[compiler-rt] [scudo] Added LRU eviction policy to secondary cache. (PR #99409)

[via llvm-commits]

================
@@ -434,6 +464,66 @@ template <typename Config> class MapAllocatorCache {
   void unmapTestOnly() { empty(); }
 
 private:
+  bool needToEvict() REQUIRES(Mutex) {
+    return (EntriesCount >= atomic_load_relaxed(&MaxEntriesCount));
+  }
+
+  void insert(const CachedBlock &Entry) REQUIRES(Mutex) {
+    DCHECK_LT(EntriesCount, atomic_load_relaxed(&MaxEntriesCount));
+
+    // Cache should be populated with valid entries when not empty
+    DCHECK_NE(AvailableHead, CachedBlock::InvalidEntry);
+
+    u32 FreeIndex = AvailableHead;
+    AvailableHead = Entries[AvailableHead].Next;
+
+    if (EntriesCount == 0) {
+      LRUTail = static_cast<u16>(FreeIndex);
+    } else {
+      // Check list order
+      if (EntriesCount > 1)
+        DCHECK_GE(Entries[LRUHead].Time, Entries[Entries[LRUHead].Next].Time);
+      Entries[LRUHead].Prev = static_cast<u16>(FreeIndex);
+    }
+
+    Entries[FreeIndex] = Entry;
+    Entries[FreeIndex].Next = LRUHead;
+    Entries[FreeIndex].Prev = CachedBlock::InvalidEntry;
+    LRUHead = static_cast<u16>(FreeIndex);
+    EntriesCount++;
+
+    // Availability stack should not have available entries when all entries
+    // are in use
+    if (EntriesCount == Config::getEntriesArraySize())
+      DCHECK(AvailableHead == CachedBlock::InvalidEntry);
+  }
+
+  void remove(uptr I) REQUIRES(Mutex) {
+    DCHECK(Entries[I].isValid());
+
+    Entries[I].invalidate();
+
+    if (I == LRUHead)
+      LRUHead = Entries[I].Next;
+    else
+      Entries[Entries[I].Prev].Next = Entries[I].Next;
+
+    if (I == LRUTail)
+      LRUTail = Entries[I].Prev;
+    else
+      Entries[Entries[I].Next].Prev = Entries[I].Prev;
+
+    Entries[I].Next = AvailableHead;
+    AvailableHead = static_cast<u16>(I);
+    EntriesCount--;
+
+    // Cache should not have valid entries when not empty
+    if (EntriesCount == 0) {
+      DCHECK(LRUHead == CachedBlock::InvalidEntry);
+      DCHECK(LRUTail == CachedBlock::InvalidEntry);
+    }
+  }
+
----------------
ChiaHungDuan wrote:

You change this to DCHECK_NE in the follow up CL, it may be better to do it here. 

I think it's fine to use DCHECK anyway because we only care if it's InvalidEntry. Knowing the wrong value may not help with figuring out what's going on. I'll leave you to make the decision

https://github.com/llvm/llvm-project/pull/99409