[SystemZ] Do not use glue to represent condition code dependencies

Currently, an instruction setting the condition code is linked to
the instruction using the condition code via a "glue" link in the
SelectionDAG.  This has a number of drawbacks; in particular, it
means the same CC cannot be used by multiple users.  It also makes
it more difficult to efficiently implement SADDO et. al.

This patch changes the back-end to represent CC dependencies as
normal values during SelectionDAG matching, along the lines of
how this is handled in the X86 back-end already.

In addition to the core mechanics of updating all relevant patterns,
this requires a number of additional changes:

- We now need to be able to spill/restore a CC value into a GPR
  if necessary.  This means providing a copyPhysReg implementation
  for moves involving CC, and defining getCrossCopyRegClass.

- Since we still prefer to avoid such spills, we provide an override
  for IsProfitableToFold to avoid creating a merged LOAD / ICMP if
  this would result in multiple users of the CC.

- combineCCMask no longer requires a single CC user, and no longer
  need to be careful about preventing invalid glue/chain cycles.

- emitSelect needs to be more careful in marking CC live-in to
  the basic block it generates.  Also, we can now optimize the
  case of multiple subsequent selects with the same condition
  just like X86 does.

llvm-svn: 331202

Author: uweigand

llvm/lib/Target/SystemZ/SystemZISelDAGToDAG.cpp

@@ -351,6 +351,7 @@ class SystemZDAGToDAGISel : public SelectionDAGISel {
   void Select(SDNode *Node) override;
   bool SelectInlineAsmMemoryOperand(const SDValue &Op, unsigned ConstraintID,
                                     std::vector<SDValue> &OutOps) override;
+  bool IsProfitableToFold(SDValue N, SDNode *U, SDNode *Root) const override;
   void PreprocessISelDAG() override;
 
   // Include the pieces autogenerated from the target description.
@@ -1445,6 +1446,52 @@ SelectInlineAsmMemoryOperand(const SDValue &Op,
   return true;
 }
 
+// IsProfitableToFold - Returns true if is profitable to fold the specific
+// operand node N of U during instruction selection that starts at Root.
+bool
+SystemZDAGToDAGISel::IsProfitableToFold(SDValue N, SDNode *U,
+                                        SDNode *Root) const {
+  // We want to avoid folding a LOAD into an ICMP node if as a result
+  // we would be forced to spill the condition code into a GPR.
+  if (N.getOpcode() == ISD::LOAD && U->getOpcode() == SystemZISD::ICMP) {
+    if (!N.hasOneUse() || !U->hasOneUse())
+      return false;
+
+    // The user of the CC value will usually be a CopyToReg into the
+    // physical CC register, which in turn is glued and chained to the
+    // actual instruction that uses the CC value.  Bail out if we have
+    // anything else than that.
+    SDNode *CCUser = *U->use_begin();
+    SDNode *CCRegUser = nullptr;
+    if (CCUser->getOpcode() == ISD::CopyToReg ||
+        cast<RegisterSDNode>(CCUser->getOperand(1))->getReg() == SystemZ::CC) {
+      for (auto *U : CCUser->uses()) {
+        if (CCRegUser == nullptr)
+          CCRegUser = U;
+        else if (CCRegUser != U)
+          return false;
+      }
+    }
+    if (CCRegUser == nullptr)
+      return false;
+
+    // If the actual instruction is a branch, the only thing that remains to be
+    // checked is whether the CCUser chain is a predecessor of the load.
+    if (CCRegUser->isMachineOpcode() &&
+        CCRegUser->getMachineOpcode() == SystemZ::BRC)
+      return !N->isPredecessorOf(CCUser->getOperand(0).getNode());
+
+    // Otherwise, the instruction may have multiple operands, and we need to
+    // verify that none of them are a predecessor of the load.  This is exactly
+    // the same check that would be done by common code if the CC setter were
+    // glued to the CC user, so simply invoke that check here.
+    if (!IsLegalToFold(N, U, CCRegUser, OptLevel, false))
+      return false;
+  }
+
+  return true;
+}
+
 namespace {
 // Represents a sequence for extracting a 0/1 value from an IPM result:
 // (((X ^ XORValue) + AddValue) >> Bit)
@@ -1543,9 +1590,9 @@ SDValue SystemZDAGToDAGISel::expandSelectBoolean(SDNode *Node) {
   int CCMask = CCMaskOp->getZExtValue();
 
   SDLoc DL(Node);
-  SDValue Glue = Node->getOperand(4);
+  SDValue CCReg = Node->getOperand(4);
   IPMConversion IPM = getIPMConversion(CCValid, CCMask);
-  SDValue Result = CurDAG->getNode(SystemZISD::IPM, DL, MVT::i32, Glue);
+  SDValue Result = CurDAG->getNode(SystemZISD::IPM, DL, MVT::i32, CCReg);
 
   if (IPM.XORValue)
     Result = CurDAG->getNode(ISD::XOR, DL, MVT::i32, Result,