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/3162Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/gc/esc.c

@@ -511,6 +511,36 @@ esc(EscState *e, Node *n, Node *up)
 
 	case OAS:
 	case OASOP:
+		// Filter out the following special case.
+		//
+		//	func (b *Buffer) Foo() {
+		//		n, m := ...
+		//		b.buf = b.buf[n:m]
+		//	}
+		//
+		// This assignment is a no-op for escape analysis,
+		// it does not store any new pointers into b that were not already there.
+		// However, without this special case b will escape, because we assign to OIND/ODOTPTR.
+		if((n->left->op == OIND || n->left->op == ODOTPTR) && n->left->left->op == ONAME && // dst is ONAME dereference
+			(n->right->op == OSLICE || n->right->op == OSLICE3 || n->right->op == OSLICESTR) && // src is slice operation
+			(n->right->left->op == OIND || n->right->left->op == ODOTPTR) && n->right->left->left->op == ONAME && // slice is applied to ONAME dereference
+			n->left->left == n->right->left->left) { // dst and src reference the same base ONAME
+			// Here we also assume that the statement will not contain calls,
+			// that is, that order will move any calls to init.
+			// Otherwise base ONAME value could change between the moments
+			// when we evaluate it for dst and for src.
+			//
+			// Note, this optimization does not apply to OSLICEARR,
+			// because it does introduce a new pointer into b that was not already there
+			// (pointer to b itself). After such assignment, if b contents escape,
+			// b escapes as well. If we ignore such OSLICEARR, we will conclude
+			// that b does not escape when b contents do.
+			if(debug['m']) {
+				warnl(n->lineno, "%S ignoring self-assignment to %hN",
+					(n->curfn && n->curfn->nname) ? n->curfn->nname->sym : S, n->left);
+			}
+			break;
+		}
 		escassign(e, n->left, n->right);
 		break;
 

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"
+}

test/escape2n.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"
+}