[vm/gc] Update initial TLAB size calculation.

First TLAB size is adjusted to account for the new space survivors.

Change-Id: I6eb4305a14c00173626069b66b01906f8ba49612
Reviewed-on: https://dart-review.googlesource.com/c/91703
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>

Author: aam

runtime/vm/heap/heap.cc

@@ -91,7 +91,7 @@ uword Heap::AllocateNew(intptr_t size) {
     return addr;
   }
 
-  intptr_t tlab_size = CalculateTLABSize();
+  intptr_t tlab_size = GetTLABSize();
   if ((tlab_size > 0) && (size > tlab_size)) {
     return AllocateOld(size, HeapPage::kData);
   }
@@ -101,8 +101,11 @@ uword Heap::AllocateNew(intptr_t size) {
     uword tlab_top = new_space_.TryAllocateNewTLAB(thread, tlab_size);
     if (tlab_top != 0) {
       addr = new_space_.TryAllocateInTLAB(thread, size);
-      ASSERT(addr != 0);
-      return addr;
+      if (addr != 0) {  // but "leftover" TLAB could end smaller than tlab_size
+        return addr;
+      }
+      // Abandon "leftover" TLAB as well so we can start from scratch.
+      AbandonRemainingTLAB(thread);
     }
   }
 
@@ -112,7 +115,7 @@ uword Heap::AllocateNew(intptr_t size) {
   // from a different thread and will be racing to allocate the requested
   // memory with other threads being released after the collection.
   CollectGarbage(kNew);
-  tlab_size = CalculateTLABSize();
+
   uword tlab_top = new_space_.TryAllocateNewTLAB(thread, tlab_size);
   if (tlab_top != 0) {
     addr = new_space_.TryAllocateInTLAB(thread, size);

runtime/vm/heap/heap.h

@@ -285,7 +285,7 @@ class Heap {
 
   static const intptr_t kNewAllocatableSize = 256 * KB;
 
-  intptr_t CalculateTLABSize() {
+  intptr_t GetTLABSize() {
     // Inspired by V8 tlab size. More than threshold for old space allocation,
     // less then minimal(initial) new semi-space.
     const intptr_t size = 512 * KB;

runtime/vm/heap/scavenger.cc

@@ -926,6 +926,15 @@ uword Scavenger::TryAllocateNewTLAB(Thread* thread, intptr_t size) {
   uword result = top_;
   intptr_t remaining = end_ - top_;
   if (remaining < size) {
+    // Grab whatever is remaining
+    size = remaining;
+  } else {
+    // Reduce TLAB size so we land at even TLAB size for future TLABs.
+    intptr_t survived_size = UsedInWords() * kWordSize;
+    size -= survived_size % size;
+  }
+  size = Utils::RoundDown(size, kObjectAlignment);
+  if (size == 0) {
     return 0;
   }
   ASSERT(to_->Contains(result));