cmd/compile: make isfat handle 1-element array, 1-field struct

This will improve liveness analysis slightly, the same logic as
isdirectiface curently does. In:

	type T struct {
	    m map[int]int
	}

        v := T{}
        v.m = make(map[int]int)

T is considered "fat", now it is not. So assigning to v.m is considered
to clobber the entire v.

This is follow up of CL 179057.

Change-Id: Id6b4807b8e8521ef5d8bcb14fedb6dceb9dbf18c
Reviewed-on: https://go-review.googlesource.com/c/go/+/179578
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/plive.go

@@ -1453,9 +1453,21 @@ func liveness(e *ssafn, f *ssa.Func, pp *Progs) LivenessMap {
 func isfat(t *types.Type) bool {
 	if t != nil {
 		switch t.Etype {
-		case TSTRUCT, TARRAY, TSLICE, TSTRING,
+		case TSLICE, TSTRING,
 			TINTER: // maybe remove later
 			return true
+		case TARRAY:
+			// Array of 1 element, check if element is fat
+			if t.NumElem() == 1 {
+				return isfat(t.Elem())
+			}
+			return true
+		case TSTRUCT:
+			// Struct with 1 field, check if field is fat
+			if t.NumFields() == 1 {
+				return isfat(t.Field(0).Type)
+			}
+			return true
 		}
 	}
 

test/live.go

@@ -659,7 +659,7 @@ func bad40() {
 
 func good40() {
 	ret := T40{}              // ERROR "stack object ret T40$"
-	ret.m = make(map[int]int) // ERROR "live at call to fastrand: .autotmp_[0-9]+ ret$" "stack object .autotmp_[0-9]+ map.hdr\[int\]int$"
+	ret.m = make(map[int]int) // ERROR "live at call to fastrand: .autotmp_[0-9]+$" "stack object .autotmp_[0-9]+ map.hdr\[int\]int$"
 	t := &ret
 	printnl() // ERROR "live at call to printnl: ret$"
 	// Note: ret is live at the printnl because the compiler moves &ret

test/live2.go

@@ -27,14 +27,14 @@ func newT40() *T40 {
 }
 
 func bad40() {
-	t := newT40() // ERROR "live at call to makemap: ret$" "stack object ret T40$" "stack object .autotmp_[0-9]+ map.hdr\[int\]int$"
+	t := newT40() // ERROR "stack object ret T40$" "stack object .autotmp_[0-9]+ map.hdr\[int\]int$"
 	printnl()     // ERROR "live at call to printnl: ret$"
 	useT40(t)
 }
 
 func good40() {
 	ret := T40{}                  // ERROR "stack object ret T40$"
-	ret.m = make(map[int]int, 42) // ERROR "live at call to makemap: ret$" "stack object .autotmp_[0-9]+ map.hdr\[int\]int$"
+	ret.m = make(map[int]int, 42) // ERROR "stack object .autotmp_[0-9]+ map.hdr\[int\]int$"
 	t := &ret
 	printnl() // ERROR "live at call to printnl: ret$"
 	useT40(t)