AMDGPU: Pull fneg/fabs out of a select

Allows better source modifier usage.

llvm-svn: 291729

1 files changed, 74 insertions, 0 deletions

diff --git a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
index a700aa85929..894056ea4d0 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
@@ -2686,8 +2686,82 @@ SDValue AMDGPUTargetLowering::performCtlzCombine(const SDLoc &SL, SDValue Cond,
   return SDValue();
 }
 
+static SDValue distributeOpThroughSelect(TargetLowering::DAGCombinerInfo &DCI,
+                                         unsigned Op,
+                                         const SDLoc &SL,
+                                         SDValue Cond,
+                                         SDValue N1,
+                                         SDValue N2) {
+  SelectionDAG &DAG = DCI.DAG;
+  EVT VT = N1.getValueType();
+
+  SDValue NewSelect = DAG.getNode(ISD::SELECT, SL, VT, Cond,
+                                  N1.getOperand(0), N2.getOperand(0));
+  DCI.AddToWorklist(NewSelect.getNode());
+  return DAG.getNode(Op, SL, VT, NewSelect);
+}
+
+// Pull a free FP operation out of a select so it may fold into uses.
+//
+// select c, (fneg x), (fneg y) -> fneg (select c, x, y)
+// select c, (fneg x), k -> fneg (select c, x, (fneg k))
+//
+// select c, (fabs x), (fabs y) -> fabs (select c, x, y)
+// select c, (fabs x), +k -> fabs (select c, x, k)
+static SDValue foldFreeOpFromSelect(TargetLowering::DAGCombinerInfo &DCI,
+                                    SDValue N) {
+  SelectionDAG &DAG = DCI.DAG;
+  SDValue Cond = N.getOperand(0);
+  SDValue LHS = N.getOperand(1);
+  SDValue RHS = N.getOperand(2);
+
+  EVT VT = N.getValueType();
+  if ((LHS.getOpcode() == ISD::FABS && RHS.getOpcode() == ISD::FABS) ||
+      (LHS.getOpcode() == ISD::FNEG && RHS.getOpcode() == ISD::FNEG)) {
+    return distributeOpThroughSelect(DCI, LHS.getOpcode(),
+                                     SDLoc(N), Cond, LHS, RHS);
+  }
+
+  bool Inv = false;
+  if (RHS.getOpcode() == ISD::FABS || RHS.getOpcode() == ISD::FNEG) {
+    std::swap(LHS, RHS);
+    Inv = true;
+  }
+
+  // TODO: Support vector constants.
+  ConstantFPSDNode *CRHS = dyn_cast<ConstantFPSDNode>(RHS);
+  if ((LHS.getOpcode() == ISD::FNEG || LHS.getOpcode() == ISD::FABS) && CRHS) {
+    SDLoc SL(N);
+    // If one side is an fneg/fabs and the other is a constant, we can push the
+    // fneg/fabs down. If it's an fabs, the constant needs to be non-negative.
+    SDValue NewLHS = LHS.getOperand(0);
+    SDValue NewRHS = RHS;
+
+    // TODO: Skip for operations where other combines can absord the fneg.
+
+    if (LHS.getOpcode() == ISD::FNEG)
+      NewRHS = DAG.getNode(ISD::FNEG, SL, VT, RHS);
+    else if (CRHS->isNegative())
+      return SDValue();
+
+    if (Inv)
+      std::swap(NewLHS, NewRHS);
+
+    SDValue NewSelect = DAG.getNode(ISD::SELECT, SL, VT,
+                                    Cond, NewLHS, NewRHS);
+    DCI.AddToWorklist(NewSelect.getNode());
+    return DAG.getNode(LHS.getOpcode(), SL, VT, NewSelect);
+  }
+
+  return SDValue();
+}
+
+
 SDValue AMDGPUTargetLowering::performSelectCombine(SDNode *N,
                                                    DAGCombinerInfo &DCI) const {
+  if (SDValue Folded = foldFreeOpFromSelect(DCI, SDValue(N, 0)))
+    return Folded;
+
   SDValue Cond = N->getOperand(0);
   if (Cond.getOpcode() != ISD::SETCC)
     return SDValue();