
+    TestFilename("test",
                  cl::desc("Name of the interesting-ness test to be run"),
                  cl::cat(LLVMReduceOptions));
 
@@ -157,10 +158,21 @@
 
 int main(int Argc, char **Argv) {
   InitLLVM X(Argc, Argv);
+  const StringRef ToolName(Argv[0]);
 
   cl::HideUnrelatedOptions({&LLVMReduceOptions, &getColorCategory()});
   cl::ParseCommandLineOptions(Argc, Argv, "LLVM automatic testcase reducer.\n");
 
+  if (Argc == 1) {
+    cl::PrintHelpMessage();
+    return 0;
+  }
+
+  if (PrintDeltaPasses) {
+    printDeltaPasses(outs());
+    return 0;
+  }
+
   bool ReduceModeMIR = false;
   if (InputLanguage != InputLanguages::None) {
     if (InputLanguage == InputLanguages::MIR)
@@ -169,9 +181,15 @@
     ReduceModeMIR = true;
   }
 
-  if (PrintDeltaPasses) {
-    printDeltaPasses(errs());
-    return 0;
+  if (InputFilename.empty()) {
+    WithColor::error(errs(), ToolName)
+        << "reduction testcase positional argument must be specified\n";
+    return 1;
+  }
+
+  if (TestFilename.empty()) {
+    WithColor::error(errs(), ToolName) << "--test option must be specified\n";
+    return 1;
   }
 
   if (!PreserveDebugEnvironment)
@@ -181,7 +199,7 @@
   std::unique_ptr<TargetMachine> TM;
 
   auto [OriginalProgram, InputIsBitcode] =
-      parseReducerWorkItem(Argv[0], InputFilename, Context, TM, ReduceModeMIR);
+      parseReducerWorkItem(ToolName, InputFilename, Context, TM, ReduceModeMIR);
   if (!OriginalProgram) {
     return 1;
   }
@@ -193,7 +211,7 @@
 
   // Initialize test environment
   TestRunner Tester(TestFilename, TestArguments, std::move(OriginalProgram),
-                    std::move(TM), Argv[0], OutputFilename, InputIsBitcode,
+                    std::move(TM), ToolName, OutputFilename, InputIsBitcode,
                     OutputBitcode);
 
   // This parses and writes out the testcase into a temporary file copy for the