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>
diff --git a/src/cmd/gc/esc.c b/src/cmd/gc/esc.c
index 2fdbd99..59b00bf 100644
--- a/src/cmd/gc/esc.c
+++ b/src/cmd/gc/esc.c
@@ -511,6 +511,36 @@
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;
diff --git a/test/escape2.go b/test/escape2.go
index 357ce4a..507a815 100644
--- a/test/escape2.go
+++ b/test/escape2.go
@@ -1519,3 +1519,40 @@
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"
+}
diff --git a/test/escape2n.go b/test/escape2n.go
index 3e9bb70..e514bde 100644
--- a/test/escape2n.go
+++ b/test/escape2n.go
@@ -1519,3 +1519,40 @@
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"
+}
