cmd/compile: de-virtualize interface calls

With this change, code like

    h := sha1.New()
    h.Write(buf)
    sum := h.Sum()

gets compiled into static calls rather than
interface calls, because the compiler is able
to prove that 'h' is really a *sha1.digest.

The InterCall re-write rule hits a few dozen times
during make.bash, and hundreds of times during all.bash.

The most common pattern identified by the compiler
is a constructor like

    func New() Interface { return &impl{...} }

where the constructor gets inlined into the caller,
and the result is used immediately. Examples include
{sha1,md5,crc32,crc64,...}.New, base64.NewEncoder,
base64.NewDecoder, errors.New, net.Pipe, and so on.

Some existing benchmarks that change on darwin/amd64:

Crc64/ISO4KB-8        2.67µs ± 1%    2.66µs ± 0%  -0.36%  (p=0.015 n=10+10)
Crc64/ISO1KB-8         694ns ± 0%     690ns ± 1%  -0.59%  (p=0.001 n=10+10)
Adler32KB-8            473ns ± 1%     471ns ± 0%  -0.39%  (p=0.010 n=10+9)

On architectures like amd64, the reduction in code size
appears to contribute more to benchmark improvements than just
removing the indirect call, since that branch gets predicted
accurately when called in a loop.

Updates #19361

Change-Id: I57d4dc21ef40a05ec0fbd55a9bb0eb74cdc67a3d
Reviewed-on: https://go-review.googlesource.com/38139
Run-TryBot: Philip Hofer <phofer@umich.edu>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 39 insertions, 0 deletions

diff --git a/test/devirt.go b/test/devirt.go
new file mode 100644
index 0000000000..23577098e4
--- /dev/null
+++ b/test/devirt.go
@@ -0,0 +1,39 @@
+// errorcheck -0 -d=ssa/opt/debug=3
+
+package main
+
+// Trivial interface call devirtualization test.
+
+type real struct {
+	value int
+}
+
+func (r *real) Value() int { return r.value }
+
+type Valuer interface {
+	Value() int
+}
+
+type indirectiface struct {
+	a, b, c int
+}
+
+func (i indirectiface) Value() int {
+	return i.a + i.b + i.c
+}
+
+func main() {
+	var r Valuer
+	rptr := &real{value: 3}
+	r = rptr
+
+	if r.Value() != 3 { // ERROR "de-virtualizing call$"
+		panic("not 3")
+	}
+
+	// Can't do types that aren't "direct" interfaces (yet).
+	r = indirectiface{3, 4, 5}
+	if r.Value() != 12 {
+		panic("not 12")
+	}
+}