[X86] Improve the selection of SSE3/AVX addsub instructions.

This patch teaches the backend how to canonicalize a shuffle vectors
according to the rule:

 - (shuffle (FADD A, B), (FSUB A, B), Mask) ->
       (shuffle (FSUB A, -B), (FADD A, -B), Mask)

Where 'Mask' is:
  <0,5,2,7>            ;; for v4f32 and v4f64 shuffles.
  <0,3>                ;; for v2f64 shuffles.
  <0,9,2,11,4,13,6,15> ;; for v8f32 shuffles.

In general, ISel only knows how to pattern-match a canonical
'fadd + fsub + blendi' dag node sequence into an ADDSUB instruction.

This new rule allows to convert a non-canonical dag sequence into a
canonical one that will be matched by a single ADDSUB at ISel stage.

The idea of converting a non-canonical ADDSUB into a canonical one by
swapping the first two operands of the shuffle, and then negating the
second operand of the FADD and FSUB, was originally proposed by Hal Finkel.

llvm-svn: 211771

1 files changed, 43 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index cde413f4551..ebacc425df0 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -18014,6 +18014,49 @@ static SDValue PerformShuffleCombine(SDNode *N, SelectionDAG &DAG,
   SDValue N1 = N->getOperand(1);
   EVT VT = N->getValueType(0);
 
+  // Canonicalize shuffles that perform 'addsub' on packed float vectors
+  // according to the rule:
+  //  (shuffle (FADD A, B), (FSUB A, B), Mask) ->
+  //  (shuffle (FSUB A, -B), (FADD A, -B), Mask)
+  //
+  // Where 'Mask' is:
+  //  <0,5,2,7>             -- for v4f32 and v4f64 shuffles;
+  //  <0,3>                 -- for v2f64 shuffles;
+  //  <0,9,2,11,4,13,6,15>  -- for v8f32 shuffles.
+  //
+  // This helps pattern-matching more SSE3/AVX ADDSUB instructions
+  // during ISel stage.
+  if (N->getOpcode() == ISD::VECTOR_SHUFFLE &&
+      ((Subtarget->hasSSE3() && (VT == MVT::v4f32 || VT == MVT::v2f64)) ||
+       (Subtarget->hasAVX() && (VT == MVT::v8f32 || VT == MVT::v4f64))) &&
+      N0->getOpcode() == ISD::FADD && N1->getOpcode() == ISD::FSUB &&
+      // Operands to the FADD and FSUB must be the same.
+      ((N0->getOperand(0) == N1->getOperand(0) &&
+        N0->getOperand(1) == N1->getOperand(1)) ||
+       // FADD is commutable. See if by commuting the operands of the FADD
+       // we would still be able to match the operands of the FSUB dag node.
+       (N0->getOperand(1) == N1->getOperand(0) &&
+        N0->getOperand(0) == N1->getOperand(1))) &&
+      N0->getOperand(0)->getOpcode() != ISD::UNDEF &&
+      N0->getOperand(1)->getOpcode() != ISD::UNDEF) {
+    
+    ShuffleVectorSDNode *SV = cast<ShuffleVectorSDNode>(N);
+    unsigned NumElts = VT.getVectorNumElements();
+    ArrayRef<int> Mask = SV->getMask();
+    bool CanFold = true;
+
+    for (unsigned i = 0, e = NumElts; i != e && CanFold; ++i)
+      CanFold = Mask[i] == (i & 1) ? i + NumElts : i;
+
+    if (CanFold) {
+      SDValue Op0 = N1->getOperand(0);
+      SDValue Op1 = DAG.getNode(ISD::FNEG, dl, VT, N1->getOperand(1));
+      SDValue Sub = DAG.getNode(ISD::FSUB, dl, VT, Op0, Op1);
+      SDValue Add = DAG.getNode(ISD::FADD, dl, VT, Op0, Op1);
+      return DAG.getVectorShuffle(VT, dl, Sub, Add, Mask);
+    }
+  }
+
   // Don't create instructions with illegal types after legalize types has run.
   const TargetLowering &TLI = DAG.getTargetLoweringInfo();
   if (!DCI.isBeforeLegalize() && !TLI.isTypeLegal(VT.getVectorElementType()))