Tail call miscompiled by FastISel

[nikic]

Running https://gist.github.com/nikic/b6c45e5eeac03ef1fd842353815ceba4 through llc -O0 results in an entirely empty function, while it should have a jmp set_state@PLT.

It works fine without the dbg.value, or without the tail annotation. It also works fine with -fast-isel=0.

[nikic]

It looks like the TCRETURN gets emitted properly, but then gets dropped by this piece of code:

llvm-project/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp

Lines 1564 to 1570 in 197f3c0

	// If the call was emitted as a tail call, we're done with the block.
	// We also need to delete any previously emitted instructions.
	if (HadTailCall) {
	FastIS->removeDeadCode(SavedInsertPt, FuncInfo->MBB->end());
	--BI;
	break;
	}

[nikic]

This is due to e297006 (cc @fhahn). The DBG_VALUE produced by FastISel gets moved before the TCRETURN, which means that the code that was intended to delete the instructions produced by FastISel now also kills the TCRETURN.

[nikic]

Candidate patch: https://reviews.llvm.org/D117489