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

runtime/vm/intermediate_language_ia32.cc

Index: runtime/vm/intermediate_language_ia32.cc
===================================================================
--- runtime/vm/intermediate_language_ia32.cc	(revision 17939)
+++ runtime/vm/intermediate_language_ia32.cc	(working copy)
@@ -1951,12 +1951,19 @@
 LocationSummary* BinarySmiOpInstr::MakeLocationSummary() const {
   const intptr_t kNumInputs = 2;
   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(EAX));
-    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(EBX));
+    }
     summary->set_out(Location::SameAsFirstInput());
     // Will be used for sign extension and division.
     summary->set_temp(0, Location::RegisterLocation(EDX));
@@ -1996,8 +2003,7 @@
   ASSERT(left == result);
   Label* deopt = NULL;
   if (CanDeoptimize()) {
-      deopt  = compiler->AddDeoptStub(deopt_id(),
-                                      kDeoptBinarySmiOp);
+    deopt  = compiler->AddDeoptStub(deopt_id(), kDeoptBinarySmiOp);
   }
 
   if (locs()->in(1).IsConstant()) {
@@ -2022,6 +2028,34 @@
         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));
+        __ cmpl(left, Immediate(0));

[Florian Schneider, 2013/02/01 11:48:33]

For checking x < 0, this sequence is 1 byte shorter:

__ testl(left, left);
__ j(NEGATIVE, &use_div, Assembler::kNearJump);

+        __ j(LESS, &use_div, Assembler::kNearJump);
+        // Positive division by power of two is an arithmetic left shift.

[Florian Schneider, 2013/02/01 11:48:33]

In the comment s/left/right/.

[srdjan, 2013/02/01 22:57:54]

Done.

+        intptr_t shift_count = Utils::ShiftForPowerOfTwo(value) + kSmiTagSize;
+        __ sarl(left, Immediate(shift_count));
+        __ jmp(&done, Assembler::kNearJump);
+        __ Bind(&use_div);

[Florian Schneider, 2013/02/01 11:48:33]

I think this could be improved for negative numbers: How about compiling it into
something similar to what gcc generates for integer division by a power of two:

result = left >= 0 
           ? left >> log2(right)
           : (left + (right - 1)) >> log2(right);

It can be further generalized to allow negative powers of two.

[Florian Schneider, 2013/02/04 13:04:02]

Yes, my mistake: of course the absolute value of right must be used in this
formula.

+        Register right = locs()->temp(1).reg();
+        ASSERT(left == EAX);
+        ASSERT((right != EDX) && (right != EAX));
+        ASSERT(locs()->temp(0).reg() == EDX);
+        ASSERT(result == EAX);
+        __ movl(right, Immediate(value));
+        __ SmiUntag(left);
+        __ cdq();  // Sign extend EAX -> EDX:EAX.
+        __ idivl(right);  //  EAX: quotient, EDX: remainder.
+        // Check the corner case of dividing the 'MIN_SMI' with -1, in which

[Florian Schneider, 2013/02/01 11:48:33]

Since the right side is a known constant, you can avoid checking for that corner
case in all cases except when  generating (x ~/ -1).

+        // case we cannot tag the result.
+        __ cmpl(result, Immediate(0x40000000));
+        __ j(EQUAL, deopt);
+        __ Bind(&done);
+        __ SmiTag(result);
+        break;
+      }
       case Token::kBIT_AND: {
         // No overflow check.
         __ andl(left, Immediate(imm));