fmt: avoid reflect.Value.Pointer to help escape analysis

This is part of a series of CLs that aim to reduce how often
interface arguments escape for the print functions in fmt.

Prior to this change, one reason arguments escape is because
fmtPointer calls reflect.Value.Pointer:

./print.go:551:39: parameter value leaks to <heap> for (*pp).fmtPointer with derefs=0:
./print.go:551:39:   flow: <heap> ← value:
./print.go:551:39:     from reflect.Value.Pointer(value) (call parameter) at ./print.go:555:20

printValue also has its value argument escape for this reason,
among others.

This CL changes those uses to reflect.Value.UnsafePointer instead,
which does not cause an escape.

Arguments still escape for other reasons.

Change-Id: I81c4f737f11fe835c5ccb122caee40a39b553451
Reviewed-on: https://go-review.googlesource.com/c/go/+/524939
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: t hepudds <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

Author: thepudds

src/fmt/print.go

@@ -550,7 +550,7 @@ func (p *pp) fmtPointer(value reflect.Value, verb rune) {
 	var u uintptr
 	switch value.Kind() {
 	case reflect.Chan, reflect.Func, reflect.Map, reflect.Pointer, reflect.Slice, reflect.UnsafePointer:
-		u = value.Pointer()
+		u = uintptr(value.UnsafePointer())
 	default:
 		p.badVerb(verb)
 		return
@@ -916,7 +916,7 @@ func (p *pp) printValue(value reflect.Value, verb rune, depth int) {
 	case reflect.Pointer:
 		// pointer to array or slice or struct? ok at top level
 		// but not embedded (avoid loops)
-		if depth == 0 && f.Pointer() != 0 {
+		if depth == 0 && f.UnsafePointer() != nil {
 			switch a := f.Elem(); a.Kind() {
 			case reflect.Array, reflect.Slice, reflect.Struct, reflect.Map:
 				p.buf.writeByte('&')