Provide tagged allocation top pointer

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index f753b0e1d53319136243b44d2cd5a25ec5db5890..a82b5792972d35668a61b2ece70ca252902b229b 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -4816,7 +4816,7 @@ void MacroAssembler::MakeSureDoubleAlignedHelper(Register result,
                                                  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 {
@@ -4828,7 +4828,7 @@ void MacroAssembler::MakeSureDoubleAlignedHelper(Register result,
     // 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 =
@@ -4849,7 +4849,7 @@ void MacroAssembler::UpdateAllocationTopHelper(Register result_end,
                                                AllocationFlags flags) {
   if (emit_debug_code()) {
     testp(result_end, Immediate(kObjectAlignmentMask));
-    Check(zero, kUnalignedAllocationInNewSpace);
+    Check(not_zero, kUnalignedAllocationInNewSpace);
   }
 
   ExternalReference allocation_top =
@@ -4913,12 +4913,12 @@ void MacroAssembler::Allocate(int object_size,
   // 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));
   }
 }
 
@@ -4981,8 +4981,9 @@ void MacroAssembler::Allocate(Register object_size,
   // 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);
 }