Know some simple identities. This improves codegen for (1LL << N).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@19698 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 3da2f5f..3b106d1 100644 (file)
--- a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -615,6 +615,19 @@ SDOperand SelectionDAG::getNode(unsigned Opcode, MVT::ValueType VT,
         std::swap(N1, N2);
       }
     }
+
+    switch (Opcode) {
+    default: break;
+    case ISD::SHL:    // shl  0, X -> 0
+      if (N1C->isNullValue()) return N1;
+      break;
+    case ISD::SRL:    // srl  0, X -> 0
+      if (N1C->isNullValue()) return N1;
+      break;
+    case ISD::SRA:    // sra -1, X -> -1
+      if (N1C->isAllOnesValue()) return N1;
+      break;
+    }
   }
 
   if (N2C) {