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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>
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@ -511,6 +511,36 @@ esc(EscState *e, Node *n, Node *up)
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case OAS:
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case OASOP:
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// Filter out the following special case.
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//
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// func (b *Buffer) Foo() {
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// n, m := ...
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// b.buf = b.buf[n:m]
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// }
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//
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// This assignment is a no-op for escape analysis,
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// it does not store any new pointers into b that were not already there.
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// However, without this special case b will escape, because we assign to OIND/ODOTPTR.
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if((n->left->op == OIND || n->left->op == ODOTPTR) && n->left->left->op == ONAME && // dst is ONAME dereference
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(n->right->op == OSLICE || n->right->op == OSLICE3 || n->right->op == OSLICESTR) && // src is slice operation
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(n->right->left->op == OIND || n->right->left->op == ODOTPTR) && n->right->left->left->op == ONAME && // slice is applied to ONAME dereference
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n->left->left == n->right->left->left) { // dst and src reference the same base ONAME
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// Here we also assume that the statement will not contain calls,
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// that is, that order will move any calls to init.
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// Otherwise base ONAME value could change between the moments
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// when we evaluate it for dst and for src.
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//
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// Note, this optimization does not apply to OSLICEARR,
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// because it does introduce a new pointer into b that was not already there
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// (pointer to b itself). After such assignment, if b contents escape,
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// b escapes as well. If we ignore such OSLICEARR, we will conclude
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// that b does not escape when b contents do.
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if(debug['m']) {
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warnl(n->lineno, "%S ignoring self-assignment to %hN",
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(n->curfn && n->curfn->nname) ? n->curfn->nname->sym : S, n->left);
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}
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break;
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}
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escassign(e, n->left, n->right);
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break;
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@ -1519,3 +1519,40 @@ func ptrlitEscape() {
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x := &Lit{&i} // ERROR "&Lit literal escapes to heap" "&i escapes to heap"
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sink = x
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}
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// self-assignments
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type Buffer struct {
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arr [64]byte
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buf1 []byte
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buf2 []byte
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str1 string
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str2 string
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}
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func (b *Buffer) foo() { // ERROR "b does not escape"
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b.buf1 = b.buf1[1:2] // ERROR "ignoring self-assignment to b.buf1"
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b.buf1 = b.buf1[1:2:3] // ERROR "ignoring self-assignment to b.buf1"
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b.buf1 = b.buf2[1:2] // ERROR "ignoring self-assignment to b.buf1"
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b.buf1 = b.buf2[1:2:3] // ERROR "ignoring self-assignment to b.buf1"
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}
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func (b *Buffer) bar() { // ERROR "leaking param: b"
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b.buf1 = b.arr[1:2] // ERROR "b.arr escapes to heap"
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}
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func (b *Buffer) baz() { // ERROR "b does not escape"
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b.str1 = b.str1[1:2] // ERROR "ignoring self-assignment to b.str1"
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b.str1 = b.str2[1:2] // ERROR "ignoring self-assignment to b.str1"
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}
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func (b *Buffer) bat() { // ERROR "leaking param: b"
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o := new(Buffer) // ERROR "new\(Buffer\) escapes to heap"
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o.buf1 = b.buf1[1:2]
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sink = o
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}
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func quux(sp *string, bp *[]byte) { // ERROR "sp does not escape" "bp does not escape"
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*sp = (*sp)[1:2] // ERROR "quux ignoring self-assignment to \*sp"
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*bp = (*bp)[1:2] // ERROR "quux ignoring self-assignment to \*bp"
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}
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@ -1519,3 +1519,40 @@ func ptrlitEscape() {
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x := &Lit{&i} // ERROR "&Lit literal escapes to heap" "&i escapes to heap"
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sink = x
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}
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// self-assignments
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type Buffer struct {
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arr [64]byte
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buf1 []byte
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buf2 []byte
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str1 string
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str2 string
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}
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func (b *Buffer) foo() { // ERROR "b does not escape"
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b.buf1 = b.buf1[1:2] // ERROR "ignoring self-assignment to b.buf1"
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b.buf1 = b.buf1[1:2:3] // ERROR "ignoring self-assignment to b.buf1"
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b.buf1 = b.buf2[1:2] // ERROR "ignoring self-assignment to b.buf1"
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b.buf1 = b.buf2[1:2:3] // ERROR "ignoring self-assignment to b.buf1"
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}
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func (b *Buffer) bar() { // ERROR "leaking param: b"
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b.buf1 = b.arr[1:2] // ERROR "b.arr escapes to heap"
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}
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func (b *Buffer) baz() { // ERROR "b does not escape"
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b.str1 = b.str1[1:2] // ERROR "ignoring self-assignment to b.str1"
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b.str1 = b.str2[1:2] // ERROR "ignoring self-assignment to b.str1"
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}
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func (b *Buffer) bat() { // ERROR "leaking param: b"
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o := new(Buffer) // ERROR "new\(Buffer\) escapes to heap"
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o.buf1 = b.buf1[1:2]
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sink = o
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}
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func quux(sp *string, bp *[]byte) { // ERROR "sp does not escape" "bp does not escape"
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*sp = (*sp)[1:2] // ERROR "quux ignoring self-assignment to \*sp"
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*bp = (*bp)[1:2] // ERROR "quux ignoring self-assignment to \*bp"
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}
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