DAG: computeNumSignBits for MUL

Copied directly from the IR version.

Most of the testcases I've added for this are somewhat problematic
because they really end up testing the yet to be implemented version
for MUL_I24/MUL_U24.

llvm-svn: 370099

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 6d6d65e55e6..fe3996bdb7d 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -3715,6 +3715,18 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
     Tmp = ComputeNumSignBits(Op.getOperand(0), Depth+1);
     if (Tmp == 1) return 1;  // Early out.
     return std::min(Tmp, Tmp2)-1;
+  case ISD::MUL: {
+    // The output of the Mul can be at most twice the valid bits in the inputs.
+    unsigned SignBitsOp0 = ComputeNumSignBits(Op.getOperand(0), Depth + 1);
+    if (SignBitsOp0 == 1)
+      break;
+    unsigned SignBitsOp1 = ComputeNumSignBits(Op.getOperand(1), Depth + 1);
+    if (SignBitsOp1 == 1)
+      break;
+    unsigned OutValidBits =
+        (VTBits - SignBitsOp0 + 1) + (VTBits - SignBitsOp1 + 1);
+    return OutValidBits > VTBits ? 1 : VTBits - OutValidBits + 1;
+  }
   case ISD::TRUNCATE: {
     // Check if the sign bits of source go down as far as the truncated value.
     unsigned NumSrcBits = Op.getOperand(0).getScalarValueSizeInBits();