cmd/compile: more precise analysis of method values

Previously for a method value "x.M", we always flowed x directly to
the heap, which led to the receiver argument generally needing to be
heap allocated.

This CL changes it to flow x to the closure and M's receiver
parameter. This allows receiver arguments to be stack allocated as
long as (1) the closure never escapes, *and* (2) method doesn't leak
its receiver parameter.

Within the standard library, this allows a handful of objects to be
stack allocated instead. Listed here are diagnostics that were
previously emitted by "go build -gcflags=-m std cmd" that are no
longer emitted:

archive/tar/writer.go:118:6: moved to heap: f
archive/tar/writer.go:208:6: moved to heap: f
archive/tar/writer.go:248:6: moved to heap: f
cmd/compile/internal/gc/initorder.go:252:2: moved to heap: d
cmd/compile/internal/gc/initorder.go:75:2: moved to heap: s
cmd/go/internal/generate/generate.go:206:7: &Generator literal escapes to heap
cmd/internal/obj/arm64/asm7.go:910:2: moved to heap: c
cmd/internal/obj/mips/asm0.go:415:2: moved to heap: c
cmd/internal/obj/pcln.go:294:22: new(pcinlineState) escapes to heap
cmd/internal/obj/s390x/asmz.go:459:2: moved to heap: c
crypto/tls/handshake_server.go:56:2: moved to heap: hs

Thanks to Cuong Manh Le for help coming up with this solution.

Fixes #27557.

Change-Id: I8c85d671d07fb9b53e11d2dd05949a34dbbd7e17
Reviewed-on: https://go-review.googlesource.com/c/go/+/228263
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

4 files changed, 5 insertions, 10 deletions

diff --git a/test/escape2.go b/test/escape2.go
index 4e30331380..cf24f4bebc 100644
--- a/test/escape2.go
+++ b/test/escape2.go
@@ -1386,8 +1386,7 @@ func (t *Tm) M() { // ERROR "t does not escape$"
 func foo141() {
 	var f func()
 
-	// BAD: new(Tm) should not escape
-	t := new(Tm) // ERROR "new\(Tm\) escapes to heap$"
+	t := new(Tm) // ERROR "new\(Tm\) does not escape$"
 	f = t.M      // ERROR "t.M does not escape$"
 	_ = f
 }
diff --git a/test/escape2n.go b/test/escape2n.go
index 26b0a1d8c5..f771e0aef2 100644
--- a/test/escape2n.go
+++ b/test/escape2n.go
@@ -1386,8 +1386,7 @@ func (t *Tm) M() { // ERROR "t does not escape$"
 func foo141() {
 	var f func()
 
-	// BAD: new(Tm) should not escape
-	t := new(Tm) // ERROR "new\(Tm\) escapes to heap$"
+	t := new(Tm) // ERROR "new\(Tm\) does not escape$"
 	f = t.M      // ERROR "t.M does not escape$"
 	_ = f
 }
diff --git a/test/fixedbugs/issue21709.go b/test/fixedbugs/issue21709.go
index abc9e767e5..cc5896ab53 100644
--- a/test/fixedbugs/issue21709.go
+++ b/test/fixedbugs/issue21709.go
@@ -14,8 +14,7 @@ func (s *S) Inc() {} // ERROR "s does not escape"
 var N int
 
 func F1() {
-	// BAD: s should not escape
-	var s S // ERROR "moved to heap: s"
+	var s S
 	for i := 0; i < N; i++ {
 		fs := []func(){ // ERROR "\[\]func\(\) literal does not escape"
 			s.Inc, // ERROR "s.Inc does not escape"
@@ -27,8 +26,7 @@ func F1() {
 }
 
 func F2() {
-	// BAD: s should not escape
-	var s S // ERROR "moved to heap: s"
+	var s S
 	for i := 0; i < N; i++ {
 		for _, f := range []func(){ // ERROR "\[\]func\(\) literal does not escape"
 			s.Inc, // ERROR "s.Inc does not escape"
diff --git a/test/fixedbugs/issue27557.go b/test/fixedbugs/issue27557.go
index 11a23f6932..e35ab5a169 100644
--- a/test/fixedbugs/issue27557.go
+++ b/test/fixedbugs/issue27557.go
@@ -9,8 +9,7 @@ package p
 var sink interface{}
 
 func _() {
-	// BAD: t should not escape
-	var t T         // ERROR "moved to heap"
+	var t T
 	f := t.noescape // ERROR "t.noescape does not escape"
 	f()
 }