[TableGen] Avoid generating switch with just default

Summary:
Switch with just default causes an MSVC warning (warning C4065: switch statement
contains 'default' but no 'case' labels).

Change-Id: I9ddeccdef93666256b5454b164b567b73b488461

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81021

Author: dstutt

llvm/utils/TableGen/GlobalISel/GIMatchTree.cpp

@@ -612,18 +612,23 @@ void GIMatchTreeOpcodePartitioner::emitPartitionResults(
 
 void GIMatchTreeOpcodePartitioner::generatePartitionSelectorCode(
     raw_ostream &OS, StringRef Indent) const {
-  OS << Indent << "Partition = -1;\n"
-     << Indent << "switch (MIs[" << InstrID << "]->getOpcode()) {\n";
-  for (const auto &EnumInstr : enumerate(PartitionToInstr)) {
-    if (EnumInstr.value() == nullptr)
-      OS << Indent << "default:";
-    else
-      OS << Indent << "case " << EnumInstr.value()->Namespace
-         << "::" << EnumInstr.value()->TheDef->getName() << ":";
-    OS << " Partition = " << EnumInstr.index() << "; break;\n";
+  // Make sure not to emit empty switch or switch with just default
+  if (PartitionToInstr.size() == 1 && PartitionToInstr[0] == nullptr) {
+    OS << Indent << "Partition = 0;\n";
+  } else if (PartitionToInstr.size()) {
+    OS << Indent << "Partition = -1;\n"
+       << Indent << "switch (MIs[" << InstrID << "]->getOpcode()) {\n";
+    for (const auto &EnumInstr : enumerate(PartitionToInstr)) {
+      if (EnumInstr.value() == nullptr)
+        OS << Indent << "default:";
+      else
+        OS << Indent << "case " << EnumInstr.value()->Namespace
+           << "::" << EnumInstr.value()->TheDef->getName() << ":";
+      OS << " Partition = " << EnumInstr.index() << "; break;\n";
+    }
+    OS << Indent << "}\n";
   }
-  OS << Indent << "}\n"
-     << Indent
+  OS << Indent
      << "// Default case but without conflicting with potential default case "
         "in selection.\n"
      << Indent << "if (Partition == -1) return false;\n";
@@ -775,4 +780,3 @@ void GIMatchTreeVRegDefPartitioner::generatePartitionSelectorCode(
 
   OS << Indent << "if (Partition == -1) return false;\n";
 }
-