[Thumb] Select (CMPZ X, -C) -> (CMPZ (ADDS X, C), 0)

The CMPZ #0 disappears during peepholing, leaving just a tADDi3, tADDi8 or t2ADDri. This avoids having to materialize the expensive negative constant in Thumb-1, and allows a shrinking from a 32-bit CMN to a 16-bit ADDS in Thumb-2.

llvm-svn: 281040

1 files changed, 42 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMISelDAGToDAG.cpp b/llvm/lib/Target/ARM/ARMISelDAGToDAG.cpp
index 3d3a595..30586aa 100644
--- a/llvm/lib/Target/ARM/ARMISelDAGToDAG.cpp
+++ b/llvm/lib/Target/ARM/ARMISelDAGToDAG.cpp
@@ -3126,6 +3126,48 @@ void ARMDAGToDAGISel::Select(SDNode *N) {
     CurDAG->RemoveDeadNode(N);
     return;
   }
+
+  case ARMISD::CMPZ: {
+    // select (CMPZ X, #-C) -> (CMPZ (ADDS X, #C), #0)
+    //   This allows us to avoid materializing the expensive negative constant.
+    //   The CMPZ #0 is useless and will be peepholed away but we need to keep it
+    //   for its glue output.
+    SDValue X = N->getOperand(0);
+    auto *C = dyn_cast<ConstantSDNode>(N->getOperand(1).getNode());
+    if (C && C->getSExtValue() < 0 && Subtarget->isThumb()) {
+      int64_t Addend = -C->getSExtValue();
+
+      SDNode *Add = nullptr;
+      // In T2 mode, ADDS can be better than CMN if the immediate fits in a
+      // 16-bit ADDS, which means either [0,256) for tADDi8 or [0,8) for tADDi3.
+      // Outside that range we can just use a CMN which is 32-bit but has a
+      // 12-bit immediate range.
+      if (Subtarget->isThumb2() && Addend < 1<<8) {
+        SDValue Ops[] = { X, CurDAG->getTargetConstant(Addend, dl, MVT::i32),
+                          getAL(CurDAG, dl), CurDAG->getRegister(0, MVT::i32),
+                          CurDAG->getRegister(0, MVT::i32) };
+        Add = CurDAG->getMachineNode(ARM::t2ADDri, dl, MVT::i32, Ops);
+      } else if (!Subtarget->isThumb2() && Addend < 1<<8) {
+        // FIXME: Add T1 tADDi8 code.
+        SDValue Ops[] = {CurDAG->getRegister(ARM::CPSR, MVT::i32), X,
+                         CurDAG->getTargetConstant(Addend, dl, MVT::i32),
+                         getAL(CurDAG, dl), CurDAG->getRegister(0, MVT::i32)};
+        Add = CurDAG->getMachineNode(ARM::tADDi8, dl, MVT::i32, Ops);
+      } else if (!Subtarget->isThumb2() && Addend < 1<<3) {
+        SDValue Ops[] = {CurDAG->getRegister(ARM::CPSR, MVT::i32), X,
+                         CurDAG->getTargetConstant(Addend, dl, MVT::i32),
+                         getAL(CurDAG, dl), CurDAG->getRegister(0, MVT::i32)};
+        Add = CurDAG->getMachineNode(ARM::tADDi3, dl, MVT::i32, Ops);
+      }
+      if (Add) {
+        SDValue Ops2[] = {SDValue(Add, 0), CurDAG->getConstant(0, dl, MVT::i32)};
+        CurDAG->MorphNodeTo(N, ARMISD::CMPZ, CurDAG->getVTList(MVT::Glue), Ops2);
+      }
+    }
+    // Other cases are autogenerated.
+    break;
+  }
+    
   case ARMISD::VZIP: {
     unsigned Opc = 0;
     EVT VT = N->getValueType(0);