[AArch64] fold 'isPositive' vector integer operations (PR26819)

This is one of the cases shown in:
https://llvm.org/bugs/show_bug.cgi?id=26819

Shift and negate is what InstCombine prefers to produce (and I tried to make it do more of that
in http://reviews.llvm.org/rL262424 ), so we should recognize that pattern as something that might
come from autovectorization even if it's unlikely to be produced from C NEON intrinsics.

The patch is based on the x86 equivalent:
http://reviews.llvm.org/rL262036

Differential Revision: http://reviews.llvm.org/D17834

llvm-svn: 262623

1 files changed, 30 insertions, 1 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
index faff086cc05..a7adb12093e 100644
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -7423,6 +7423,33 @@ bool AArch64TargetLowering::shouldConvertConstantLoadToIntImm(const APInt &Imm,
   return Shift < 3;
 }
 
+/// Turn vector tests of the signbit in the form of:
+///   xor (sra X, elt_size(X)-1), -1
+/// into:
+///   cmge X, X, #0
+static SDValue foldVectorXorShiftIntoCmp(SDNode *N, SelectionDAG &DAG,
+                                         const AArch64Subtarget *Subtarget) {
+  EVT VT = N->getValueType(0);
+  if (!Subtarget->hasNEON() || !VT.isVector())
+    return SDValue();
+
+  // There must be a shift right algebraic before the xor, and the xor must be a
+  // 'not' operation.
+  SDValue Shift = N->getOperand(0);
+  SDValue Ones = N->getOperand(1);
+  if (Shift.getOpcode() != AArch64ISD::VASHR || !Shift.hasOneUse() ||
+      !ISD::isBuildVectorAllOnes(Ones.getNode()))
+    return SDValue();
+
+  // The shift should be smearing the sign bit across each vector element.
+  auto *ShiftAmt = dyn_cast<ConstantSDNode>(Shift.getOperand(1));
+  EVT ShiftEltTy = Shift.getValueType().getVectorElementType();
+  if (!ShiftAmt || ShiftAmt->getZExtValue() != ShiftEltTy.getSizeInBits() - 1)
+    return SDValue();
+
+  return DAG.getNode(AArch64ISD::CMGEz, SDLoc(N), VT, Shift.getOperand(0));
+}
+
 // Generate SUBS and CSEL for integer abs.
 static SDValue performIntegerAbsCombine(SDNode *N, SelectionDAG &DAG) {
   EVT VT = N->getValueType(0);
@@ -7451,13 +7478,15 @@ static SDValue performIntegerAbsCombine(SDNode *N, SelectionDAG &DAG) {
   return SDValue();
 }
 
-// performXorCombine - Attempts to handle integer ABS.
 static SDValue performXorCombine(SDNode *N, SelectionDAG &DAG,
                                  TargetLowering::DAGCombinerInfo &DCI,
                                  const AArch64Subtarget *Subtarget) {
   if (DCI.isBeforeLegalizeOps())
     return SDValue();
 
+  if (SDValue Cmp = foldVectorXorShiftIntoCmp(N, DAG, Subtarget))
+    return Cmp;
+
   return performIntegerAbsCombine(N, DAG);
 }