Use SAR for positive divident by a power-of two constant divisor.

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
===================================================================
--- runtime/vm/intermediate_language_x64.cc	(revision 17939)
+++ runtime/vm/intermediate_language_x64.cc	(working copy)
@@ -1786,6 +1786,7 @@
     Immediate(reinterpret_cast<int64_t>(constant.raw())).is_int32();
 }
 
+
 LocationSummary* BinarySmiOpInstr::MakeLocationSummary() const {
   const intptr_t kNumInputs = 2;
 
@@ -1805,12 +1806,19 @@
   }
 
   if (op_kind() == Token::kTRUNCDIV) {
-    const intptr_t kNumTemps = 1;
+    const intptr_t kNumTemps = RightIsPowerOfTwoConstant() ? 2 : 1;
     LocationSummary* summary =
         new LocationSummary(kNumInputs, kNumTemps, LocationSummary::kNoCall);
     // Both inputs must be writable because they will be untagged.
     summary->set_in(0, Location::RegisterLocation(RAX));
-    summary->set_in(1, Location::WritableRegister());
+    if (kNumTemps == 1) {
+      summary->set_in(1, Location::WritableRegister());
+    } else {
+      ConstantInstr* right_constant = right()->definition()->AsConstant();
+      summary->set_in(1, Location::Constant(right_constant->value()));
+      // Temporary to hold divisor constant.
+      summary->set_temp(1, Location::RegisterLocation(RBX));
+    }
     summary->set_out(Location::SameAsFirstInput());
     // Will be used for sign extension and division.
     summary->set_temp(0, Location::RegisterLocation(RDX));
@@ -1877,6 +1885,34 @@
         if (deopt != NULL) __ j(OVERFLOW, deopt);
         break;
       }
+      case Token::kTRUNCDIV: {
+        Label use_div, done;
+        const intptr_t value = Smi::Cast(constant).Value();
+        ASSERT((value > 0) && Utils::IsPowerOfTwo(value));
+        __ cmpq(left, Immediate(0));
+        __ j(LESS, &use_div, Assembler::kNearJump);
+        // Positive division by power of two is an arithmetic left shift.
+        intptr_t shift_count = Utils::ShiftForPowerOfTwo(value) + kSmiTagSize;
+        __ sarq(left, Immediate(shift_count));
+        __ jmp(&done, Assembler::kNearJump);
+        __ Bind(&use_div);
+        Register right = locs()->temp(1).reg();
+        ASSERT(left == RAX);
+        ASSERT((right != RDX) && (right != RAX));
+        ASSERT(locs()->temp(0).reg() == RDX);
+        ASSERT(result == RAX);
+        __ movq(right, Immediate(value));
+        __ SmiUntag(left);
+        __ cqo();  // Sign extend RAX -> RDX:RAX.
+        __ idivq(right);  //  RAX: quotient, RDX: remainder.
+        // Check the corner case of dividing the 'MIN_SMI' with -1, in which
+        // case we cannot tag the result.
+        __ cmpq(result, Immediate(0x4000000000000000));
+        __ j(EQUAL, deopt);
+        __ Bind(&done);
+        __ SmiTag(result);
+        break;
+      }
       case Token::kBIT_AND: {
         // No overflow check.
         __ andq(left, Immediate(imm));