cmd/compile: add support for a go:noinline directive

Some tests need to disable inlining of a function.  It's currently done
in one of a few ways (adding a function call, an empty switch, or a
defer).  Add support for a less fragile 'go:noinline' directive that
prevents inlining.

Fixes #12312

Change-Id: Ife444e13361b4a927709d81aa41e448f32eec8d4
Reviewed-on: https://go-review.googlesource.com/13911
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -657,10 +657,11 @@ var instrumenting bool
 // Pending annotations for next func declaration.
 var (
 	noescape       bool
+	noinline       bool
+	norace         bool
 	nosplit        bool
 	nowritebarrier bool
 	systemstack    bool
-	norace         bool
 )
 
 var debuglive int

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

@@ -1393,6 +1393,7 @@ xfndcl:
 		$$.Noescape = noescape;
 		$$.Func.Norace = norace;
 		$$.Func.Nosplit = nosplit;
+		$$.Func.Noinline = noinline;
 		$$.Func.Nowritebarrier = nowritebarrier;
 		$$.Func.Systemstack = systemstack;
 		funcbody($$);
@@ -1579,8 +1580,9 @@ xdcl_list:
 		if nsyntaxerrors == 0 {
 			testdclstack();
 		}
-		nointerface = false
 		noescape = false
+		noinline = false
+		nointerface = false
 		norace = false
 		nosplit = false
 		nowritebarrier = false

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

@@ -106,6 +106,11 @@ func caninl(fn *Node) {
 		Fatalf("caninl no nname %v", Nconv(fn, obj.FmtSign))
 	}
 
+	// If marked "go:noinline", don't inline
+	if fn.Func.Noinline {
+		return
+	}
+
 	// If fn has no body (is defined outside of Go), cannot inline it.
 	if fn.Nbody == nil {
 		return

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

@@ -1662,6 +1662,11 @@ func getlinepragma() int {
 			return c
 		}
 
+		if verb == "go:noinline" {
+			noinline = true
+			return c
+		}
+
 		if verb == "go:systemstack" {
 			if compiling_runtime == 0 {
 				Yyerror("//go:systemstack only allowed in runtime")

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

@@ -171,6 +171,7 @@ type Func struct {
 
 	Norace         bool // func must not have race detector annotations
 	Nosplit        bool // func should not execute on separate stack
+	Noinline       bool // func should not be inlined
 	Nowritebarrier bool // emit compiler error instead of write barrier
 	Dupok          bool // duplicate definitions ok
 	Wrapper        bool // is method wrapper

test/inline.go

@@ -22,3 +22,12 @@ func add1(p unsafe.Pointer, x uintptr) unsafe.Pointer { // ERROR "can inline add
 func f(x *byte) *byte { // ERROR "can inline f" "leaking param: x to result"
 	return add2(x, 1) // ERROR "inlining call to add2" "inlining call to add1"
 }
+
+//go:noinline
+func g(x int) int {
+	return x + 1
+}
+
+func h(x int) int { // ERROR "can inline h"
+	return x + 2
+}