[TableGen] New tblgen Instruction bit to disable DAGISel pattern imports

Added a new DAGISelShouldIgnore property to class Instruction in Target.td, similar to FastISelShouldIgnore. This allows one to avoid a record's DAGISel .td implementation and .inc generation.

Author: jofrn

llvm/include/llvm/Target/Target.td

@@ -777,6 +777,10 @@ class Instruction : InstructionEncoding {
   /// SelectionDAG can.
   bit FastISelShouldIgnore = false;
 
+  /// Should DAGISel ignore this instruction. In cases where lowering may
+  /// be done elsewhere or is unneeded, DAGISel may skip over them.
+  bit DAGISelShouldIgnore = false;
+
   /// HasPositionOrder: Indicate tablegen to sort the instructions by record
   /// ID, so that instruction that is defined earlier can be sorted earlier
   /// in the assembly matching table.

llvm/utils/TableGen/Common/CodeGenInstruction.cpp

@@ -465,6 +465,7 @@ CodeGenInstruction::CodeGenInstruction(Record *R)
   isConvergent = R->getValueAsBit("isConvergent");
   hasNoSchedulingInfo = R->getValueAsBit("hasNoSchedulingInfo");
   FastISelShouldIgnore = R->getValueAsBit("FastISelShouldIgnore");
+  DAGISelShouldIgnore = R->getValueAsBit("DAGISelShouldIgnore");
   variadicOpsAreDefs = R->getValueAsBit("variadicOpsAreDefs");
   isAuthenticated = R->getValueAsBit("isAuthenticated");
 

llvm/utils/TableGen/Common/CodeGenInstruction.h

@@ -282,6 +282,7 @@ class CodeGenInstruction {
   bool isConvergent : 1;
   bool hasNoSchedulingInfo : 1;
   bool FastISelShouldIgnore : 1;
+  bool DAGISelShouldIgnore : 1;
   bool hasChain : 1;
   bool hasChain_Inferred : 1;
   bool variadicOpsAreDefs : 1;

llvm/utils/TableGen/DAGISelEmitter.cpp

@@ -165,8 +165,21 @@ void DAGISelEmitter::run(raw_ostream &OS) {
   // Add all the patterns to a temporary list so we can sort them.
   Records.startTimer("Sort patterns");
   std::vector<const PatternToMatch *> Patterns;
-  for (const PatternToMatch &PTM : CGP.ptms())
+  for (const PatternToMatch &PTM : CGP.ptms()) {
+
+    // Disable import of patterns marked as ignore.
+    const TreePatternNode &Dst = PTM.getDstPattern();
+    if (!Dst.isLeaf()) {
+      const Record *Op = Dst.getOperator();
+      const bool shouldIgnore =
+          Op->isSubClassOf("Instruction") &&
+          CGP.getTargetInfo().getInstruction(Op).DAGISelShouldIgnore;
+      if (shouldIgnore)
+        continue;
+    }
+
     Patterns.push_back(&PTM);
+  }
 
   // We want to process the matches in order of minimal cost.  Sort the patterns
   // so the least cost one is at the start.