[SelectionDAG] Improve knownbits handling of UMIN/UMAX (PR31293)

This patch improves the knownbits logic for unsigned integer min/max opcodes.

For UMIN we know that the result will have the maximum of the inputs' known leading zero bits in the result, similarly for UMAX the maximum of the inputs' leading one bits.

This is particularly useful for simplifying clamping patterns,. e.g. as SSE doesn't have a uitofp instruction we want to use sitofp instead where possible and for that we need to confirm that the top bit is not set.

Differential Revision: https://reviews.llvm.org/D28853

llvm-svn: 292528

1 files changed, 33 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index be3eef9234b..d7b4b4291a1 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2687,12 +2687,42 @@ void SelectionDAG::computeKnownBits(SDValue Op, APInt &KnownZero,
     KnownOne = KnownOne2.byteSwap();
     break;
   }
-  case ISD::SMIN:
-  case ISD::SMAX:
-  case ISD::UMIN:
+  case ISD::UMIN: {
+    computeKnownBits(Op.getOperand(0), KnownZero, KnownOne, DemandedElts,
+                     Depth + 1);
+    computeKnownBits(Op.getOperand(1), KnownZero2, KnownOne2, DemandedElts,
+                     Depth + 1);
+
+    // UMIN - we know that the result will have the maximum of the
+    // known zero leading bits of the inputs.
+    unsigned LeadZero = KnownZero.countLeadingOnes();
+    LeadZero = std::max(LeadZero, KnownZero2.countLeadingOnes());
+
+    KnownZero &= KnownZero2;
+    KnownOne &= KnownOne2;
+    KnownZero |= APInt::getHighBitsSet(BitWidth, LeadZero);
+    break;
+  }
   case ISD::UMAX: {
     computeKnownBits(Op.getOperand(0), KnownZero, KnownOne, DemandedElts,
                      Depth + 1);
+    computeKnownBits(Op.getOperand(1), KnownZero2, KnownOne2, DemandedElts,
+                     Depth + 1);
+
+    // UMAX - we know that the result will have the maximum of the
+    // known one leading bits of the inputs.
+    unsigned LeadOne = KnownOne.countLeadingOnes();
+    LeadOne = std::max(LeadOne, KnownOne2.countLeadingOnes());
+
+    KnownZero &= KnownZero2;
+    KnownOne &= KnownOne2;
+    KnownOne |= APInt::getHighBitsSet(BitWidth, LeadOne);
+    break;
+  }
+  case ISD::SMIN:
+  case ISD::SMAX: {
+    computeKnownBits(Op.getOperand(0), KnownZero, KnownOne, DemandedElts,
+                     Depth + 1);
     // If we don't know any bits, early out.
     if (!KnownOne && !KnownZero)
       break;