[mips] Emit the .insn directive for empty basic blocks.

[vkalintiris]

Summary

In microMIPS, labels need to know whether they are on code or data. This is
indicated with STO_MIPS_MICROMIPS and can be inferred by being followed
by instructions. For empty basic blocks, we can ensure this by emitting the
.insn directive after the label.

Also, this fixes some failures in our out-of-tree MIPS buildbots, for the
exception handling regression tests under: SingleSource/Regression/C++/EH

[dsanders]

LGTM with a couple nits.

Also, this fixes some failures in our out-of-tree MIPS buildbots, for the
exception handling regression tests under: SingleSource/Regression/C++/EH

This should mention that it's the microMIPS buildbots in particular.

test/CodeGen/Mips/insn-zero-size-bb.ll

5–6

Nit: This comment isn't quite right since we emit .insn for empty BB's on all subtargets. I think you mean it only matters for microMIPS and MIPS16.

lib/Target/Mips/MipsAsmPrinter.h

public:
 
   void EmitInstruction(const MachineInstr *MI) override;
   void printSavedRegsBitmask();
   void emitFrameDirective();
   const char *getCurrentABIString() const;
   void EmitFunctionEntryLabel() override;
   void EmitFunctionBodyStart() override;
   void EmitFunctionBodyEnd() override;
+  void EmitBasicBlockEnd(const MachineBasicBlock &MBB) override;
   bool isBlockOnlyReachableByFallthrough(
                                    const MachineBasicBlock* MBB) const override;
   bool PrintAsmOperand(const MachineInstr *MI, unsigned OpNo,
                        unsigned AsmVariant, const char *ExtraCode,
                        raw_ostream &O) override;
   bool PrintAsmMemoryOperand(const MachineInstr *MI, unsigned OpNum,
                              unsigned AsmVariant, const char *ExtraCode,
                              raw_ostream &O) override;

lib/Target/Mips/MipsAsmPrinter.cpp

void MipsAsmPrinter::EmitFunctionBodyEnd() {
   // Make sure to terminate any constant pools that were at the end
   // of the function.
   if (!InConstantPool)
     return;
   InConstantPool = false;
   OutStreamer->EmitDataRegion(MCDR_DataRegionEnd);
 }
 
+void MipsAsmPrinter::EmitBasicBlockEnd(const MachineBasicBlock &MBB) {
+  MipsTargetStreamer &TS = getTargetStreamer();
+  if (MBB.size() == 0)
+    TS.emitDirectiveInsn();
+}
+
 /// isBlockOnlyReachableByFallthough - Return true if the basic block has
 /// exactly one predecessor and the control transfer mechanism between
 /// the predecessor and this block is a fall-through.
 bool MipsAsmPrinter::isBlockOnlyReachableByFallthrough(const MachineBasicBlock*
                                                        MBB) const {
   // The predecessor has to be immediately before this block.
   const MachineBasicBlock *Pred = *MBB->pred_begin();
 

test/CodeGen/Mips/insn-zero-size-bb.ll

+; RUN: llc < %s -march=mips -mcpu=mips32 | FileCheck %s
+; RUN: llc < %s -march=mips -mcpu=mips32r3 -mattr=+micromips | FileCheck %s
+; RUN: llc < %s -march=mips -mcpu=mips16 | FileCheck %s
+
+; Verify that when we are in micromips or mips16, we do emit the .insn
+; directive for zero-sized (empty) basic blocks.
+
+declare i32 @foo(...)
+declare void @bar()
+
+define void @main() {
+entry:
+  invoke void @bar() #0
+          to label %unreachable unwind label %return
+
+unreachable:
+; CHECK:          ${{.*}}: # %unreachable
+; CHECK-NEXT:         .insn
+  unreachable
+
+return:
+  %0 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)* @foo to i8*)
+          catch i8* null
+  ret void
+}
+
+attributes #0 = { noreturn }