reflect: make all flag.mustBe* methods inlinable

mustBe was barely over budget, so manually inlining the first flag.kind
call is enough. Add a TODO to reverse that in the future, once the
compiler gets better.

mustBeExported and mustBeAssignable were over budget by a larger amount,
so add slow path functions instead. This is the same strategy used in
the sync package for common methods like Once.Do, for example.

Lots of exported reflect.Value methods call these assert-like unexported
methods, so avoiding the function call overhead in the common case does
shave off a percent from most exported APIs.

Finally, add the methods to TestIntendedInlining.

While at it, replace a couple of uses of the 0 Kind with its descriptive
name, Invalid.

name     old time/op    new time/op    delta
Call-8     68.0ns ± 1%    66.8ns ± 1%  -1.81%  (p=0.000 n=10+9)
PtrTo-8    8.00ns ± 2%    7.83ns ± 0%  -2.19%  (p=0.000 n=10+9)

Updates #7818.

Change-Id: Ic1603b640519393f6b50dd91ec3767753eb9e761
Reviewed-on: https://go-review.googlesource.com/c/go/+/166462
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -133,14 +133,11 @@ func TestIntendedInlining(t *testing.T) {
 			"Value.pointer",
 			"add",
 			"align",
+			"flag.mustBe",
+			"flag.mustBeAssignable",
+			"flag.mustBeExported",
 			"flag.kind",
 			"flag.ro",
-
-			// TODO: these use panic, which gets their budgets
-			// slightly over the limit
-			// "flag.mustBe",
-			// "flag.mustBeAssignable",
-			// "flag.mustBeExported",
 		},
 		"regexp": {
 			"(*bitState).push",

src/reflect/value.go

@@ -203,7 +203,8 @@ type nonEmptyInterface struct {
 // v.flag.mustBe(Bool), which will only bother to copy the
 // single important word for the receiver.
 func (f flag) mustBe(expected Kind) {
-	if f.kind() != expected {
+	// TODO(mvdan): use f.kind() again once mid-stack inlining gets better
+	if Kind(f&flagKindMask) != expected {
 		panic(&ValueError{methodName(), f.kind()})
 	}
 }
@@ -211,8 +212,14 @@ func (f flag) mustBe(expected Kind) {
 // mustBeExported panics if f records that the value was obtained using
 // an unexported field.
 func (f flag) mustBeExported() {
+	if f == 0 || f&flagRO != 0 {
+		f.mustBeExportedSlow()
+	}
+}
+
+func (f flag) mustBeExportedSlow() {
 	if f == 0 {
-		panic(&ValueError{methodName(), 0})
+		panic(&ValueError{methodName(), Invalid})
 	}
 	if f&flagRO != 0 {
 		panic("reflect: " + methodName() + " using value obtained using unexported field")
@@ -223,6 +230,12 @@ func (f flag) mustBeExported() {
 // which is to say that either it was obtained using an unexported field
 // or it is not addressable.
 func (f flag) mustBeAssignable() {
+	if f&flagRO != 0 || f&flagAddr == 0 {
+		f.mustBeAssignableSlow()
+	}
+}
+
+func (f flag) mustBeAssignableSlow() {
 	if f == 0 {
 		panic(&ValueError{methodName(), Invalid})
 	}
@@ -981,7 +994,7 @@ func (v Value) Interface() (i interface{}) {
 
 func valueInterface(v Value, safe bool) interface{} {
 	if v.flag == 0 {
-		panic(&ValueError{"reflect.Value.Interface", 0})
+		panic(&ValueError{"reflect.Value.Interface", Invalid})
 	}
 	if safe && v.flag&flagRO != 0 {
 		// Do not allow access to unexported values via Interface,