[libc] Add a config option to disable slab reclaiming (#151599)

Summary: Without slab reclaiming this interface is much simpler and it can speed up cases with a lot of churn. Basically, wastes memory for performance.

Author: jhuber6

libc/src/__support/GPU/allocator.cpp

@@ -43,6 +43,9 @@ constexpr static uint32_t MAX_TRIES = 1024;
 // The number of previously allocated slabs we will keep in memory.
 constexpr static uint32_t CACHED_SLABS = 8;
 
+// Configuration for whether or not we will return unused slabs to memory.
+constexpr static bool RECLAIM = true;
+
 static_assert(!(ARRAY_SIZE & (ARRAY_SIZE - 1)), "Must be a power of two");
 
 namespace impl {
@@ -399,7 +402,7 @@ struct GuardPtr {
  // and obtain exclusive rights to deconstruct it. If the CAS failed either
  // another thread resurrected the counter and we quit, or a parallel read
  // helped us invalidating it. For the latter, claim that flag and return.
- if (counter.fetch_sub(n, cpp::MemoryOrder::RELAXED) == n) {
+ if (counter.fetch_sub(n, cpp::MemoryOrder::RELAXED) == n && RECLAIM) {
  uint32_t expected = 0;
  if (counter.compare_exchange_strong(expected, INVALID,
  cpp::MemoryOrder::RELAXED,
@@ -417,8 +420,9 @@ struct GuardPtr {
  // thread.
  uint64_t read() {
  auto val = counter.load(cpp::MemoryOrder::RELAXED);
- if (val == 0 && counter.compare_exchange_strong(
- val, INVALID | HELPED, cpp::MemoryOrder::RELAXED))
+ if (val == 0 && RECLAIM &&
+ counter.compare_exchange_strong(val, INVALID | HELPED,
+ cpp::MemoryOrder::RELAXED))
  return 0;
  return (val & INVALID) ? 0 : val;
  }
@@ -463,7 +467,7 @@ struct GuardPtr {
  return nullptr;
 
  cpp::atomic_thread_fence(cpp::MemoryOrder::ACQUIRE);
- return ptr.load(cpp::MemoryOrder::RELAXED);
+ return RECLAIM ? ptr.load(cpp::MemoryOrder::RELAXED) : expected;
  }
 
  // Finalize the associated memory and signal that it is ready to use by