mirror
/
go
mirror of https://github.com/golang/go.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

slices: handle aliasing cases in Insert/Replace 

Handle cases where the inserted slice is actually part of the slice
that is being inserted into.

Requires a bit more work, but no more allocations. (Compare to #494536.)

Not entirely sure this is worth the complication.

Fixes #60138

Change-Id: Ia72c872b04309b99025e6ca5a4a326ebed2abb69
Reviewed-on: https://go-review.googlesource.com/c/go/+/494817
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
This commit is contained in:
Keith Randall 2023-05-12 20:23:57 -07:00
parent 099f5a985f
commit 882cc4d596
3 changed files with 320 additions and 22 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
src/go/build/deps_test.go
View File

@ -46,9 +46,13 @@ var depsRules = `
internal/goexperiment, internal/goos,
internal/goversion, internal/nettrace, internal/platform,
log/internal,
maps, slices, unicode/utf8, unicode/utf16, unicode,
maps, unicode/utf8, unicode/utf16, unicode,
unsafe;
# slices depends on unsafe for overlapping check.
unsafe
< slices;
# These packages depend only on internal/goarch and unsafe.
internal/goarch, unsafe
< internal/abi;

265
src/slices/slices.go
View File

@ -5,6 +5,10 @@
// Package slices defines various functions useful with slices of any type.
package slices
import (
"unsafe"
)
// Equal reports whether two slices are equal: the same length and all
// elements equal. If the lengths are different, Equal returns false.
// Otherwise, the elements are compared in increasing index order, and the
@ -81,22 +85,83 @@ func ContainsFunc[E any](s []E, f func(E) bool) bool {
// Insert panics if i is out of range.
// This function is O(len(s) + len(v)).
func Insert[S ~[]E, E any](s S, i int, v ...E) S {
tot := len(s) + len(v)
if tot <= cap(s) {
s2 := s[:tot]
copy(s2[i+len(v):], s[i:])
m := len(v)
if m == 0 {
return s
}
n := len(s)
if i == n {
return append(s, v...)
}
if n+m > cap(s) {
// Use append rather than make so that we bump the size of
// the slice up to the next storage class.
// This is what Grow does but we don't call Grow because
// that might copy the values twice.
s2 := append(S(nil), make(S, n+m)...)
copy(s2, s[:i])
copy(s2[i:], v)
copy(s2[i+m:], s[i:])
return s2
}
// Use append rather than make so that we bump the size of
// the slice up to the next storage class.
// This is what Grow does but we don't call Grow because
// that might copy the values twice.
s2 := append(S(nil), make(S, tot)...)
copy(s2, s[:i])
copy(s2[i:], v)
copy(s2[i+len(v):], s[i:])
return s2
s = s[:n+m]
// before:
// s: aaaaaaaabbbbccccccccdddd
// ^ ^ ^ ^
// i i+m n n+m
// after:
// s: aaaaaaaavvvvbbbbcccccccc
// ^ ^ ^ ^
// i i+m n n+m
//
// a are the values that don't move in s.
// v are the values copied in from v.
// b and c are the values from s that are shifted up in index.
// d are the values that get overwritten, never to be seen again.
if !overlaps(v, s[i+m:]) {
// Easy case - v does not overlap either the c or d regions.
// (It might be in some of a or b, or elsewhere entirely.)
// The data we copy up doesn't write to v at all, so just do it.
copy(s[i+m:], s[i:])
// Now we have
// s: aaaaaaaabbbbbbbbcccccccc
// ^ ^ ^ ^
// i i+m n n+m
// Note the b values are duplicated.
copy(s[i:], v)
// Now we have
// s: aaaaaaaavvvvbbbbcccccccc
// ^ ^ ^ ^
// i i+m n n+m
// That's the result we want.
return s
}
// The hard case - v overlaps c or d. We can't just shift up
// the data because we'd move or clobber the values we're trying
// to insert.
// So instead, write v on top of d, then rotate.
copy(s[n:], v)
// Now we have
// s: aaaaaaaabbbbccccccccvvvv
// ^ ^ ^ ^
// i i+m n n+m
rotateRight(s[i:], m)
// Now we have
// s: aaaaaaaavvvvbbbbcccccccc
// ^ ^ ^ ^
// i i+m n n+m
// That's the result we want.
return s
}
// Delete removes the elements s[i:j] from s, returning the modified slice.
@ -143,18 +208,89 @@ func DeleteFunc[S ~[]E, E any](s S, del func(E) bool) S {
// modified slice. Replace panics if s[i:j] is not a valid slice of s.
func Replace[S ~[]E, E any](s S, i, j int, v ...E) S {
_ = s[i:j] // verify that i:j is a valid subslice
if i == j {
return Insert(s, i, v...)
}
if j == len(s) {
return append(s[:i], v...)
}
tot := len(s[:i]) + len(v) + len(s[j:])
if tot <= cap(s) {
s2 := s[:tot]
copy(s2[i+len(v):], s[j:])
if tot > cap(s) {
// Too big to fit, allocate and copy over.
s2 := append(S(nil), make(S, tot)...) // See Insert
copy(s2, s[:i])
copy(s2[i:], v)
copy(s2[i+len(v):], s[j:])
return s2
}
s2 := make(S, tot)
copy(s2, s[:i])
copy(s2[i:], v)
copy(s2[i+len(v):], s[j:])
return s2
r := s[:tot]
if i+len(v) <= j {
// Easy, as v fits in the deleted portion.
copy(r[i:], v)
if i+len(v) != j {
copy(r[i+len(v):], s[j:])
}
return r
}
// We are expanding (v is bigger than j-i).
// The situation is something like this:
// (example has i=4,j=8,len(s)=16,len(v)=6)
// s: aaaaxxxxbbbbbbbbyy
// ^ ^ ^ ^
// i j len(s) tot
// a: prefix of s
// x: deleted range
// b: more of s
// y: area to expand into
if !overlaps(r[i+len(v):], v) {
// Easy, as v is not clobbered by the first copy.
copy(r[i+len(v):], s[j:])
copy(r[i:], v)
return r
}
// This is a situation where we don't have a single place to which
// we can copy v. Parts of it need to go to two different places.
// We want to copy the prefix of v into y and the suffix into x, then
// rotate |y| spots to the right.
//
// v[2:] v[:2]
// | |
// s: aaaavvvvbbbbbbbbvv
// ^ ^ ^ ^
// i j len(s) tot
//
// If either of those two destinations don't alias v, then we're good.
y := len(v) - (j - i) // length of y portion
if !overlaps(r[i:j], v) {
copy(r[i:j], v[y:])
copy(r[len(s):], v[:y])
rotateRight(r[i:], y)
return r
}
if !overlaps(r[len(s):], v) {
copy(r[len(s):], v[:y])
copy(r[i:j], v[y:])
rotateRight(r[i:], y)
return r
}
// Now we know that v overlaps both x and y.
// That means that the entirety of b is *inside* v.
// So we don't need to preserve b at all; instead we
// can copy v first, then copy the b part of v out of
// v to the right destination.
k := startIdx(v, s[j:])
copy(r[i:], v)
copy(r[i+len(v):], r[i+k:])
return r
}
// Clone returns a copy of the slice.
@ -224,3 +360,90 @@ func Grow[S ~[]E, E any](s S, n int) S {
func Clip[S ~[]E, E any](s S) S {
return s[:len(s):len(s)]
}
// Rotation algorithm explanation:
//
// rotate left by 2
// start with
// 0123456789
// split up like this
// 01 234567 89
// swap first 2 and last 2
// 89 234567 01
// join first parts
// 89234567 01
// recursively rotate first left part by 2
// 23456789 01
// join at the end
// 2345678901
//
// rotate left by 8
// start with
// 0123456789
// split up like this
// 01 234567 89
// swap first 2 and last 2
// 89 234567 01
// join last parts
// 89 23456701
// recursively rotate second part left by 6
// 89 01234567
// join at the end
// 8901234567
// TODO: There are other rotate algorithms.
// This algorithm has the desirable property that it moves each element exactly twice.
// The triple-reverse algorithm is simpler and more cache friendly, but takes more writes.
// The follow-cycles algorithm can be 1-write but it is not very cache friendly.
// rotateLeft rotates b left by n spaces.
// s_final[i] = s_orig[i+r], wrapping around.
func rotateLeft[S ~[]E, E any](s S, r int) {
for r != 0 && r != len(s) {
if r*2 <= len(s) {
swap(s[:r], s[len(s)-r:])
s = s[:len(s)-r]
} else {
swap(s[:len(s)-r], s[r:])
s, r = s[len(s)-r:], r*2-len(s)
}
}
}
func rotateRight[S ~[]E, E any](s S, r int) {
rotateLeft(s, len(s)-r)
}
// swap swaps the contents of x and y. x and y must be equal length and disjoint.
func swap[S ~[]E, E any](x, y S) {
for i := 0; i < len(x); i++ {
x[i], y[i] = y[i], x[i]
}
}
// overlaps reports whether the memory ranges a[0:len(a)] and b[0:len(b)] overlap.
func overlaps[S ~[]E, E any](a, b S) bool {
if len(a) == 0 || len(b) == 0 {
return false
}
elemSize := unsafe.Sizeof(a[0])
if elemSize == 0 {
return false
}
// TODO: use a runtime/unsafe facility once one becomes available. See issue 12445.
// Also see crypto/internal/alias/alias.go:AnyOverlap
return uintptr(unsafe.Pointer(&a[0])) <= uintptr(unsafe.Pointer(&b[len(b)-1]))+(elemSize-1) &&
uintptr(unsafe.Pointer(&b[0])) <= uintptr(unsafe.Pointer(&a[len(a)-1]))+(elemSize-1)
}
// startIdx returns the index in haystack where the needle starts.
// prerequisite: the needle must be aliased entirely inside the haystack.
func startIdx[S ~[]E, E any](haystack, needle S) int {
p := &needle[0]
for i := range haystack {
if p == &haystack[i] {
return i
}
}
// TODO: what if the overlap is by a non-integral number of Es?
panic("needle not found")
}

71
src/slices/slices_test.go
View File

@ -302,6 +302,31 @@ func TestInsert(t *testing.T) {
}
}
func TestInsertOverlap(t *testing.T) {
const N = 10
a := make([]int, N)
want := make([]int, 2*N)
for n := 0; n <= N; n++ { // length
for i := 0; i <= n; i++ { // insertion point
for x := 0; x <= N; x++ { // start of inserted data
for y := x; y <= N; y++ { // end of inserted data
for k := 0; k < N; k++ {
a[k] = k
}
want = want[:0]
want = append(want, a[:i]...)
want = append(want, a[x:y]...)
want = append(want, a[i:n]...)
got := Insert(a[:n], i, a[x:y]...)
if !Equal(got, want) {
t.Errorf("Insert with overlap failed n=%d i=%d x=%d y=%d, got %v want %v", n, i, x, y, got, want)
}
}
}
}
}
}
var deleteTests = []struct {
s []int
i, j int
@ -662,6 +687,33 @@ func TestReplacePanics(t *testing.T) {
}
}
func TestReplaceOverlap(t *testing.T) {
const N = 10
a := make([]int, N)
want := make([]int, 2*N)
for n := 0; n <= N; n++ { // length
for i := 0; i <= n; i++ { // insertion point 1
for j := i; j <= n; j++ { // insertion point 2
for x := 0; x <= N; x++ { // start of inserted data
for y := x; y <= N; y++ { // end of inserted data
for k := 0; k < N; k++ {
a[k] = k
}
want = want[:0]
want = append(want, a[:i]...)
want = append(want, a[x:y]...)
want = append(want, a[j:n]...)
got := Replace(a[:n], i, j, a[x:y]...)
if !Equal(got, want) {
t.Errorf("Insert with overlap failed n=%d i=%d j=%d x=%d y=%d, got %v want %v", n, i, j, x, y, got, want)
}
}
}
}
}
}
}
func BenchmarkReplace(b *testing.B) {
cases := []struct {
name string
@ -710,3 +762,22 @@ func BenchmarkReplace(b *testing.B) {
}
}
func TestRotate(t *testing.T) {
const N = 10
s := make([]int, 0, N)
for n := 0; n < N; n++ {
for r := 0; r < n; r++ {
s = s[:0]
for i := 0; i < n; i++ {
s = append(s, i)
}
rotateLeft(s, r)
for i := 0; i < n; i++ {
if s[i] != (i+r)%n {
t.Errorf("expected n=%d r=%d i:%d want:%d got:%d", n, r, i, (i+r)%n, s[i])
}
}
}
}
}