TableGen/GlobalISel: Don't take reference to temporary values

[arsenm]

Summary

These return temporary Optional<> values which are immediately
destroyed. I'm not sure why no sanitizers seem to have caught this,
but I encountered crashes on these in a future patch.

[dsanders]

LGTM

[arsenm]

eafa8dbefe30c85e58efc5309793f2d7c2109e35

llvm/utils/TableGen/GlobalISelEmitter.cpp

if (OpName == "EXTRACT_SUBREG") {
     if (!SuperClass)
       return failedImport(
         "Cannot infer register class from EXTRACT_SUBREG operand #0");
 
     auto SubIdx = inferSubRegIndexForNode(Dst->getChild(1));
     if (!SubIdx)
       return failedImport("EXTRACT_SUBREG child #1 is not a subreg index");
 
-    const auto &SrcRCDstRCPair =
+    const auto SrcRCDstRCPair =
       (*SuperClass)->getMatchingSubClassWithSubRegs(CGRegs, *SubIdx);
     assert(SrcRCDstRCPair->second && "Couldn't find a matching subclass");
     M.insertAction<ConstrainOperandToRegClassAction>(
       InsertPt, DstMIBuilder.getInsnID(), 0, *SrcRCDstRCPair->second);
     M.insertAction<ConstrainOperandToRegClassAction>(
       InsertPt, DstMIBuilder.getInsnID(), 1, *SrcRCDstRCPair->first);
 
     // We're done with this pattern!  It's eligible for GISel emission; return
if (DefInit *SubRegInit =
       Record *RCDef = getInitValueAsRegClass(Dst->getChild(0)->getLeafValue());
       if (!RCDef)
         return failedImport("EXTRACT_SUBREG child #0 could not "
                             "be coerced to a register class");
 
       CodeGenRegisterClass *RC = CGRegs.getRegClass(RCDef);
       CodeGenSubRegIndex *SubIdx = CGRegs.getSubRegIdx(SubRegInit->getDef());
 
-      const auto &SrcRCDstRCPair =
+      const auto SrcRCDstRCPair =
           RC->getMatchingSubClassWithSubRegs(CGRegs, SubIdx);
       if (SrcRCDstRCPair.hasValue()) {
         assert(SrcRCDstRCPair->second && "Couldn't find a matching subclass");
         if (SrcRCDstRCPair->first != RC)
           return failedImport("EXTRACT_SUBREG requires an additional COPY");
       }
 
       DstMIBuilder.addRenderer<CopySubRegRenderer>(Dst->getChild(0)->getName(),
if (DstIName == "EXTRACT_SUBREG") {
     // to pick a register class so constrain the result to a register class
     // that can hold the correct MVT.
     //
     // FIXME: This may introduce an extra copy if the chosen class doesn't
     //        actually contain the subregisters.
     assert(Src->getExtTypes().size() == 1 &&
              "Expected Src of EXTRACT_SUBREG to have one result type");
 
-    const auto &SrcRCDstRCPair =
+    const auto SrcRCDstRCPair =
       (*SuperClass)->getMatchingSubClassWithSubRegs(CGRegs, *SubIdx);
     assert(SrcRCDstRCPair->second && "Couldn't find a matching subclass");
     M.addAction<ConstrainOperandToRegClassAction>(0, 0, *SrcRCDstRCPair->second);
     M.addAction<ConstrainOperandToRegClassAction>(0, 1, *SrcRCDstRCPair->first);
 
     // We're done with this pattern!  It's eligible for GISel emission; return
     // it.
     ++NumPatternImported;