// path: /usr/local/go/src/runtime/sizeclasses.go
const _NumSizeClasses = 67
var class_to_size = [_NumSizeClasses]uint16{0, 8, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240, 256, 288, 320, 352, 384, 416, 448, 480, 512, 576, 640, 704, 768, 896, 1024, 1152, 1280, 1408, 1536,1792, 2048, 2304, 2688, 3072, 3200, 3456, 4096, 4864, 5376, 6144, 6528, 6784, 6912, 8192, 9472, 9728, 10240, 10880, 12288, 13568, 14336, 16384, 18432, 19072, 20480, 21760, 24576, 27264, 28672, 32768}

// Allocate a span to use in an mcache.
func (c *mcentral) cacheSpan() *mspan {
   // Deduct credit for this span allocation and sweep if necessary.
   spanBytes := uintptr(class_to_allocnpages[c.spanclass.sizeclass()]) * _PageSize
   deductSweepCredit(spanBytes, 0)

   traceDone := false
   if trace.enabled {
      traceGCSweepStart()
   }
   
   spanBudget := 100

   var s *mspan
   sl := newSweepLocker()
   sg := sl.sweepGen

   // Try partial swept spans first.
   if s = c.partialSwept(sg).pop(); s != nil {
      goto havespan
   }

   // Now try partial unswept spans.
   for ; spanBudget >= 0; spanBudget-- {
      s = c.partialUnswept(sg).pop()
      if s == nil {
         break
      }
      if s, ok := sl.tryAcquire(s); ok {
         // We got ownership of the span, so let's sweep it and use it.
         s.sweep(true)
         sl.dispose()
         goto havespan
      }
   }
   // Now try full unswept spans, sweeping them and putting them into the
   // right list if we fail to get a span.
   for ; spanBudget >= 0; spanBudget-- {
      s = c.fullUnswept(sg).pop()
      if s == nil {
         break
      }
      if s, ok := sl.tryAcquire(s); ok {
         // We got ownership of the span, so let's sweep it.
         s.sweep(true)
         // Check if there's any free space.
         freeIndex := s.nextFreeIndex()
         if freeIndex != s.nelems {
            s.freeindex = freeIndex
            sl.dispose()
            goto havespan
         }
         // Add it to the swept list, because sweeping didn't give us any free space.
         c.fullSwept(sg).push(s.mspan)
      }
      // See comment for partial unswept spans.
   }
   sl.dispose()
   if trace.enabled {
      traceGCSweepDone()
      traceDone = true
   }

   // We failed to get a span from the mcentral so get one from mheap.
   s = c.grow()
   if s == nil {
      return nil
   }

   // At this point s is a span that should have free slots.
havespan:
   if trace.enabled && !traceDone {
      traceGCSweepDone()
   }
   n := int(s.nelems) - int(s.allocCount)
   if n == 0 || s.freeindex == s.nelems || uintptr(s.allocCount) == s.nelems {
      throw("span has no free objects")
   }
   freeByteBase := s.freeindex &^ (64 - 1)
   whichByte := freeByteBase / 8
   // Init alloc bits cache.
   s.refillAllocCache(whichByte)

   // Adjust the allocCache so that s.freeindex corresponds to the low bit in
   // s.allocCache.
   s.allocCache >>= s.freeindex % 64

   return s
}