improve allocation accounting for incremental mark

Add an assertion that allocated_bytes >= 0 in IncrementalMark::Step and then
make it pass. We were not being diligent in maintaining top_on_previous_step_
and as a result inaccurate, and even negative values of allocated_bytes were
being reported to Step.

BUG=
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/1274453002

Cr-Commit-Position: refs/heads/master@{#30778}

src/heap/incremental-marking.cc

@@ -901,6 +901,8 @@ intptr_t IncrementalMarking::Step(intptr_t allocated_bytes,
                                   CompletionAction action,
                                   ForceMarkingAction marking,
                                   ForceCompletionAction completion) {
+  DCHECK(allocated_bytes >= 0);
+
   if (heap_->gc_state() != Heap::NOT_IN_GC || !FLAG_incremental_marking ||
       (state_ != SWEEPING && state_ != MARKING)) {
     return 0;

src/heap/spaces.cc

@@ -1451,6 +1451,7 @@ void NewSpace::UpdateAllocationInfo() {
 
 
 void NewSpace::ResetAllocationInfo() {
+  Address old_top = allocation_info_.top();
   to_space_.Reset();
   UpdateAllocationInfo();
   pages_used_ = 0;
@@ -1459,6 +1460,12 @@ void NewSpace::ResetAllocationInfo() {
   while (it.has_next()) {
     Bitmap::Clear(it.next());
   }
+  if (top_on_previous_step_) {
+    int bytes_allocated = static_cast<int>(old_top - top_on_previous_step_);
+    heap()->incremental_marking()->Step(bytes_allocated,
+                                        IncrementalMarking::GC_VIA_STACK_GUARD);
+    top_on_previous_step_ = allocation_info_.top();
+  }
 }
 
 
@@ -1537,13 +1544,15 @@ bool NewSpace::EnsureAllocation(int size_in_bytes,
       return false;
     }
 
-    // Do a step for the bytes allocated on the last page.
-    int bytes_allocated = static_cast<int>(old_top - top_on_previous_step_);
-    heap()->incremental_marking()->Step(bytes_allocated,
-                                        IncrementalMarking::GC_VIA_STACK_GUARD);
-    old_top = allocation_info_.top();
-    top_on_previous_step_ = old_top;
+    if (top_on_previous_step_) {
+      // Do a step for the bytes allocated on the last page.
+      int bytes_allocated = static_cast<int>(old_top - top_on_previous_step_);
+      heap()->incremental_marking()->Step(
+          bytes_allocated, IncrementalMarking::GC_VIA_STACK_GUARD);
+      top_on_previous_step_ = allocation_info_.top();
+    }
 
+    old_top = allocation_info_.top();
     high = to_space_.page_high();
     filler_size = Heap::GetFillToAlign(old_top, alignment);
     aligned_size_in_bytes = size_in_bytes + filler_size;
@@ -1555,12 +1564,14 @@ bool NewSpace::EnsureAllocation(int size_in_bytes,
     // Either the limit has been lowered because linear allocation was disabled
     // or because incremental marking wants to get a chance to do a step. Set
     // the new limit accordingly.
-    Address new_top = old_top + aligned_size_in_bytes;
-    int bytes_allocated = static_cast<int>(new_top - top_on_previous_step_);
-    heap()->incremental_marking()->Step(bytes_allocated,
-                                        IncrementalMarking::GC_VIA_STACK_GUARD);
+    if (top_on_previous_step_) {
+      Address new_top = old_top + aligned_size_in_bytes;
+      int bytes_allocated = static_cast<int>(new_top - top_on_previous_step_);
+      heap()->incremental_marking()->Step(
+          bytes_allocated, IncrementalMarking::GC_VIA_STACK_GUARD);
+      top_on_previous_step_ = new_top;
+    }
     UpdateInlineAllocationLimit(aligned_size_in_bytes);
-    top_on_previous_step_ = new_top;
   }
   return true;
 }

src/heap/spaces.h

@@ -2427,7 +2427,8 @@ class NewSpace : public Space {
         to_space_(heap, kToSpace),
         from_space_(heap, kFromSpace),
         reservation_(),
-        inline_allocation_limit_step_(0) {}
+        inline_allocation_limit_step_(0),
+        top_on_previous_step_(0) {}
 
   // Sets up the new space using the given chunk.
   bool SetUp(int reserved_semispace_size_, int max_semi_space_size);
@@ -2609,7 +2610,7 @@ class NewSpace : public Space {
   void LowerInlineAllocationLimit(intptr_t step) {
     inline_allocation_limit_step_ = step;
     UpdateInlineAllocationLimit(0);
-    top_on_previous_step_ = allocation_info_.top();
+    top_on_previous_step_ = step ? allocation_info_.top() : 0;
   }
 
   // Get the extent of the inactive semispace (for use as a marking stack,