PartitionAlloc: Increase the number of pages per bucket

third_party/WebKit/Source/wtf/PartitionAlloc.cpp

Index: third_party/WebKit/Source/wtf/PartitionAlloc.cpp
diff --git a/third_party/WebKit/Source/wtf/PartitionAlloc.cpp b/third_party/WebKit/Source/wtf/PartitionAlloc.cpp
index fbecfbb6349d92b1daffa9df8ac5308997c0128a..fedf8c91d426e67734c0e53eb9cf32a9c8620bab 100644
--- a/third_party/WebKit/Source/wtf/PartitionAlloc.cpp
+++ b/third_party/WebKit/Source/wtf/PartitionAlloc.cpp
@@ -62,7 +62,7 @@ void (*PartitionRootBase::gOomHandlingFunction)() = nullptr;
 PartitionAllocHooks::AllocationHook* PartitionAllocHooks::m_allocationHook = nullptr;
 PartitionAllocHooks::FreeHook* PartitionAllocHooks::m_freeHook = nullptr;
 
-static uint16_t partitionBucketNumSystemPages(size_t size)
+static uint8_t partitionBucketNumSystemPages(size_t size)
 {
     // This works out reasonably for the current bucket sizes of the generic
     // allocator, and the current values of partition page size and constants.
@@ -78,7 +78,9 @@ static uint16_t partitionBucketNumSystemPages(size_t size)
     uint16_t bestPages = 0;
     if (size > kMaxSystemPagesPerSlotSpan * kSystemPageSize) {
         ASSERT(!(size % kSystemPageSize));
-        return static_cast<uint16_t>(size / kSystemPageSize);
+        bestPages = static_cast<uint16_t>(size / kSystemPageSize);
+        RELEASE_ASSERT(bestPages < (1 << 8));

[bashi, 2016/01/26 23:52:37]

RELEASE_ASSERT(bestPages <= kMaxSystemPagesPerSlotSpan) ?

[haraken, 2016/01/27 01:19:32]

No, what I really want to assert here is that the following static_cast<uint8_t>
doesn't lose information (i.e., bestPages is less than 1<<8). Given the current
configuration of PartitionAlloc, it is statically guaranteed, but it may change
if someone changes some constant value of PartitionAlloc. This assertion is here
to catch it.

[bashi, 2016/01/27 01:34:07]

I think it's not the most important thing to ensure bestPages is less than 1<<8.
What we really want to do here is to ensure that we don't exceed the maximum
number of |PartitionBucket::numSystemPagesPerSlotSpan|, which is logically
determined by the size and relationship of system pages and slot spans.  8 bits
are just enough bits to store that maximum number.

+        return static_cast<uint8_t>(bestPages);
     }
     ASSERT(size <= kMaxSystemPagesPerSlotSpan * kSystemPageSize);
     for (uint16_t i = kNumSystemPagesPerPartitionPage - 1; i <= kMaxSystemPagesPerSlotSpan; ++i) {
@@ -97,7 +99,8 @@ static uint16_t partitionBucketNumSystemPages(size_t size)
         }
     }
     ASSERT(bestPages > 0);
-    return bestPages;
+    RELEASE_ASSERT(bestPages < (1 << 8));

[bashi, 2016/01/26 23:52:37]

ditto.

+    return static_cast<uint8_t>(bestPages);
 }
 
 static void partitionAllocBaseInit(PartitionRootBase* root)