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Source file src/runtime/msize.go

     1	// Copyright 2009 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	// Malloc small size classes.
     6	//
     7	// See malloc.go for overview.
     8	// See also mksizeclasses.go for how we decide what size classes to use.
     9	
    10	package runtime
    11	
    12	// sizeToClass(0 <= n <= MaxSmallSize) returns the size class,
    13	//	1 <= sizeclass < NumSizeClasses, for n.
    14	//	Size class 0 is reserved to mean "not small".
    15	//
    16	// The sizeToClass lookup is implemented using two arrays,
    17	// one mapping sizes <= 1024 to their class and one mapping
    18	// sizes >= 1024 and <= MaxSmallSize to their class.
    19	// All objects are 8-aligned, so the first array is indexed by
    20	// the size divided by 8 (rounded up).  Objects >= 1024 bytes
    21	// are 128-aligned, so the second array is indexed by the
    22	// size divided by 128 (rounded up).  The arrays are constants
    23	// in sizeclass.go generated by mksizeclass.go.
    24	func sizeToClass(size uint32) uint32 {
    25		if size > _MaxSmallSize {
    26			throw("invalid size")
    27		}
    28		if size > smallSizeMax-8 {
    29			return uint32(size_to_class128[(size-smallSizeMax+largeSizeDiv-1)/largeSizeDiv])
    30		}
    31		return uint32(size_to_class8[(size+smallSizeDiv-1)/smallSizeDiv])
    32	}
    33	
    34	// Returns size of the memory block that mallocgc will allocate if you ask for the size.
    35	func roundupsize(size uintptr) uintptr {
    36		if size < _MaxSmallSize {
    37			if size <= smallSizeMax-8 {
    38				return uintptr(class_to_size[size_to_class8[(size+smallSizeDiv-1)/smallSizeDiv]])
    39			} else {
    40				return uintptr(class_to_size[size_to_class128[(size-smallSizeMax+largeSizeDiv-1)/largeSizeDiv]])
    41			}
    42		}
    43		if size+_PageSize < size {
    44			return size
    45		}
    46		return round(size, _PageSize)
    47	}
    48	

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