[llvm] [SelectionDAG] Handle `fneg`/`fabs`/`fcopysign` in `SimplifyDemandedBits` (PR #139239)

[Jay Foad via llvm-commits]

================
@@ -2966,6 +2966,81 @@ bool TargetLowering::SimplifyDemandedBits(
     }
     break;
   }
+  case ISD::FABS: {
+    SDValue Op0 = Op.getOperand(0);
+    APInt SignMask = APInt::getSignMask(BitWidth);
+
+    if (!DemandedBits.intersects(SignMask))
+      return TLO.CombineTo(Op, Op0);
+
+    if (SimplifyDemandedBits(Op0, DemandedBits, DemandedElts, Known, TLO,
+                             Depth + 1))
+      return true;
+
+    if (Known.isNonNegative())
+      return TLO.CombineTo(Op, Op0);
+    if (Known.isNegative())
+      return TLO.CombineTo(
+          Op, TLO.DAG.getNode(ISD::FNEG, dl, VT, Op0, Op->getFlags()));
+
+    Known.Zero |= SignMask;
+    Known.One &= ~SignMask;
+
+    break;
+  }
+  case ISD::FCOPYSIGN: {
+    SDValue Op0 = Op.getOperand(0);
+    SDValue Op1 = Op.getOperand(1);
+
+    unsigned BitWidth0 = Op0.getScalarValueSizeInBits();
+    unsigned BitWidth1 = Op1.getScalarValueSizeInBits();
+    APInt SignMask0 = APInt::getSignMask(BitWidth0);
+    APInt SignMask1 = APInt::getSignMask(BitWidth1);
+
+    if (!DemandedBits.intersects(SignMask0))
+      return TLO.CombineTo(Op, Op0);
+
+    if (SimplifyDemandedBits(Op0, ~SignMask0 & DemandedBits, DemandedElts,
+                             Known, TLO, Depth + 1) ||
+        SimplifyDemandedBits(Op1, SignMask1, DemandedElts, Known2, TLO,
+                             Depth + 1))
+      return true;
+
+    if ((Known.isNonNegative() && Known2.isNonNegative()) ||
+        (Known.isNegative() && Known2.isNegative()))
+      return TLO.CombineTo(Op, Op0);
+
+    if (Known2.isNonNegative())
+      return TLO.CombineTo(
+          Op, TLO.DAG.getNode(ISD::FABS, dl, VT, Op0, Op->getFlags()));
+
+    if (Known2.isNegative())
+      return TLO.CombineTo(
+          Op, TLO.DAG.getNode(ISD::FNEG, dl, VT,
+                              TLO.DAG.getNode(ISD::FABS, SDLoc(Op0), VT, Op0)));
+
+    Known.Zero &= ~SignMask0;
+    Known.One &= ~SignMask0;
+    break;
+  }
+  case ISD::FNEG: {
+    SDValue Op0 = Op.getOperand(0);
+    APInt SignMask = APInt::getSignMask(BitWidth);
+
+    if (!DemandedBits.intersects(SignMask))
+      return TLO.CombineTo(Op, Op0);
+
+    if (SimplifyDemandedBits(Op0, DemandedBits, DemandedElts, Known, TLO,
+                             Depth + 1))
+      return true;
+
+    if (Known.isNonNegative() || Known.isNegative()) {
----------------
jayfoad wrote:

```suggestion
    if (!Known.isSignUnknown()) {
```

https://github.com/llvm/llvm-project/pull/139239