cmd/compile: add types.SoleComponent, use in convFuncName

The specialized conversion functions care only
about a type's layout in memory, so e.g.
[1]string is equivalent to string.

Add types.SoleComponent to assist with such use cases,
and use it for the specialized conversion functions.

Increases the number of convTstring calls by ~1%.

Change-Id: I09a392909f2037387b30642781e65f707a048af5
Reviewed-on: https://go-review.googlesource.com/c/148577
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -398,10 +398,14 @@ func convFuncName(from, to *types.Type) (fnname string, needsaddr bool) {
 			return "convT32", false
 		case from.Size() == 8 && from.Align == types.Types[TUINT64].Align && !types.Haspointers(from):
 			return "convT64", false
-		case from.IsString():
-			return "convTstring", false
-		case from.IsSlice():
-			return "convTslice", false
+		}
+		if sc := from.SoleComponent(); sc != nil {
+			switch {
+			case sc.IsString():
+				return "convTstring", false
+			case sc.IsSlice():
+				return "convTslice", false
+			}
 		}
 
 		switch tkind {

src/cmd/compile/internal/types/type.go

@@ -1395,6 +1395,28 @@ func (t *Type) NumComponents(countBlank componentsIncludeBlankFields) int64 {
 	return 1
 }
 
+// SoleComponent returns the only primitive component in t,
+// if there is exactly one. Otherwise, it returns nil.
+// Components are counted as in NumComponents, including blank fields.
+func (t *Type) SoleComponent() *Type {
+	switch t.Etype {
+	case TSTRUCT:
+		if t.IsFuncArgStruct() {
+			Fatalf("SoleComponent func arg struct")
+		}
+		if t.NumFields() != 1 {
+			return nil
+		}
+		return t.Field(0).Type.SoleComponent()
+	case TARRAY:
+		if t.NumElem() != 1 {
+			return nil
+		}
+		return t.Elem().SoleComponent()
+	}
+	return t
+}
+
 // ChanDir returns the direction of a channel type t.
 // The direction will be one of Crecv, Csend, or Cboth.
 func (t *Type) ChanDir() ChanDir {