src/runtime/mksizeclasses.go - go - Git at Google

 // Copyright 2016 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.

 // +build ignore

 // Generate tables for small malloc size classes.
 //
 // See malloc.go for overview.
 //
 // The size classes are chosen so that rounding an allocation
 // request up to the next size class wastes at most 12.5% (1.125x).
 //
 // Each size class has its own page count that gets allocated
 // and chopped up when new objects of the size class are needed.
 // That page count is chosen so that chopping up the run of
 // pages into objects of the given size wastes at most 12.5% (1.125x)
 // of the memory. It is not necessary that the cutoff here be
 // the same as above.
 //
 // The two sources of waste multiply, so the worst possible case
 // for the above constraints would be that allocations of some
 // size might have a 26.6% (1.266x) overhead.
 // In practice, only one of the wastes comes into play for a
 // given size (sizes < 512 waste mainly on the round-up,
 // sizes > 512 waste mainly on the page chopping).
 //
 // TODO(rsc): Compute max waste for any given size.

 package main

 import (
 	"bytes"
 	"flag"
 	"fmt"
 	"go/format"
 	"io"
 	"io/ioutil"
 	"log"
 	"os"
 )

 // Generate msize.go

 var stdout = flag.Bool("stdout", false, "write to stdout instead of sizeclasses.go")

 func main() {
 	flag.Parse()

 	var b bytes.Buffer
 	fmt.Fprintln(&b, "// Code generated by mksizeclasses.go; DO NOT EDIT.")
 	fmt.Fprintln(&b, "//go:generate go run mksizeclasses.go")
 	fmt.Fprintln(&b)
 	fmt.Fprintln(&b, "package runtime")
 	classes := makeClasses()

 	printComment(&b, classes)

 	printClasses(&b, classes)

 	out, err := format.Source(b.Bytes())
 	if err != nil {
 		log.Fatal(err)
 	}
 	if *stdout {
 		_, err = os.Stdout.Write(out)
 	} else {
 		err = ioutil.WriteFile("sizeclasses.go", out, 0666)
 	}
 	if err != nil {
 		log.Fatal(err)
 	}
 }

 const (
 	// Constants that we use and will transfer to the runtime.
 	maxSmallSize = 32 << 10
 	smallSizeDiv = 8
 	smallSizeMax = 1024
 	largeSizeDiv = 128
 	pageShift    = 13

 	// Derived constants.
 	pageSize = 1 << pageShift
 )

 type class struct {
 	size   int // max size
 	npages int // number of pages

 	mul    int
 	shift  uint
 	shift2 uint
 	mask   int
 }

 func powerOfTwo(x int) bool {
 	return x != 0 && x&(x-1) == 0
 }

 func makeClasses() []class {
 	var classes []class

 	classes = append(classes, class{}) // class #0 is a dummy entry

 	align := 8
 	for size := align; size <= maxSmallSize; size += align {
 		if powerOfTwo(size) { // bump alignment once in a while
 			if size >= 2048 {
 				align = 256
 			} else if size >= 128 {
 				align = size / 8
 			} else if size >= 16 {
 				align = 16 // required for x86 SSE instructions, if we want to use them
 			}
 		}
 		if !powerOfTwo(align) {
 			panic("incorrect alignment")
 		}

 		// Make the allocnpages big enough that
 		// the leftover is less than 1/8 of the total,
 		// so wasted space is at most 12.5%.
 		allocsize := pageSize
 		for allocsize%size > allocsize/8 {
 			allocsize += pageSize
 		}
 		npages := allocsize / pageSize

 		// If the previous sizeclass chose the same
 		// allocation size and fit the same number of
 		// objects into the page, we might as well
 		// use just this size instead of having two
 		// different sizes.
 		if len(classes) > 1 && npages == classes[len(classes)-1].npages && allocsize/size == allocsize/classes[len(classes)-1].size {
 			classes[len(classes)-1].size = size
 			continue
 		}
 		classes = append(classes, class{size: size, npages: npages})
 	}

 	// Increase object sizes if we can fit the same number of larger objects
 	// into the same number of pages. For example, we choose size 8448 above
 	// with 6 objects in 7 pages. But we can well use object size 9472,
 	// which is also 6 objects in 7 pages but +1024 bytes (+12.12%).
 	// We need to preserve at least largeSizeDiv alignment otherwise
 	// sizeToClass won't work.
 	for i := range classes {
 		if i == 0 {
 			continue
 		}
 		c := &classes[i]
 		psize := c.npages * pageSize
 		new_size := (psize / (psize / c.size)) &^ (largeSizeDiv - 1)
 		if new_size > c.size {
 			c.size = new_size
 		}
 	}

 	if len(classes) != 67 {
 		panic("number of size classes has changed")
 	}

 	for i := range classes {
 		computeDivMagic(&classes[i])
 	}

 	return classes
 }

 // computeDivMagic computes some magic constants to implement
 // the division required to compute object number from span offset.
 // n / c.size is implemented as n >> c.shift * c.mul >> c.shift2
 // for all 0 <= n < c.npages * pageSize
 func computeDivMagic(c *class) {
 	// divisor
 	d := c.size
 	if d == 0 {
 		return
 	}

 	// maximum input value for which the formula needs to work.
 	max := c.npages*pageSize - 1

 	if powerOfTwo(d) {
 		// If the size is a power of two, heapBitsForObject can divide even faster by masking.
 		// Compute this mask.
 		if max >= 1<<16 {
 			panic("max too big for power of two size")
 		}
 		c.mask = 1<<16 - d
 	}

 	// Compute pre-shift by factoring power of 2 out of d.
 	for d%2 == 0 {
 		c.shift++
 		d >>= 1
 		max >>= 1
 	}

 	// Find the smallest k that works.
 	// A small k allows us to fit the math required into 32 bits
 	// so we can use 32-bit multiplies and shifts on 32-bit platforms.
 nextk:
 	for k := uint(0); ; k++ {
 		mul := (int(1)<<k + d - 1) / d //  ⌈2^k / d⌉

 		// Test to see if mul works.
 		for n := 0; n <= max; n++ {
 			if n*mul>>k != n/d {
 				continue nextk
 			}
 		}
 		if mul >= 1<<16 {
 			panic("mul too big")
 		}
 		if uint64(mul)*uint64(max) >= 1<<32 {
 			panic("mul*max too big")
 		}
 		c.mul = mul
 		c.shift2 = k
 		break
 	}

 	// double-check.
 	for n := 0; n <= max; n++ {
 		if n*c.mul>>c.shift2 != n/d {
 			fmt.Printf("d=%d max=%d mul=%d shift2=%d n=%d\n", d, max, c.mul, c.shift2, n)
 			panic("bad multiply magic")
 		}
 		// Also check the exact computations that will be done by the runtime,
 		// for both 32 and 64 bit operations.
 		if uint32(n)*uint32(c.mul)>>uint8(c.shift2) != uint32(n/d) {
 			fmt.Printf("d=%d max=%d mul=%d shift2=%d n=%d\n", d, max, c.mul, c.shift2, n)
 			panic("bad 32-bit multiply magic")
 		}
 		if uint64(n)*uint64(c.mul)>>uint8(c.shift2) != uint64(n/d) {
 			fmt.Printf("d=%d max=%d mul=%d shift2=%d n=%d\n", d, max, c.mul, c.shift2, n)
 			panic("bad 64-bit multiply magic")
 		}
 	}
 }

 func printComment(w io.Writer, classes []class) {
 	fmt.Fprintf(w, "// %-5s  %-9s  %-10s  %-7s  %-11s\n", "class", "bytes/obj", "bytes/span", "objects", "waste bytes")
 	for i, c := range classes {
 		if i == 0 {
 			continue
 		}
 		spanSize := c.npages * pageSize
 		objects := spanSize / c.size
 		waste := spanSize - c.size*(spanSize/c.size)
 		fmt.Fprintf(w, "// %5d  %9d  %10d  %7d  %11d\n", i, c.size, spanSize, objects, waste)
 	}
 	fmt.Fprintf(w, "\n")
 }

 func printClasses(w io.Writer, classes []class) {
 	fmt.Fprintln(w, "const (")
 	fmt.Fprintf(w, "_MaxSmallSize = %d\n", maxSmallSize)
 	fmt.Fprintf(w, "smallSizeDiv = %d\n", smallSizeDiv)
 	fmt.Fprintf(w, "smallSizeMax = %d\n", smallSizeMax)
 	fmt.Fprintf(w, "largeSizeDiv = %d\n", largeSizeDiv)
 	fmt.Fprintf(w, "_NumSizeClasses = %d\n", len(classes))
 	fmt.Fprintf(w, "_PageShift = %d\n", pageShift)
 	fmt.Fprintln(w, ")")

 	fmt.Fprint(w, "var class_to_size = [_NumSizeClasses]uint16 {")
 	for _, c := range classes {
 		fmt.Fprintf(w, "%d,", c.size)
 	}
 	fmt.Fprintln(w, "}")

 	fmt.Fprint(w, "var class_to_allocnpages = [_NumSizeClasses]uint8 {")
 	for _, c := range classes {
 		fmt.Fprintf(w, "%d,", c.npages)
 	}
 	fmt.Fprintln(w, "}")

 	fmt.Fprintln(w, "type divMagic struct {")
 	fmt.Fprintln(w, "  shift uint8")
 	fmt.Fprintln(w, "  shift2 uint8")
 	fmt.Fprintln(w, "  mul uint16")
 	fmt.Fprintln(w, "  baseMask uint16")
 	fmt.Fprintln(w, "}")
 	fmt.Fprint(w, "var class_to_divmagic = [_NumSizeClasses]divMagic {")
 	for _, c := range classes {
 		fmt.Fprintf(w, "{%d,%d,%d,%d},", c.shift, c.shift2, c.mul, c.mask)
 	}
 	fmt.Fprintln(w, "}")

 	// map from size to size class, for small sizes.
 	sc := make([]int, smallSizeMax/smallSizeDiv+1)
 	for i := range sc {
 		size := i * smallSizeDiv
 		for j, c := range classes {
 			if c.size >= size {
 				sc[i] = j
 				break
 			}
 		}
 	}
 	fmt.Fprint(w, "var size_to_class8 = [smallSizeMax/smallSizeDiv+1]uint8 {")
 	for _, v := range sc {
 		fmt.Fprintf(w, "%d,", v)
 	}
 	fmt.Fprintln(w, "}")

 	// map from size to size class, for large sizes.
 	sc = make([]int, (maxSmallSize-smallSizeMax)/largeSizeDiv+1)
 	for i := range sc {
 		size := smallSizeMax + i*largeSizeDiv
 		for j, c := range classes {
 			if c.size >= size {
 				sc[i] = j
 				break
 			}
 		}
 	}
 	fmt.Fprint(w, "var size_to_class128 = [(_MaxSmallSize-smallSizeMax)/largeSizeDiv+1]uint8 {")
 	for _, v := range sc {
 		fmt.Fprintf(w, "%d,", v)
 	}
 	fmt.Fprintln(w, "}")
 }
	// Copyright 2016 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.

	// +build ignore

	// Generate tables for small malloc size classes.
	//
	// See malloc.go for overview.
	//
	// The size classes are chosen so that rounding an allocation
	// request up to the next size class wastes at most 12.5% (1.125x).
	//
	// Each size class has its own page count that gets allocated
	// and chopped up when new objects of the size class are needed.
	// That page count is chosen so that chopping up the run of
	// pages into objects of the given size wastes at most 12.5% (1.125x)
	// of the memory. It is not necessary that the cutoff here be
	// the same as above.
	//
	// The two sources of waste multiply, so the worst possible case
	// for the above constraints would be that allocations of some
	// size might have a 26.6% (1.266x) overhead.
	// In practice, only one of the wastes comes into play for a
	// given size (sizes < 512 waste mainly on the round-up,
	// sizes > 512 waste mainly on the page chopping).
	//
	// TODO(rsc): Compute max waste for any given size.

	package main

	import (
	"bytes"
	"flag"
	"fmt"
	"go/format"
	"io"
	"io/ioutil"
	"log"
	"os"
	)

	// Generate msize.go

	var stdout = flag.Bool("stdout", false, "write to stdout instead of sizeclasses.go")

	func main() {
	flag.Parse()

	var b bytes.Buffer
	fmt.Fprintln(&b, "// Code generated by mksizeclasses.go; DO NOT EDIT.")
	fmt.Fprintln(&b, "//go:generate go run mksizeclasses.go")
	fmt.Fprintln(&b)
	fmt.Fprintln(&b, "package runtime")
	classes := makeClasses()

	printComment(&b, classes)

	printClasses(&b, classes)

	out, err := format.Source(b.Bytes())
	if err != nil {
	log.Fatal(err)
	}
	if *stdout {
	_, err = os.Stdout.Write(out)
	} else {
	err = ioutil.WriteFile("sizeclasses.go", out, 0666)
	}
	if err != nil {
	log.Fatal(err)
	}
	}

	const (
	// Constants that we use and will transfer to the runtime.
	maxSmallSize = 32 << 10
	smallSizeDiv = 8
	smallSizeMax = 1024
	largeSizeDiv = 128
	pageShift = 13

	// Derived constants.
	pageSize = 1 << pageShift
	)

	type class struct {
	size int // max size
	npages int // number of pages

	mul int
	shift uint
	shift2 uint
	mask int
	}

	func powerOfTwo(x int) bool {
	return x != 0 && x&(x-1) == 0
	}

	func makeClasses() []class {
	var classes []class

	classes = append(classes, class{}) // class #0 is a dummy entry

	align := 8
	for size := align; size <= maxSmallSize; size += align {
	if powerOfTwo(size) { // bump alignment once in a while
	if size >= 2048 {
	align = 256
	} else if size >= 128 {
	align = size / 8
	} else if size >= 16 {
	align = 16 // required for x86 SSE instructions, if we want to use them
	}
	}
	if !powerOfTwo(align) {
	panic("incorrect alignment")
	}

	// Make the allocnpages big enough that
	// the leftover is less than 1/8 of the total,
	// so wasted space is at most 12.5%.
	allocsize := pageSize
	for allocsize%size > allocsize/8 {
	allocsize += pageSize
	}
	npages := allocsize / pageSize

	// If the previous sizeclass chose the same
	// allocation size and fit the same number of
	// objects into the page, we might as well
	// use just this size instead of having two
	// different sizes.
	if len(classes) > 1 && npages == classes[len(classes)-1].npages && allocsize/size == allocsize/classes[len(classes)-1].size {
	classes[len(classes)-1].size = size
	continue
	}
	classes = append(classes, class{size: size, npages: npages})
	}

	// Increase object sizes if we can fit the same number of larger objects
	// into the same number of pages. For example, we choose size 8448 above
	// with 6 objects in 7 pages. But we can well use object size 9472,
	// which is also 6 objects in 7 pages but +1024 bytes (+12.12%).
	// We need to preserve at least largeSizeDiv alignment otherwise
	// sizeToClass won't work.
	for i := range classes {
	if i == 0 {
	continue
	}
	c := &classes[i]
	psize := c.npages * pageSize
	new_size := (psize / (psize / c.size)) &^ (largeSizeDiv - 1)
	if new_size > c.size {
	c.size = new_size
	}
	}

	if len(classes) != 67 {
	panic("number of size classes has changed")
	}

	for i := range classes {
	computeDivMagic(&classes[i])
	}

	return classes
	}

	// computeDivMagic computes some magic constants to implement
	// the division required to compute object number from span offset.
	// n / c.size is implemented as n >> c.shift * c.mul >> c.shift2
	// for all 0 <= n < c.npages * pageSize
	func computeDivMagic(c *class) {
	// divisor
	d := c.size
	if d == 0 {
	return
	}

	// maximum input value for which the formula needs to work.
	max := c.npages*pageSize - 1

	if powerOfTwo(d) {
	// If the size is a power of two, heapBitsForObject can divide even faster by masking.
	// Compute this mask.
	if max >= 1<<16 {
	panic("max too big for power of two size")
	}
	c.mask = 1<<16 - d
	}

	// Compute pre-shift by factoring power of 2 out of d.
	for d%2 == 0 {
	c.shift++
	d >>= 1
	max >>= 1
	}

	// Find the smallest k that works.
	// A small k allows us to fit the math required into 32 bits
	// so we can use 32-bit multiplies and shifts on 32-bit platforms.
	nextk:
	for k := uint(0); ; k++ {
	mul := (int(1)<<k + d - 1) / d // ⌈2^k / d⌉

	// Test to see if mul works.
	for n := 0; n <= max; n++ {
	if n*mul>>k != n/d {
	continue nextk
	}
	}
	if mul >= 1<<16 {
	panic("mul too big")
	}
	if uint64(mul)*uint64(max) >= 1<<32 {
	panic("mul*max too big")
	}
	c.mul = mul
	c.shift2 = k
	break
	}

	// double-check.
	for n := 0; n <= max; n++ {
	if n*c.mul>>c.shift2 != n/d {
	fmt.Printf("d=%d max=%d mul=%d shift2=%d n=%d\n", d, max, c.mul, c.shift2, n)
	panic("bad multiply magic")
	}
	// Also check the exact computations that will be done by the runtime,
	// for both 32 and 64 bit operations.
	if uint32(n)*uint32(c.mul)>>uint8(c.shift2) != uint32(n/d) {
	fmt.Printf("d=%d max=%d mul=%d shift2=%d n=%d\n", d, max, c.mul, c.shift2, n)
	panic("bad 32-bit multiply magic")
	}
	if uint64(n)*uint64(c.mul)>>uint8(c.shift2) != uint64(n/d) {
	fmt.Printf("d=%d max=%d mul=%d shift2=%d n=%d\n", d, max, c.mul, c.shift2, n)
	panic("bad 64-bit multiply magic")
	}
	}
	}

	func printComment(w io.Writer, classes []class) {
	fmt.Fprintf(w, "// %-5s %-9s %-10s %-7s %-11s\n", "class", "bytes/obj", "bytes/span", "objects", "waste bytes")
	for i, c := range classes {
	if i == 0 {
	continue
	}
	spanSize := c.npages * pageSize
	objects := spanSize / c.size
	waste := spanSize - c.size*(spanSize/c.size)
	fmt.Fprintf(w, "// %5d %9d %10d %7d %11d\n", i, c.size, spanSize, objects, waste)
	}
	fmt.Fprintf(w, "\n")
	}

	func printClasses(w io.Writer, classes []class) {
	fmt.Fprintln(w, "const (")
	fmt.Fprintf(w, "_MaxSmallSize = %d\n", maxSmallSize)
	fmt.Fprintf(w, "smallSizeDiv = %d\n", smallSizeDiv)
	fmt.Fprintf(w, "smallSizeMax = %d\n", smallSizeMax)
	fmt.Fprintf(w, "largeSizeDiv = %d\n", largeSizeDiv)
	fmt.Fprintf(w, "_NumSizeClasses = %d\n", len(classes))
	fmt.Fprintf(w, "_PageShift = %d\n", pageShift)
	fmt.Fprintln(w, ")")

	fmt.Fprint(w, "var class_to_size = [_NumSizeClasses]uint16 {")
	for _, c := range classes {
	fmt.Fprintf(w, "%d,", c.size)
	}
	fmt.Fprintln(w, "}")

	fmt.Fprint(w, "var class_to_allocnpages = [_NumSizeClasses]uint8 {")
	for _, c := range classes {
	fmt.Fprintf(w, "%d,", c.npages)
	}
	fmt.Fprintln(w, "}")

	fmt.Fprintln(w, "type divMagic struct {")
	fmt.Fprintln(w, " shift uint8")
	fmt.Fprintln(w, " shift2 uint8")
	fmt.Fprintln(w, " mul uint16")
	fmt.Fprintln(w, " baseMask uint16")
	fmt.Fprintln(w, "}")
	fmt.Fprint(w, "var class_to_divmagic = [_NumSizeClasses]divMagic {")
	for _, c := range classes {
	fmt.Fprintf(w, "{%d,%d,%d,%d},", c.shift, c.shift2, c.mul, c.mask)
	}
	fmt.Fprintln(w, "}")

	// map from size to size class, for small sizes.
	sc := make([]int, smallSizeMax/smallSizeDiv+1)
	for i := range sc {
	size := i * smallSizeDiv
	for j, c := range classes {
	if c.size >= size {
	sc[i] = j
	break
	}
	}
	}
	fmt.Fprint(w, "var size_to_class8 = [smallSizeMax/smallSizeDiv+1]uint8 {")
	for _, v := range sc {
	fmt.Fprintf(w, "%d,", v)
	}
	fmt.Fprintln(w, "}")

	// map from size to size class, for large sizes.
	sc = make([]int, (maxSmallSize-smallSizeMax)/largeSizeDiv+1)
	for i := range sc {
	size := smallSizeMax + i*largeSizeDiv
	for j, c := range classes {
	if c.size >= size {
	sc[i] = j
	break
	}
	}
	}
	fmt.Fprint(w, "var size_to_class128 = [(_MaxSmallSize-smallSizeMax)/largeSizeDiv+1]uint8 {")
	for _, v := range sc {
	fmt.Fprintf(w, "%d,", v)
	}
	fmt.Fprintln(w, "}")
	}