Changes to improve the performance of reading a snapshot.

runtime/vm/pages.cc

Index: runtime/vm/pages.cc
===================================================================
--- runtime/vm/pages.cc	(revision 37116)
+++ runtime/vm/pages.cc	(working copy)
@@ -225,17 +225,17 @@
   ASSERT(size >= kObjectAlignment);
   ASSERT(Utils::IsAligned(size, kObjectAlignment));
   uword result = 0;
-  SpaceUsage after_allocation = usage_;
-  after_allocation.used_in_words += size >> kWordSizeLog2;
+  intptr_t after_capacity_in_words = usage_.capacity_in_words;
   if (size < kAllocatablePageSize) {
     const bool is_protected = (type == HeapPage::kExecutable)
         && FLAG_write_protect_code;
     result = freelist_[type].TryAllocate(size, is_protected);
     if (result == 0) {
       // Can we grow by one page?
-      after_allocation.capacity_in_words += kPageSizeInWords;
-      if ((!page_space_controller_.NeedsGarbageCollection(after_allocation) ||
-           growth_policy == kForceGrowth) &&
+      after_capacity_in_words += kPageSizeInWords;
+      const bool needs_gc = page_space_controller_.NeedsGarbageCollection(
+          after_capacity_in_words);
+      if ((!needs_gc || growth_policy == kForceGrowth) &&
           CanIncreaseCapacityInWords(kPageSizeInWords)) {
         HeapPage* page = AllocatePage(type);
         ASSERT(page != NULL);
@@ -256,9 +256,10 @@
       // On overflow we fail to allocate.
       return 0;
     }
-    after_allocation.capacity_in_words += page_size_in_words;
-    if ((!page_space_controller_.NeedsGarbageCollection(after_allocation) ||
-         growth_policy == kForceGrowth) &&
+    after_capacity_in_words += page_size_in_words;
+    const bool needs_gc = page_space_controller_.NeedsGarbageCollection(
+          after_capacity_in_words);
+    if ((!needs_gc || growth_policy == kForceGrowth) &&
         CanIncreaseCapacityInWords(page_size_in_words)) {
       HeapPage* page = AllocateLargePage(size, type);
       if (page != NULL) {
@@ -267,7 +268,8 @@
     }
   }
   if (result != 0) {
-    usage_ = after_allocation;
+    usage_.used_in_words += (size >> kWordSizeLog2);
+    usage_.capacity_in_words = after_capacity_in_words;
     if (FLAG_compiler_stats && (type == HeapPage::kExecutable)) {
       CompilerStats::code_allocated += size;
     }
@@ -585,7 +587,8 @@
 PageSpaceController::~PageSpaceController() {}
 
 
-bool PageSpaceController::NeedsGarbageCollection(SpaceUsage after) const {
+bool PageSpaceController::NeedsGarbageCollection(
+    intptr_t after_capacity_in_words) const {
   if (!is_enabled_) {
     return false;
   }
@@ -593,7 +596,7 @@
     return false;
   }
   intptr_t capacity_increase_in_words =
-    after.capacity_in_words - last_usage_.capacity_in_words;
+    after_capacity_in_words - last_usage_.capacity_in_words;
   ASSERT(capacity_increase_in_words >= 0);
   capacity_increase_in_words =
       Utils::RoundUp(capacity_increase_in_words, PageSpace::kPageSizeInWords);