Optimize the multiplication of two 32-bit unsigned integers.

runtime/vm/intermediate_language_arm.cc

Index: runtime/vm/intermediate_language_arm.cc
===================================================================
--- runtime/vm/intermediate_language_arm.cc	(revision 38266)
+++ runtime/vm/intermediate_language_arm.cc	(working copy)
@@ -6130,9 +6130,26 @@
       }
       break;
     }
-    default:
-      UNREACHABLE();
-    break;
+    case Token::kMUL: {
+      // We only support the multiplication of two positive 32-bit integers
+      // resulting in a positive 64-bit integer fitting in a mint.
+      // We deopt in all other cases.
+      // This guarantees that the multiplication of 16-bit unsigned integers,
+      // as used in bignum arithmetic, will always succeed.
+      if (TargetCPUFeatures::arm_version() == ARMv7) {
+        __ orrs(out_hi, left_hi, Operand(right_hi));
+        __ b(deopt, NE);
+        __ smull(out_lo, out_hi, left_lo, right_lo);
+        if (can_overflow()) {
+          __ TestImmediate(out_hi, 0xC0000000);
+          __ b(deopt, NE);
+        }
+      } else {
+        __ b(deopt);
+      }
+      break;
+    }
+    default: UNREACHABLE();
   }
   if (FLAG_throw_on_javascript_int_overflow) {
     EmitJavascriptIntOverflowCheck(compiler, deopt, out_lo, out_hi);