Provide tagged allocation top pointer

src/x64/macro-assembler-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/base/bits.h"
 #include "src/base/division-by-constant.h"
 #include "src/bootstrapper.h"
 #include "src/codegen.h"
@@ skipping 4798 matching lines @@
   }
 }
 
 
 void MacroAssembler::MakeSureDoubleAlignedHelper(Register result,
                                                  Register scratch,
                                                  Label* gc_required,
                                                  AllocationFlags flags) {
   if (kPointerSize == kDoubleSize) {
     if (FLAG_debug_code) {
-      testl(result, Immediate(kDoubleAlignmentMask));
+      testl(result, Immediate(kDoubleAlignmentMask - kHeapObjectTag));

[Michael Lippautz, 2016/04/28 16:58:08]

Don't really like this one (and below) but also don't know a better way.

[Hannes Payer (out of office), 2016/04/29 14:13:31]

I think that is fine. Maybe have a constant for that case, e.g.
kDoubleAlignmentMaskTagged or something like that.

       Check(zero, kAllocationIsNotDoubleAligned);
     }
   } else {
     // Align the next allocation. Storing the filler map without checking top
     // is safe in new-space because the limit of the heap is aligned there.
     DCHECK(kPointerSize * 2 == kDoubleSize);
     DCHECK(kPointerAlignment * 2 == kDoubleAlignment);
     // Make sure scratch is not clobbered by this function as it might be
     // used in UpdateAllocationTopHelper later.
     DCHECK(!scratch.is(kScratchRegister));
     Label aligned;
-    testl(result, Immediate(kDoubleAlignmentMask));
+    testl(result, Immediate(kDoubleAlignmentMask - kHeapObjectTag));
     j(zero, &aligned, Label::kNear);
     if ((flags & PRETENURE) != 0) {
       ExternalReference allocation_limit =
           AllocationUtils::GetAllocationLimitReference(isolate(), flags);
       cmpp(result, ExternalOperand(allocation_limit));
       j(above_equal, gc_required);
     }
     LoadRoot(kScratchRegister, Heap::kOnePointerFillerMapRootIndex);
     movp(Operand(result, 0), kScratchRegister);
     addp(result, Immediate(kDoubleSize / 2));
     bind(&aligned);
   }
 }
 
 
 void MacroAssembler::UpdateAllocationTopHelper(Register result_end,
                                                Register scratch,
                                                AllocationFlags flags) {
   if (emit_debug_code()) {
     testp(result_end, Immediate(kObjectAlignmentMask));
-    Check(zero, kUnalignedAllocationInNewSpace);
+    Check(not_zero, kUnalignedAllocationInNewSpace);
   }
 
   ExternalReference allocation_top =
       AllocationUtils::GetAllocationTopReference(isolate(), flags);
 
   // Update new top.
   if (scratch.is_valid()) {
     // Scratch already contains address of allocation top.
     movp(Operand(scratch, 0), result_end);
   } else {
@@ skipping 43 matching lines @@
     movp(top_reg, result);
   }
   addp(top_reg, Immediate(object_size));
   Operand limit_operand = ExternalOperand(allocation_limit);
   cmpp(top_reg, limit_operand);
   j(above, gc_required);
 
   // Update allocation top.
   UpdateAllocationTopHelper(top_reg, scratch, flags);
 
+  // Untag result_end.
+  STATIC_ASSERT(kHeapObjectTag == 1);
+  decp(result_end);

[Benedikt Meurer, 2016/05/01 17:30:03]

Hm, who needs this result_end anyways? Shouldn't that also be returned as tagged
value?
To me it seems highly confusing that the result is tagged, but the result_end is
not (I suppose that was always the case, but since we're touching this now, it
might be worth fixing/removing).

+
   if (top_reg.is(result)) {
-    subp(result, Immediate(object_size - kHeapObjectTag));
-  } else {
-    // Tag the result.
-    DCHECK(kHeapObjectTag == 1);
-    incp(result);
+    subp(result, Immediate(object_size));
   }
 }
 
 
 void MacroAssembler::Allocate(int header_size,
                               ScaleFactor element_size,
                               Register element_count,
                               Register result,
                               Register result_end,
                               Register scratch,
@@ skipping 42 matching lines @@
   }
   addp(result_end, result);
   j(carry, gc_required);
   Operand limit_operand = ExternalOperand(allocation_limit);
   cmpp(result_end, limit_operand);
   j(above, gc_required);
 
   // Update allocation top.
   UpdateAllocationTopHelper(result_end, scratch, flags);
 
-  // Tag the result.
-  addp(result, Immediate(kHeapObjectTag));
+  // Untag result_end.
+  STATIC_ASSERT(kHeapObjectTag == 1);
+  decp(result_end);
 }
 
 
 void MacroAssembler::AllocateHeapNumber(Register result,
                                         Register scratch,
                                         Label* gc_required,
                                         MutableMode mode) {
   // Allocate heap number in new space.
   Allocate(HeapNumber::kSize, result, scratch, no_reg, gc_required,
            NO_ALLOCATION_FLAGS);
@@ skipping 686 matching lines @@
   movl(rax, dividend);
   shrl(rax, Immediate(31));
   addl(rdx, rax);
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64