transform obscured FP sign bit ops into a fabs/fneg using TLI hook

This is effectively a revert of:
http://reviews.llvm.org/rL249702 - [InstCombine] transform masking off of an FP sign bit into a fabs() intrinsic call (PR24886)
and:
http://reviews.llvm.org/rL249701 - [ValueTracking] teach computeKnownBits that a fabs() clears sign bits
and a reimplementation as a DAG combine for targets that have IEEE754-compliant fabs/fneg instructions.

This is intended to resolve the objections raised on the dev list:
http://lists.llvm.org/pipermail/llvm-dev/2016-April/098154.html
and:
https://llvm.org/bugs/show_bug.cgi?id=24886#c4

In the interest of patch minimalism, I've only partly enabled AArch64. PowerPC, MIPS, x86 and others can enable later.

Differential Revision: http://reviews.llvm.org/D19391

llvm-svn: 271573

1 files changed, 46 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index d3378483499..e7ee596beec 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -7351,6 +7351,49 @@ static unsigned getPPCf128HiElementSelector(const SelectionDAG &DAG) {
   return DAG.getDataLayout().isBigEndian() ? 1 : 0;
 }
 
+static SDValue foldBitcastedFPLogic(SDNode *N, SelectionDAG &DAG,
+                                    const TargetLowering &TLI) {
+  // If this is not a bitcast to an FP type or if the target doesn't have
+  // IEEE754-compliant FP logic, we're done.
+  EVT VT = N->getValueType(0);
+  if (!VT.isFloatingPoint() || !TLI.hasBitPreservingFPLogic(VT))
+    return SDValue();
+
+  // TODO: Use splat values for the constant-checking below and remove this
+  // restriction.
+  SDValue N0 = N->getOperand(0);
+  EVT SourceVT = N0.getValueType();
+  if (SourceVT.isVector())
+    return SDValue();
+
+  unsigned FPOpcode;
+  APInt SignMask;
+  switch (N0.getOpcode()) {
+  case ISD::AND:
+    FPOpcode = ISD::FABS;
+    SignMask = ~APInt::getSignBit(SourceVT.getSizeInBits());
+    break;
+  case ISD::XOR:
+    FPOpcode = ISD::FNEG;
+    SignMask = APInt::getSignBit(SourceVT.getSizeInBits());
+    break;
+  // TODO: ISD::OR --> ISD::FNABS?
+  default:
+    return SDValue();
+  }
+
+  // Fold (bitcast int (and (bitcast fp X to int), 0x7fff...) to fp) -> fabs X
+  // Fold (bitcast int (xor (bitcast fp X to int), 0x8000...) to fp) -> fneg X
+  SDValue LogicOp0 = N0.getOperand(0);
+  ConstantSDNode *LogicOp1 = dyn_cast<ConstantSDNode>(N0.getOperand(1));
+  if (LogicOp1 && LogicOp1->getAPIntValue() == SignMask &&
+      LogicOp0.getOpcode() == ISD::BITCAST &&
+      LogicOp0->getOperand(0).getValueType() == VT)
+    return DAG.getNode(FPOpcode, SDLoc(N), VT, LogicOp0->getOperand(0));
+
+  return SDValue();
+}
+
 SDValue DAGCombiner::visitBITCAST(SDNode *N) {
   SDValue N0 = N->getOperand(0);
   EVT VT = N->getValueType(0);
@@ -7415,6 +7458,9 @@ SDValue DAGCombiner::visitBITCAST(SDNode *N) {
     }
   }
 
+  if (SDValue V = foldBitcastedFPLogic(N, DAG, TLI))
+    return V;
+
   // fold (bitconvert (fneg x)) -> (xor (bitconvert x), signbit)
   // fold (bitconvert (fabs x)) -> (and (bitconvert x), (not signbit))
   //