Recognize pattern (a << b) & c with c being a positive Smi and allow left shift to truncate the res…

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
===================================================================
--- runtime/vm/intermediate_language.cc	(revision 18843)
+++ runtime/vm/intermediate_language.cc	(working copy)
@@ -66,7 +66,6 @@
 }
 
 
-
 CheckClassInstr::CheckClassInstr(Value* value,
                                  intptr_t deopt_id,
                                  const ICData& unary_checks)
@@ -138,7 +137,8 @@
   BinarySmiOpInstr* other_op = other->AsBinarySmiOp();
   ASSERT(other_op != NULL);
   return (op_kind() == other_op->op_kind()) &&
-      (overflow_ == other_op->overflow_);
+      (overflow_ == other_op->overflow_) &&
+      (is_truncating_ == other_op->is_truncating_);
 }
 
 
@@ -882,6 +882,14 @@
       return (right_range == NULL)
           || !right_range->IsWithin(0, RangeBoundary::kPlusInfinity);
     }
+    case Token::kSHL: {
+      Range* right_range = this->right()->definition()->range();
+      if ((right_range != NULL) && is_truncating()) {
+        // Can deoptimize if right can be negative.
+        return !right_range->IsWithin(0, RangeBoundary::kPlusInfinity);
+      }
+      return true;
+    }
     default:
       return overflow_;
   }