[SelectionDAG] Add partial BITCAST support to computeKnownBits

Adds support for bitcasting a little endian 'small element' vector to 'large element' scalar/vector (e.g. v16i8 to v4i32 or v2i32 to i64), which is required for PR30845. We extract the knownbits for each 'small element' part and concatenate the results together.

We can add support for big endian and 'large element' scalar/vector to 'small element' vector bitcasting once we have test cases for them.

Differential Revision: https://reviews.llvm.org/D27129

llvm-svn: 289200

1 files changed, 44 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index f276902d2e6..cbe5a2454ce 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2148,6 +2148,50 @@ void SelectionDAG::computeKnownBits(SDValue Op, APInt &KnownZero,
     }
     break;
   }
+  case ISD::BITCAST: {
+    SDValue N0 = Op.getOperand(0);
+    unsigned SubBitWidth = N0.getScalarValueSizeInBits();
+
+    // Ignore bitcasts from floating point.
+    if (!N0.getValueType().isInteger())
+      break;
+
+    // Fast handling of 'identity' bitcasts.
+    if (BitWidth == SubBitWidth) {
+      computeKnownBits(N0, KnownZero, KnownOne, DemandedElts, Depth + 1);
+      break;
+    }
+
+    // Support big-endian targets when it becomes useful.
+    bool IsLE = getDataLayout().isLittleEndian();
+    if (!IsLE)
+      break;
+
+    // Bitcast 'small element' vector to 'large element' scalar/vector.
+    if ((BitWidth % SubBitWidth) == 0) {
+      assert(N0.getValueType().isVector() && "Expected bitcast from vector");
+
+      // Collect known bits for the (larger) output by collecting the known
+      // bits from each set of sub elements and shift these into place.
+      // We need to separately call computeKnownBits for each set of
+      // sub elements as the knownbits for each is likely to be different.
+      unsigned SubScale = BitWidth / SubBitWidth;
+      APInt SubDemandedElts(NumElts * SubScale, 0);
+      for (unsigned i = 0; i != NumElts; ++i)
+        if (DemandedElts[i])
+          SubDemandedElts.setBit(i * SubScale);
+
+      for (unsigned i = 0; i != SubScale; ++i) {
+        computeKnownBits(N0, KnownZero2, KnownOne2, SubDemandedElts.shl(i),
+                         Depth + 1);
+        KnownOne |= KnownOne2.zext(BitWidth).shl(SubBitWidth * i);
+        KnownZero |= KnownZero2.zext(BitWidth).shl(SubBitWidth * i);
+      }
+    }
+
+    // TODO - support ((SubBitWidth % BitWidth) == 0) when it becomes useful.
+    break;
+  }
   case ISD::AND:
     // If either the LHS or the RHS are Zero, the result is zero.
     computeKnownBits(Op.getOperand(1), KnownZero, KnownOne, DemandedElts,