cmd/compile: use devirtualization in escape analysis #33160
Description
#19361 introduced devirtualization when the concrete type is known statically. This would be extremely useful to compensate for the unfortunate design of the hash.Hash functions, which make it impossible not to make the input escape.
However, it looks like it doesn't figure out the sha256.New return value, even if that function gets inlined.
package sha256 // import "crypto/sha256"
func New() hash.Hash
New returns a new hash.Hash computing the SHA256 checksum. The Hash also
implements encoding.BinaryMarshaler and encoding.BinaryUnmarshaler to
marshal and unmarshal the internal state of the hash.
type Hash interface {
io.Writer
Sum(b []byte) []byte
Reset()
Size() int
BlockSize() int
}
package main
import "crypto/sha256"
func main() {
s := make([]byte, 128)
h := sha256.New()
h.Write(s)
h.Sum(nil)
}
$ gotip version
go version devel +5bc46cb7 Wed Jul 17 17:34:32 2019 +0000 darwin/amd64
$ gotip build -gcflags -m=2 .
# x
./x.go:5:6: cannot inline main: function too complex: cost 204 exceeds budget 80
./x.go:7:17: inlining call to sha256.New func() hash.Hash { var sha256.d·2 *sha256.digest; sha256.d·2 = <N>; sha256.d·2 = new(sha256.digest); sha256.d·2.Reset(); return hash.Hash(sha256.d·2) }
./x.go:6:11: make([]byte, 128) escapes to heap
./x.go:7:17: new(sha256.digest) escapes to heap
./x.go:7:17: hash.Hash(sha256.d·2) escapes to heap
If there is some tweak that can be made in the sha256 package to make this work, I'm happy to make this change, otherwise it would be nice to extend the optimization.
/cc @randall77
