[compiler-rt] [scudo] Store more blocks in each TransferBatch (PR #70390)

[Christopher Ferris via llvm-commits]

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@@ -945,39 +930,64 @@ template <typename Config> class SizeClassAllocator64 {
       // Block used by `BatchGroup` is from BatchClassId. Turn the block into
       // `TransferBatch` with single block.
       TransferBatchT *TB = reinterpret_cast<TransferBatchT *>(BG);
-      TB->clear();
-      TB->add(
-          compactPtr(SizeClassMap::BatchClassId, reinterpret_cast<uptr>(TB)));
+      ToArray[0] =
+          compactPtr(SizeClassMap::BatchClassId, reinterpret_cast<uptr>(TB));
       Region->FreeListInfo.PoppedBlocks += 1;
-      return TB;
+      return 1U;
     }
 
+    // So far, instead of always fill blocks to `MaxBlockCount`, we only exmaine
+    // single `TransferBatch` to minimize the time spent in the primary
+    // allocator. Besides, the sizes of `TransferBatch` and
+    // `CacheT::getMaxCached()` may also impact the times of accessing the
+    // primary allocator.
+    // TODO(chiahungduan): Evaluate if we want to always prepare `MaxBlockCount`
+    // blocks and/or adjust the size of `TransferBatch` according to
+    // `CacheT::getMaxCached()`.
     TransferBatchT *B = Batches.front();
-    Batches.pop_front();
     DCHECK_NE(B, nullptr);
     DCHECK_GT(B->getCount(), 0U);
 
-    if (Batches.empty()) {
-      BatchGroupT *BG = Region->FreeListInfo.BlockList.front();
-      Region->FreeListInfo.BlockList.pop_front();
-
-      // We don't keep BatchGroup with zero blocks to avoid empty-checking while
-      // allocating. Note that block used by constructing BatchGroup is recorded
-      // as free blocks in the last element of BatchGroup::Batches. Which means,
-      // once we pop the last TransferBatch, the block is implicitly
-      // deallocated.
+    // BachClassId should always take all blocks in the TransferBatch. Read the
+    // comment in `pushBatchClassBlocks()` for more details.
+    const u16 PopCount = ClassId == SizeClassMap::BatchClassId
+                             ? B->getCount()
+                             : Min(MaxBlockCount, B->getCount());
+    B->moveNToArray(ToArray, PopCount);
+
+    // TODO(chiahungduan): The deallocation of unused BatchClassId blocks can be
+    // done without holding `FLLock`.
+    if (B->empty()) {
+      Batches.pop_front();
+      // `TransferBatch` of BatchClassId is self-contained, no need to
+      // deallocate. Read the comment in `pushBatchClassBlocks()` for more
+      // details.
       if (ClassId != SizeClassMap::BatchClassId)
-        C->deallocate(SizeClassMap::BatchClassId, BG);
+        C->deallocate(SizeClassMap::BatchClassId, B);
+
+      if (Batches.empty()) {
+        BatchGroupT *BG = Region->FreeListInfo.BlockList.front();
+        Region->FreeListInfo.BlockList.pop_front();
+
+        // We don't keep BatchGroup with zero blocks to avoid empty-checking
+        // while allocating. Note that block used by constructing BatchGroup is
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cferris1000 wrote:

that block used by -> that the block used for

https://github.com/llvm/llvm-project/pull/70390