cmd/gc: ignore re-slicing in escape analysis

Escape analysis treats everything assigned to OIND/ODOTPTR as escaping.
As the result b escapes in the following code:

	func (b *Buffer) Foo() {
		n, m := ...
		b.buf = b.buf[n:m]
	}

This change recognizes such assignments and ignores them.

Update issue #9043.
Update issue #7921.

There are two similar cases in std lib that benefit from this optimization.
First is in archive/zip:

type readBuf []byte
func (b *readBuf) uint32() uint32 {
	v := binary.LittleEndian.Uint32(*b)
	*b = (*b)[4:]
	return v
}

Second is in time:

type data struct {
	p     []byte
	error bool
}

func (d *data) read(n int) []byte {
	if len(d.p) < n {
		d.p = nil
		d.error = true
		return nil
	}
	p := d.p[0:n]
	d.p = d.p[n:]
	return p
}

benchmark                         old ns/op     new ns/op     delta
BenchmarkCompressedZipGarbage     32431724      32217851      -0.66%

benchmark                         old allocs     new allocs     delta
BenchmarkCompressedZipGarbage     153            143            -6.54%

Change-Id: Ia6cd32744e02e36d6d8c19f402f8451101711626
Reviewed-on: https://go-review.googlesource.com/3162
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 37 insertions, 0 deletions

diff --git a/test/escape2.go b/test/escape2.go
index 357ce4a8a8..507a815044 100644
--- a/test/escape2.go
+++ b/test/escape2.go
@@ -1519,3 +1519,40 @@ func ptrlitEscape() {
 	x := &Lit{&i} // ERROR "&Lit literal escapes to heap" "&i escapes to heap"
 	sink = x
 }
+
+// self-assignments
+
+type Buffer struct {
+	arr  [64]byte
+	buf1 []byte
+	buf2 []byte
+	str1 string
+	str2 string
+}
+
+func (b *Buffer) foo() { // ERROR "b does not escape"
+	b.buf1 = b.buf1[1:2]   // ERROR "ignoring self-assignment to b.buf1"
+	b.buf1 = b.buf1[1:2:3] // ERROR "ignoring self-assignment to b.buf1"
+	b.buf1 = b.buf2[1:2]   // ERROR "ignoring self-assignment to b.buf1"
+	b.buf1 = b.buf2[1:2:3] // ERROR "ignoring self-assignment to b.buf1"
+}
+
+func (b *Buffer) bar() { // ERROR "leaking param: b"
+	b.buf1 = b.arr[1:2] // ERROR "b.arr escapes to heap"
+}
+
+func (b *Buffer) baz() { // ERROR "b does not escape"
+	b.str1 = b.str1[1:2] // ERROR "ignoring self-assignment to b.str1"
+	b.str1 = b.str2[1:2] // ERROR "ignoring self-assignment to b.str1"
+}
+
+func (b *Buffer) bat() { // ERROR "leaking param: b"
+	o := new(Buffer) // ERROR "new\(Buffer\) escapes to heap"
+	o.buf1 = b.buf1[1:2]
+	sink = o
+}
+
+func quux(sp *string, bp *[]byte) { // ERROR "sp does not escape" "bp does not escape"
+	*sp = (*sp)[1:2] // ERROR "quux ignoring self-assignment to \*sp"
+	*bp = (*bp)[1:2] // ERROR "quux ignoring self-assignment to \*bp"
+}