Support double allocations when folding allocation.

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 53dedda7ac7212ea282d8e731107e82db1870804..7dc039a84a64e14f7c98dc553420fc74239e277e 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -3857,8 +3857,7 @@ void MacroAssembler::Allocate(int object_size,
   // Load address of new object into result.
   LoadAllocationTopHelper(result, scratch, flags);
 
-  // Align the next allocation. Storing the filler map without checking top is
-  // always safe because the limit of the heap is always aligned.
+  // On X64 each allocation has to be double aligned.
   if (((flags & DOUBLE_ALIGNMENT) != 0) && FLAG_debug_code) {
     testq(result, Immediate(kDoubleAlignmentMask));
     Check(zero, "Allocation is not double aligned");
@@ -3936,8 +3935,7 @@ void MacroAssembler::Allocate(Register object_size,
   // Load address of new object into result.
   LoadAllocationTopHelper(result, scratch, flags);
 
-  // Align the next allocation. Storing the filler map without checking top is
-  // always safe because the limit of the heap is always aligned.
+  // On X64 each allocation has to be double aligned.
   if (((flags & DOUBLE_ALIGNMENT) != 0) && FLAG_debug_code) {
     testq(result, Immediate(kDoubleAlignmentMask));
     Check(zero, "Allocation is not double aligned");