[MIR-Canon] Fixing case where MachineFunction is empty.

In cases where the machine function is empty: bail on the RPO traversal.

Differential Revision: https://reviews.llvm.org/D62617

llvm-svn: 362158

llvm/lib/CodeGen/MIRCanonicalizerPass.cpp

@@ -104,6 +104,8 @@ INITIALIZE_PASS_END(MIRCanonicalizer, "mir-canonicalizer",
                     "Rename Register Operands Canonically", false, false)
 
 static std::vector<MachineBasicBlock *> GetRPOList(MachineFunction &MF) {
+  if (MF.empty())
+    return {};
   ReversePostOrderTraversal<MachineBasicBlock *> RPOT(&*MF.begin());
   std::vector<MachineBasicBlock *> RPOList;
   for (auto MBB : RPOT) {

llvm/test/CodeGen/MIR/AArch64/empty-MF.mir

@@ -0,0 +1,11 @@
+# RUN: llc -mtriple aarch64-- -run-pass mir-canonicalizer %s -o - | FileCheck %s
+# CHECK: ...
+# CHECK-NEXT: ---
+# CHECK-NEXT: name:            foo
+# CHECK: body:             |
+# CHECK-EMPTY:
+# CHECK-NEXT: ...
+---
+name: foo
+body:             |
+...