[X86] Add INSERT_SUBVECTOR to ComputeNumSignBits

This adds support for calculating sign bits of insert_subvector. I based it on the computeKnownBits.

My motivating case is propagating sign bits information across basic blocks on AVX targets where concatenating using insert_subvector is common.

Differential Revision: https://reviews.llvm.org/D56283

llvm-svn: 350432

1 files changed, 35 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 519f4d82f6a..3cff4c5c0fc 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -3693,7 +3693,7 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
     }
     return ComputeNumSignBits(Src, Depth + 1);
   }
-  case ISD::CONCAT_VECTORS:
+  case ISD::CONCAT_VECTORS: {
     // Determine the minimum number of sign bits across all demanded
     // elts of the input vectors. Early out if the result is already 1.
     Tmp = std::numeric_limits<unsigned>::max();
@@ -3711,6 +3711,40 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
     assert(Tmp <= VTBits && "Failed to determine minimum sign bits");
     return Tmp;
   }
+  case ISD::INSERT_SUBVECTOR: {
+    // If we know the element index, demand any elements from the subvector and
+    // the remainder from the src its inserted into, otherwise demand them all.
+    SDValue Src = Op.getOperand(0);
+    SDValue Sub = Op.getOperand(1);
+    auto *SubIdx = dyn_cast<ConstantSDNode>(Op.getOperand(2));
+    unsigned NumSubElts = Sub.getValueType().getVectorNumElements();
+    if (SubIdx && SubIdx->getAPIntValue().ule(NumElts - NumSubElts)) {
+      Tmp = std::numeric_limits<unsigned>::max();
+      uint64_t Idx = SubIdx->getZExtValue();
+      APInt DemandedSubElts = DemandedElts.extractBits(NumSubElts, Idx);
+      if (!!DemandedSubElts) {
+        Tmp = ComputeNumSignBits(Sub, DemandedSubElts, Depth + 1);
+        if (Tmp == 1) return 1; // early-out
+      }
+      APInt SubMask = APInt::getBitsSet(NumElts, Idx, Idx + NumSubElts);
+      APInt DemandedSrcElts = DemandedElts & ~SubMask;
+      if (!!DemandedSrcElts) {
+        Tmp2 = ComputeNumSignBits(Src, DemandedSrcElts, Depth + 1);
+        Tmp = std::min(Tmp, Tmp2);
+      }
+      assert(Tmp <= VTBits && "Failed to determine minimum sign bits");
+      return Tmp;
+    }
+
+    // Not able to determine the index so just assume worst case.
+    Tmp = ComputeNumSignBits(Sub, Depth + 1);
+    if (Tmp == 1) return 1; // early-out
+    Tmp2 = ComputeNumSignBits(Src, Depth + 1);
+    Tmp = std::min(Tmp, Tmp2);
+    assert(Tmp <= VTBits && "Failed to determine minimum sign bits");
+    return Tmp;
+  }
+  }
 
   // If we are looking at the loaded value of the SDNode.
   if (Op.getResNo() == 0) {