cmd/compile: restore zero-copy string->[]byte optimization

This CL implements the remainder of the zero-copy string->[]byte
conversion optimization initially attempted in go.dev/cl/520395, but
fixes the tracking of mutations due to ODEREF/ODOTPTR assignments, and
adds more comprehensive tests that I should have included originally.

However, this CL also keeps it behind the -d=zerocopy flag. The next
CL will enable it by default (for easier rollback).

Updates #2205.

Change-Id: Ic330260099ead27fc00e2680a59c6ff23cb63c2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/520599
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

1 files changed, 77 insertions, 0 deletions

diff --git a/test/escape_mutations.go b/test/escape_mutations.go
new file mode 100644
index 0000000000..4365fc1ec3
--- /dev/null
+++ b/test/escape_mutations.go
@@ -0,0 +1,77 @@
+// errorcheck -0 -m -d=escapemutationscalls,zerocopy -l
+
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "fmt"
+
+type B struct {
+	x  int
+	px *int
+	pb *B
+}
+
+func F1(b *B) { // ERROR "mutates param: b derefs=0"
+	b.x = 1
+}
+
+func F2(b *B) { // ERROR "mutates param: b derefs=1"
+	*b.px = 1
+}
+
+func F2a(b *B) { // ERROR "mutates param: b derefs=0"
+	b.px = nil
+}
+
+func F3(b *B) { // ERROR "leaking param: b"
+	fmt.Println(b) // ERROR "\.\.\. argument does not escape"
+}
+
+func F4(b *B) { // ERROR "leaking param content: b"
+	fmt.Println(*b) // ERROR "\.\.\. argument does not escape" "\*b escapes to heap"
+}
+
+func F4a(b *B) { // ERROR "leaking param content: b" "mutates param: b derefs=0"
+	b.x = 2
+	fmt.Println(*b) // ERROR "\.\.\. argument does not escape" "\*b escapes to heap"
+}
+
+func F5(b *B) { // ERROR "leaking param: b"
+	sink = b
+}
+
+func F6(b *B) int { // ERROR "b does not escape, mutate, or call"
+	return b.x
+}
+
+var sink any
+
+func M() {
+	var b B // ERROR "moved to heap: b"
+	F1(&b)
+	F2(&b)
+	F2a(&b)
+	F3(&b)
+	F4(&b)
+}
+
+func g(s string) { // ERROR "s does not escape, mutate, or call"
+	sink = &([]byte(s))[10] // ERROR "\(\[\]byte\)\(s\) escapes to heap"
+}
+
+func h(out []byte, s string) { // ERROR "mutates param: out derefs=0" "s does not escape, mutate, or call"
+	copy(out, []byte(s)) // ERROR "zero-copy string->\[\]byte conversion" "\(\[\]byte\)\(s\) does not escape"
+}
+
+func i(s string) byte { // ERROR "s does not escape, mutate, or call"
+	p := []byte(s) // ERROR "zero-copy string->\[\]byte conversion" "\(\[\]byte\)\(s\) does not escape"
+	return p[20]
+}
+
+func j(s string, x byte) { // ERROR "s does not escape, mutate, or call"
+	p := []byte(s) // ERROR "\(\[\]byte\)\(s\) does not escape"
+	p[20] = x
+}