[SelectionDAG] Add ability for computeKnownBits to peek through bitcasts from 'large element' scalar/vector to 'small element' vector.

Extension to D27129 which already supported bitcasts from 'small element' vector to 'large element' scalar/vector types.

llvm-svn: 289329

1 files changed, 23 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index d80048c1396..45c9c59fe21 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2186,7 +2186,29 @@ void SelectionDAG::computeKnownBits(SDValue Op, APInt &KnownZero,
       }
     }
 
-    // TODO - support ((SubBitWidth % BitWidth) == 0) when it becomes useful.
+    // Bitcast 'large element' scalar/vector to 'small element' vector.
+    if ((SubBitWidth % BitWidth) == 0) {
+      assert(Op.getValueType().isVector() && "Expected bitcast to vector");
+
+      // Collect known bits for the (smaller) output by collecting the known
+      // bits from the overlapping larger input elements and extracting the
+      // sub sections we actually care about.
+      unsigned SubScale = SubBitWidth / BitWidth;
+      APInt SubDemandedElts(NumElts / SubScale, 0);
+      for (unsigned i = 0; i != NumElts; ++i)
+        if (DemandedElts[i])
+          SubDemandedElts.setBit(i / SubScale);
+
+      computeKnownBits(N0, KnownZero2, KnownOne2, SubDemandedElts, Depth + 1);
+
+      KnownZero = KnownOne = APInt::getAllOnesValue(BitWidth);
+      for (unsigned i = 0; i != NumElts; ++i)
+        if (DemandedElts[i]) {
+          unsigned Offset = (i % SubScale) * BitWidth;
+          KnownOne &= KnownOne2.lshr(Offset).trunc(BitWidth);
+          KnownZero &= KnownZero2.lshr(Offset).trunc(BitWidth);
+        }
+    }
     break;
   }
   case ISD::AND: