[SelectionDAG] Don't subject ISD:Constant to the depth limit in TargetLowering::SimplifyDemandedBits.

[craig.topper]

Summary

We shouldn't recurse any further but it doesn't mean we shouldn't be able to give the known bits for a constant. The caller would probably like that we always return the right answer for a constant RHS. This matches what InstCombine does in this case.

I don't have a test case because this showed up while trying to revive D31724.

[arsenm]

lib/CodeGen/SelectionDAG/TargetLowering.cpp

519

Should also apply to ConstantFP?

[craig.topper]

Doesn't look like SimplifyDemandedBits or computeKnownBits currently handling ConstantFP. We probably don't cross any fp->integer boundaries when recursing. I definitely see an early out in ISD::BITCAST handling if its a cast from FP.

[RKSimon]

In D38967#898866, @craig.topper wrote:

Doesn't look like SimplifyDemandedBits or computeKnownBits currently handling ConstantFP. We probably don't cross any fp->integer boundaries when recursing. I definitely see an early out in ISD::BITCAST handling if its a cast from FP.

I think it's mostly as we don't have a use case for handling known bits of floats, so we just early out.

[RKSimon]

LGTM - worth doing the same for SelectionDAG::ComputeNumSignBits and SelectionDAG::computeKnownBits ?

lib/CodeGen/SelectionDAG/TargetLowering.cpp

assert(Op.getScalarValueSizeInBits() == BitWidth &&
          "Mask size mismatches value type size!");
   APInt NewMask = DemandedMask;
   SDLoc dl(Op);
   auto &DL = TLO.DAG.getDataLayout();
 
   // Don't know anything.
   Known = KnownBits(BitWidth);
 
+  if (Op.getOpcode() == ISD::Constant) {
+    // We know all of the bits for a constant!
+    Known.One = cast<ConstantSDNode>(Op)->getAPIntValue();
+    Known.Zero = ~Known.One;
+    return false;
+  }
+
   // Other users may use these bits.
   if (!Op.getNode()->hasOneUse() && !AssumeSingleUse) {
     if (Depth != 0) {
       // If not at the root, Just compute the Known bits to
       // simplify things downstream.
       TLO.DAG.computeKnownBits(Op, Known, Depth);
       return false;
     }
     // If this is the root being simplified, allow it to have multiple uses,
     // just set the NewMask to all bits.
     NewMask = APInt::getAllOnesValue(BitWidth);
   } else if (DemandedMask == 0) {
     // Not demanding any bits from Op.
     if (!Op.isUndef())
       return TLO.CombineTo(Op, TLO.DAG.getUNDEF(Op.getValueType()));
     return false;
   } else if (Depth == 6) {        // Limit search depth.
     return false;
   }
 
   KnownBits Known2, KnownOut;
   switch (Op.getOpcode()) {
-  case ISD::Constant:
-    // We know all of the bits for a constant!
-    Known.One = cast<ConstantSDNode>(Op)->getAPIntValue();
-    Known.Zero = ~Known.One;
-    return false;   // Don't fall through, will infinitely loop.
   case ISD::BUILD_VECTOR:
     // Collect the known bits that are shared by every constant vector element.
     Known.Zero.setAllBits(); Known.One.setAllBits();
     for (SDValue SrcOp : Op->ops()) {
       if (!isa<ConstantSDNode>(SrcOp)) {
         // We can only handle all constant values - bail out with no known bits.
         Known = KnownBits(BitWidth);
         return false;