src/runtime/msize.go

// 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)
}