[scudo] Quarantine optimization

Summary:
It turns out that the previous code construct was not optimizing the allocation
and deallocation of batches. The class id was read as a class member (even
though a precomputed one) and nothing else was optimized. By changing the
construct this way, the compiler actually optimizes most of the allocation and
deallocation away to only work with a single class id, which not only saves some
CPU but also some code footprint.

Reviewers: alekseyshl, dvyukov

Reviewed By: dvyukov

Subscribers: dvyukov, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D46961

llvm-svn: 332502

compiler-rt/lib/scudo/scudo_allocator.cpp

@@ -197,16 +197,17 @@ struct QuarantineCallback {
   // that the batches are indeed serviced by the Primary.
   // TODO(kostyak): figure out the best way to protect the batches.
   void *Allocate(uptr Size) {
+    const uptr BatchClassId = SizeClassMap::ClassID(sizeof(QuarantineBatch));
     return getBackendAllocator().allocatePrimary(Cache_, BatchClassId);
   }
 
   void Deallocate(void *Ptr) {
+    const uptr BatchClassId = SizeClassMap::ClassID(sizeof(QuarantineBatch));
     getBackendAllocator().deallocatePrimary(Cache_, Ptr, BatchClassId);
   }
 
   AllocatorCache *Cache_;
   COMPILER_CHECK(sizeof(QuarantineBatch) < SizeClassMap::kMaxSize);
-  const uptr BatchClassId = SizeClassMap::ClassID(sizeof(QuarantineBatch));
 };
 
 typedef Quarantine<QuarantineCallback, void> ScudoQuarantine;