[TableGen] New default operand "undef_tied_input"

This is a new special identifier which you can use as a default in
OperandWithDefaultOps. The idea is that you use it for an input
operand of an instruction that's tied to an output operand, and its
semantics are that (in the default case) the input operand's value is
not used at all.

The detailed effect is that when instruction selection emits the
instruction in the form of a pre-regalloc MachineInstr, it creates an
IMPLICIT_DEF node to use as that input.

If you're creating an MCInst with explicit register names, then the
right handling would be to set the input operand to the same register
as the output one (honouring the tie) and to add the 'undef' flag
indicating that that register is deemed to acquire a new don't-care
definition just before we read it. But I haven't done that in this
commit, because there was no need to - no Tablegen backend seems to
autogenerate default fields in an MCInst.

Patch by: Simon Tatham

Differential Revision: https://reviews.llvm.org/D60696

llvm-svn: 362064

llvm/utils/TableGen/CodeGenDAGPatterns.cpp

@@ -2132,7 +2132,7 @@ static TypeSetByHwMode getImplicitType(Record *R, unsigned ResNo,
 
   if (R->getName() == "node" || R->getName() == "srcvalue" ||
       R->getName() == "zero_reg" || R->getName() == "immAllOnesV" ||
-      R->getName() == "immAllZerosV") {
+      R->getName() == "immAllZerosV" || R->getName() == "undef_tied_input") {
     // Placeholder.
     return TypeSetByHwMode(); // Unknown.
   }

llvm/utils/TableGen/DAGISelMatcherGen.cpp

@@ -691,6 +691,17 @@ void MatcherGen::EmitResultLeafAsOperand(const TreePatternNode *N,
       return;
     }
 
+    if (Def->getName() == "undef_tied_input") {
+      std::array<MVT::SimpleValueType, 1> ResultVTs = { N->getSimpleType(0) };
+      std::array<unsigned, 0> InstOps;
+      auto IDOperandNo = NextRecordedOperandNo++;
+      AddMatcher(new EmitNodeMatcher("TargetOpcode::IMPLICIT_DEF",
+                                     ResultVTs, InstOps, false, false, false,
+                                     false, -1, IDOperandNo));
+      ResultOps.push_back(IDOperandNo);
+      return;
+    }
+
     // Handle a reference to a register class. This is used
     // in COPY_TO_SUBREG instructions.
     if (Def->isSubClassOf("RegisterOperand"))

llvm/utils/TableGen/GlobalISelEmitter.cpp

@@ -3037,7 +3037,8 @@ private:
   importExplicitUseRenderer(action_iterator InsertPt, RuleMatcher &Rule,
                             BuildMIAction &DstMIBuilder,
                             TreePatternNode *DstChild);
-  Error importDefaultOperandRenderers(BuildMIAction &DstMIBuilder,
+  Error importDefaultOperandRenderers(action_iterator InsertPt, RuleMatcher &M,
+                                      BuildMIAction &DstMIBuilder,
                                       DagInit *DefaultOps) const;
   Error
   importImplicitDefRenderers(BuildMIAction &DstMIBuilder,
@@ -3777,7 +3778,8 @@ Expected<action_iterator> GlobalISelEmitter::importExplicitUseRenderers(
     // end up with too many rendered operands.
     if (DstIOperand.Rec->isSubClassOf("OperandWithDefaultOps")) {
       DagInit *DefaultOps = DstIOperand.Rec->getValueAsDag("DefaultOps");
-      if (auto Error = importDefaultOperandRenderers(DstMIBuilder, DefaultOps))
+      if (auto Error = importDefaultOperandRenderers(
+            InsertPt, M, DstMIBuilder, DefaultOps))
         return std::move(Error);
       ++NumDefaultOps;
       continue;
@@ -3802,19 +3804,38 @@ Expected<action_iterator> GlobalISelEmitter::importExplicitUseRenderers(
 }
 
 Error GlobalISelEmitter::importDefaultOperandRenderers(
-    BuildMIAction &DstMIBuilder, DagInit *DefaultOps) const {
+    action_iterator InsertPt, RuleMatcher &M, BuildMIAction &DstMIBuilder,
+    DagInit *DefaultOps) const {
   for (const auto *DefaultOp : DefaultOps->getArgs()) {
+    Optional<LLTCodeGen> OpTyOrNone = None;
+
     // Look through ValueType operators.
     if (const DagInit *DefaultDagOp = dyn_cast<DagInit>(DefaultOp)) {
       if (const DefInit *DefaultDagOperator =
               dyn_cast<DefInit>(DefaultDagOp->getOperator())) {
         if (DefaultDagOperator->getDef()->isSubClassOf("ValueType"))
+          OpTyOrNone = MVTToLLT(getValueType(
+                                  DefaultDagOperator->getDef()));
           DefaultOp = DefaultDagOp->getArg(0);
       }
     }
 
     if (const DefInit *DefaultDefOp = dyn_cast<DefInit>(DefaultOp)) {
-      DstMIBuilder.addRenderer<AddRegisterRenderer>(DefaultDefOp->getDef());
+      auto Def = DefaultDefOp->getDef();
+      if (Def->getName() == "undef_tied_input") {
+        unsigned TempRegID = M.allocateTempRegID();
+        M.insertAction<MakeTempRegisterAction>(
+          InsertPt, OpTyOrNone.getValue(), TempRegID);
+        InsertPt = M.insertAction<BuildMIAction>(
+          InsertPt, M.allocateOutputInsnID(),
+          &Target.getInstruction(RK.getDef("IMPLICIT_DEF")));
+        BuildMIAction &IDMIBuilder = *static_cast<BuildMIAction *>(
+          InsertPt->get());
+        IDMIBuilder.addRenderer<TempRegRenderer>(TempRegID);
+        DstMIBuilder.addRenderer<TempRegRenderer>(TempRegID);
+      } else {
+        DstMIBuilder.addRenderer<AddRegisterRenderer>(Def);
+      }
       continue;
     }