TableGen: Allow conditional instruction pattern in multiclass.

Define a 'null_frag' SDPatternOperator node, which if referenced in an
instruction Pattern, results in the pattern being collapsed to be as-if
'[]' had been specified instead. This allows supporting a multiclass
definition where some instaniations have ISel patterns associated and
others do not.

For example,
multiclass myMulti<RegisterClass rc, SDPatternOperator OpNode = null_frag> {
  def _x : myI<(outs rc:), (ins rc:), []>;
  def _r : myI<(outs rc:), (ins rc:), [(set rc:, (OpNode rc:))]>;
}

defm foo : myMulti<GRa, not>;
defm bar : myMulti<GRb>;

llvm-svn: 160333

llvm/include/llvm/Target/TargetSelectionDAG.td

@@ -595,6 +595,13 @@ def not  : PatFrag<(ops node:$in), (xor node:$in, -1)>;
 def vnot : PatFrag<(ops node:$in), (xor node:$in, immAllOnesV)>;
 def ineg : PatFrag<(ops node:$in), (sub 0, node:$in)>;
 
+// null_frag - The null pattern operator is used in multiclass instantiations
+// which accept an SDPatternOperator for use in matching patterns for internal
+// definitions. When expanding a pattern, if the null fragment is referenced
+// in the expansion, the pattern is discarded and it is as-if '[]' had been
+// specified. This allows multiclasses to have the isel patterns be optional.
+def null_frag : SDPatternOperator;
+
 // load fragments.
 def unindexedload : PatFrag<(ops node:$ptr), (ld node:$ptr), [{
   return cast<LoadSDNode>(N)->getAddressingMode() == ISD::UNINDEXED;

llvm/utils/TableGen/CodeGenDAGPatterns.cpp

@@ -2520,6 +2520,37 @@ static void InferFromPattern(const CodeGenInstruction &Inst,
     IsVariadic = true;  // Can warn if we want.
 }
 
+/// hasNullFragReference - Return true if the DAG has any reference to the
+/// null_frag operator.
+static bool hasNullFragReference(DagInit *DI) {
+  DefInit *OpDef = dynamic_cast<DefInit*>(DI->getOperator());
+  if (!OpDef) return false;
+  Record *Operator = OpDef->getDef();
+
+  // If this is the null fragment, return true.
+  if (Operator->getName() == "null_frag") return true;
+  // If any of the arguments reference the null fragment, return true.
+  for (unsigned i = 0, e = DI->getNumArgs(); i != e; ++i) {
+    DagInit *Arg = dynamic_cast<DagInit*>(DI->getArg(i));
+    if (Arg && hasNullFragReference(Arg))
+      return true;
+  }
+
+  return false;
+}
+
+/// hasNullFragReference - Return true if any DAG in the list references
+/// the null_frag operator.
+static bool hasNullFragReference(ListInit *LI) {
+  for (unsigned i = 0, e = LI->getSize(); i != e; ++i) {
+    DagInit *DI = dynamic_cast<DagInit*>(LI->getElement(i));
+    assert(DI && "non-dag in an instruction Pattern list?!");
+    if (hasNullFragReference(DI))
+      return true;
+  }
+  return false;
+}
+
 /// ParseInstructions - Parse all of the instructions, inlining and resolving
 /// any fragments involved.  This populates the Instructions list with fully
 /// resolved instructions.
@@ -2534,8 +2565,11 @@ void CodeGenDAGPatterns::ParseInstructions() {
 
     // If there is no pattern, only collect minimal information about the
     // instruction for its operand list.  We have to assume that there is one
-    // result, as we have no detailed info.
-    if (!LI || LI->getSize() == 0) {
+    // result, as we have no detailed info. A pattern which references the
+    // null_frag operator is as-if no pattern were specified. Normally this
+    // is from a multiclass expansion w/ a SDPatternOperator passed in as
+    // null_frag.
+    if (!LI || LI->getSize() == 0 || hasNullFragReference(LI)) {
       std::vector<Record*> Results;
       std::vector<Record*> Operands;