[TableGen][CodeEmitterGen] Avoid empty OpNum switches in getOperandBitOffset()

[iii]

Summary

getOperandBitOffset() causes the following warning on MSVC:

E:\llvm\ninja\lib\Target\SystemZ\SystemZGenMCCodeEmitter.inc(15414): warning C4060: switch statement contains no 'case' or 'default' labels

Do not emit empty OpNum switches.

[uweigand]

LGTM. As this is fixing a build failure on some platforms, please go ahead and commit.

[RKSimon]

Sorry, this just replaces the warning with:

warning C4065: switch statement contains 'default' but no 'case' labels

You're going to have to avoid emitting empty cases entirely.

[iii]

• Avoid emitting empty cases entirely.

[RKSimon]

LGTM - cheers

llvm/utils/TableGen/CodeEmitterGen.cpp

 void CodeEmitterGen::addInstructionCasesForEncoding(
     Record *R, Record *EncodingDef, CodeGenTarget &Target, std::string &Case,
     std::string &BitOffsetCase) {
   BitsInit *BI = EncodingDef->getValueAsBitsInit("Inst");
 
   // Loop over all of the fields in the instruction, determining which are the
   // operands to the instruction.
   bool Success = true;
+  size_t OrigBitOffsetCaseSize = BitOffsetCase.size();
   BitOffsetCase += "      switch (OpNum) {\n";
+  size_t BitOffsetCaseSizeBeforeLoop = BitOffsetCase.size();
   for (const RecordVal &RV : EncodingDef->getValues()) {
     // Ignore fixed fields in the record, we're looking for values like:
     //    bits<5> RST = { ?, ?, ?, ?, ? };
     if (RV.isNonconcreteOK() || RV.getValue()->isComplete())
       continue;
 
     Success &= addCodeToMergeInOperand(R, BI, std::string(RV.getName()), Case,
                                        BitOffsetCase, Target);
   }
+  // Avoid empty switches.
+  if (BitOffsetCase.size() == BitOffsetCaseSizeBeforeLoop)
+    BitOffsetCase.resize(OrigBitOffsetCaseSize);
+  else
     BitOffsetCase += "      }\n";
 
   if (!Success) {
     // Dump the record, so we can see what's going on...
     std::string E;
     raw_string_ostream S(E);
     S << "Dumping record for previous error:\n";
     S << *R;
     PrintNote(E);