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slices: for Insert and Replace, grow slices like append does

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At least when we're inserting/replacing near the end of a slice, when
we have to grow it use the same multiplicative growth factor that the
runtime uses for append.

Before this CL, we would grow the slice one page (8192 bytes) at a time
for large slices. This would cause O(n^2) work when appending near the
end should only take O(n) work.

This doesn't fix the problem if you insert/replace near the start of the
array, but maybe that doesn't need fixing because it is O(n^2) anyway.

Fixes #60134

Change-Id: If05376bc512ab839769180e5ce4cb929f47363b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/495296
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
This commit is contained in:
Keith Randall 2023-05-16 15:16:06 -07:00
parent 882cc4d596
commit 6d1c507bfc
2 changed files with 38 additions and 4 deletions
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6
src/slices/slices.go
View File

@ -98,8 +98,7 @@ func Insert[S ~[]E, E any](s S, i int, v ...E) S {
// 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])
s2 := append(s[:i], make(S, n+m-i)...)
copy(s2[i:], v)
copy(s2[i+m:], s[i:])
return s2
@ -219,8 +218,7 @@ func Replace[S ~[]E, E any](s S, i, j int, v ...E) S {
tot := len(s[:i]) + len(v) + len(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])
s2 := append(s[:i], make(S, tot-i)...) // See Insert
copy(s2[i:], v)
copy(s2[i+len(v):], s[j:])
return s2

36
src/slices/slices_test.go
View File

@ -781,3 +781,39 @@ func TestRotate(t *testing.T) {
}
}
}
func TestInsertGrowthRate(t *testing.T) {
b := make([]byte, 1)
maxCap := cap(b)
nGrow := 0
const N = 1e6
for i := 0; i < N; i++ {
b = Insert(b, len(b)-1, 0)
if cap(b) > maxCap {
maxCap = cap(b)
nGrow++
}
}
want := int(math.Log(N) / math.Log(1.25)) // 1.25 == growth rate for large slices
if nGrow > want {
t.Errorf("too many grows. got:%d want:%d", nGrow, want)
}
}
func TestReplaceGrowthRate(t *testing.T) {
b := make([]byte, 2)
maxCap := cap(b)
nGrow := 0
const N = 1e6
for i := 0; i < N; i++ {
b = Replace(b, len(b)-2, len(b)-1, 0, 0)
if cap(b) > maxCap {
maxCap = cap(b)
nGrow++
}
}
want := int(math.Log(N) / math.Log(1.25)) // 1.25 == growth rate for large slices
if nGrow > want {
t.Errorf("too many grows. got:%d want:%d", nGrow, want)
}
}