[DAG] Teach computeKnownBits and ComputeNumSignBits in SelectionDAG to look through EXTRACT_VECTOR_ELT.

Summary: Both computeKnownBits and ComputeNumSignBits can now do a simple
look-through of EXTRACT_VECTOR_ELT. It will compute the result based
on the known bits (or known sign bits) for the vector that the element
is extracted from.

Reviewers: bogner, tstellarAMD, mkuper

Subscribers: wdng, RKSimon, jyknight, llvm-commits, nhaehnle

Differential Revision: https://reviews.llvm.org/D25007

llvm-svn: 283347

1 files changed, 34 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index e9680ff521d..6ed8999766b 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2448,6 +2448,26 @@ void SelectionDAG::computeKnownBits(SDValue Op, APInt &KnownZero,
     KnownOne = KnownOne.trunc(BitWidth);
     break;
   }
+  case ISD::EXTRACT_VECTOR_ELT: {
+    // At the moment we keep this simple and skip tracking the specific
+    // element. This way we get the lowest common denominator for all elements
+    // of the vector.
+    // TODO: get information for given vector element
+    const unsigned BitWidth = Op.getValueSizeInBits();
+    const unsigned EltBitWidth = Op.getOperand(0).getScalarValueSizeInBits();
+    // If BitWidth > EltBitWidth the value is anyext:ed. So we do not know
+    // anything about the extended bits.
+    if (BitWidth > EltBitWidth) {
+      KnownZero = KnownZero.trunc(EltBitWidth);
+      KnownOne = KnownOne.trunc(EltBitWidth);
+    }
+    computeKnownBits(Op.getOperand(0), KnownZero, KnownOne, Depth+1);
+    if (BitWidth > EltBitWidth) {
+      KnownZero = KnownZero.zext(BitWidth);
+      KnownOne = KnownOne.zext(BitWidth);
+    }
+    break;
+  }
   case ISD::BSWAP: {
     computeKnownBits(Op.getOperand(0), KnownZero2, KnownOne2, Depth+1);
     KnownZero = KnownZero2.byteSwap();
@@ -2715,6 +2735,20 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, unsigned Depth) const {
     // result. Otherwise it gives either negative or > bitwidth result
     return std::max(std::min(KnownSign - rIndex * BitWidth, BitWidth), 0);
   }
+  case ISD::EXTRACT_VECTOR_ELT: {
+    // At the moment we keep this simple and skip tracking the specific
+    // element. This way we get the lowest common denominator for all elements
+    // of the vector.
+    // TODO: get information for given vector element
+    const unsigned BitWidth = Op.getValueSizeInBits();
+    const unsigned EltBitWidth = Op.getOperand(0).getScalarValueSizeInBits();
+    // If BitWidth > EltBitWidth the value is anyext:ed, and we do not know
+    // anything about sign bits. But if the sizes match we can derive knowledge
+    // about sign bits from the vector operand.
+    if (BitWidth == EltBitWidth)
+      return ComputeNumSignBits(Op.getOperand(0), Depth+1);
+    break;
+  }
   }
 
   // If we are looking at the loaded value of the SDNode.