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Source file src/runtime/msize.go

Documentation: runtime

  // Copyright 2009 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // Malloc small size classes.
  //
  // See malloc.go for overview.
  // See also mksizeclasses.go for how we decide what size classes to use.
  
  package runtime
  
  // sizeToClass(0 <= n <= MaxSmallSize) returns the size class,
  //	1 <= sizeclass < NumSizeClasses, for n.
  //	Size class 0 is reserved to mean "not small".
  //
  // The sizeToClass lookup is implemented using two arrays,
  // one mapping sizes <= 1024 to their class and one mapping
  // sizes >= 1024 and <= MaxSmallSize to their class.
  // All objects are 8-aligned, so the first array is indexed by
  // the size divided by 8 (rounded up).  Objects >= 1024 bytes
  // are 128-aligned, so the second array is indexed by the
  // size divided by 128 (rounded up).  The arrays are constants
  // in sizeclass.go generated by mksizeclass.go.
  func sizeToClass(size uint32) uint32 {
  	if size > _MaxSmallSize {
  		throw("invalid size")
  	}
  	if size > smallSizeMax-8 {
  		return uint32(size_to_class128[(size-smallSizeMax+largeSizeDiv-1)/largeSizeDiv])
  	}
  	return uint32(size_to_class8[(size+smallSizeDiv-1)/smallSizeDiv])
  }
  
  // Returns size of the memory block that mallocgc will allocate if you ask for the size.
  func roundupsize(size uintptr) uintptr {
  	if size < _MaxSmallSize {
  		if size <= smallSizeMax-8 {
  			return uintptr(class_to_size[size_to_class8[(size+smallSizeDiv-1)/smallSizeDiv]])
  		} else {
  			return uintptr(class_to_size[size_to_class128[(size-smallSizeMax+largeSizeDiv-1)/largeSizeDiv]])
  		}
  	}
  	if size+_PageSize < size {
  		return size
  	}
  	return round(size, _PageSize)
  }
  

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