llc doesn't output anything when used with `-stop-after`/`-stop-before`

[danilaml]

https://github.com/llvm/llvm-project/blob/9eeb0293e27c0f1b44304fd9681dcd88fd9edc0a/llvm/docs/MIRLangRef.rst#mir-testing-guide

Uses this command as an example to produce .mir:
llc -stop-after=machine-cp bug-trigger.ll > test.mir

However, when I try it, llc produces empty stdout. I have to pass an output file explicitly for it to work, i.e.
llc -stop-after=machine-cp bug-trigger.ll -o test.mir

Creating an issue, because I'm not sure if this was an intentional change (in which cases the docs need to be updated) or not (in which case it needs to be fixed).

[MatzeB]

I don't think this is intentional. Debugging and patches welcome :)

[danilaml]

Hm, looks like it's been the case since at least ~2016.

[JOE1994]

It seems it's been like this at least since 2010 ( 7ba6f22 )
If the input file name fed to llc is foo.ll with no output filename set explicitly, OutputFilename is set to foo.s.

llvm-project/llvm/tools/llc/llc.cpp

Lines 127 to 196 in 7ba6f22

	static formatted_raw_ostream *GetOutputStream(const char *TargetName,
	Triple::OSType OS,
	const char *ProgName) {
	// If we don't yet have an output filename, make one.
	if (OutputFilename.empty()) {
	if (InputFilename == "-")
	OutputFilename = "-";
	else {
	OutputFilename = GetFileNameRoot(InputFilename);
	switch (FileType) {
	default: assert(0 && "Unknown file type");
	case TargetMachine::CGFT_AssemblyFile:
	if (TargetName[0] == 'c') {
	if (TargetName[1] == 0)
	OutputFilename += ".cbe.c";
	else if (TargetName[1] == 'p' && TargetName[2] == 'p')
	OutputFilename += ".cpp";
	else
	OutputFilename += ".s";
	} else
	OutputFilename += ".s";
	break;
	case TargetMachine::CGFT_ObjectFile:
	if (OS == Triple::Win32)
	OutputFilename += ".obj";
	else
	OutputFilename += ".o";
	break;
	case TargetMachine::CGFT_Null:
	OutputFilename += ".null";
	break;
	}
	}
	}
	// Decide if we need "binary" output.
	bool Binary = false;
	switch (FileType) {
	default: assert(0 && "Unknown file type");
	case TargetMachine::CGFT_AssemblyFile:
	break;
	case TargetMachine::CGFT_ObjectFile:
	case TargetMachine::CGFT_Null:
	Binary = true;
	break;
	}
	// Make sure that the Out file gets unlinked from the disk if we get a
	// SIGINT
	if (OutputFilename != "-")
	sys::RemoveFileOnSignal(sys::Path(OutputFilename));
	// Open the file.
	std::string error;
	unsigned OpenFlags = 0;
	if (Binary) OpenFlags |= raw_fd_ostream::F_Binary;
	raw_fd_ostream *FDOut = new raw_fd_ostream(OutputFilename.c_str(), error,
	OpenFlags);
	if (!error.empty()) {
	errs() << error << '\n';
	delete FDOut;
	return 0;
	}
	formatted_raw_ostream *Out =
	new formatted_raw_ostream(*FDOut, formatted_raw_ostream::DELETE_STREAM);
	return Out;
	}

OTOH, the MIR testing guide section was written in 2015 by ea788c4 .

It seems the documentation was first written with a mistake but wasn't corrected ever since.

I'll create a PR to update the "MIR testing guide" documentation section.

[JOE1994]

Oh documention was already fixed last year by dabf849 👍 👍