cmd/gc: add way to specify 'noescape' for extern funcs

A new comment directive //go:noescape instructs the compiler
that the following external (no body) func declaration should be
treated as if none of its arguments escape to the heap.

Fixes #4099.

R=golang-dev, dave, minux.ma, daniel.morsing, remyoudompheng, adg, agl, iant
CC=golang-dev
https://golang.org/cl/7289048

src/cmd/gc/doc.go

@@ -25,6 +25,8 @@ other packages. It is therefore not necessary when compiling client C of
 package P to read the files of P's dependencies, only the compiled output
 of P.
 
+Command Line
+
 Usage:
 	go tool 6g [flags] file...
 The specified files must be Go source files and all part of the same package.
@@ -48,7 +50,7 @@ Flags:
 		disable optimizations
 	-S
 		write assembly language text to standard output (code only)
-	-SS
+	-S -S
 		write assembly language text to standard output (code and data)
 	-u
 		disallow importing packages not marked as safe
@@ -60,5 +62,28 @@ Flags:
 There are also a number of debugging flags; run the command with no arguments
 to get a usage message.
 
+Compiler Directives
+
+The compiler accepts two compiler directives in the form of // comments at the
+beginning of a line. To distinguish them from non-directive comments, the directives
+require no space between the slashes and the name of the directive. However, since
+they are comments, tools unaware of the directive convention or of a particular
+directive can skip over a directive like any other comment.
+
+    //line path/to/file:linenumber
+
+The //line directive specifies that the source line that follows should be recorded
+as having come from the given file path and line number. Successive lines are
+recorded using increasing line numbers, until the next directive. This directive
+typically appears in machine-generated code, so that compilers and debuggers
+will show lines in the original input to the generator.
+
+    //go:noescape
+
+The //go:noescape directive specifies that the next declaration in the file, which
+must be a func without a body (meaning that it has an implementation not written
+in Go) does not allow any of the pointers passed as arguments to escape into the
+heap or into the values returned from the function. This information can be used as
+during the compiler's escape analysis of Go code calling the function.
 */
 package documentation

src/cmd/gc/esc.c

@@ -330,7 +330,10 @@ escfunc(EscState *e, Node *func)
 		case PPARAM:
 			if(ll->n->type && !haspointers(ll->n->type))
 				break;
-			ll->n->esc = EscNone;	// prime for escflood later
+			if(curfn->nbody == nil && !curfn->noescape)
+				ll->n->esc = EscHeap;
+			else
+				ll->n->esc = EscNone;	// prime for escflood later
 			e->noesc = list(e->noesc, ll->n);
 			ll->n->escloopdepth = 1; 
 			break;
@@ -1109,13 +1112,21 @@ esctag(EscState *e, Node *func)
 {
 	Node *savefn;
 	NodeList *ll;
-
+	Type *t;
+
 	USED(e);
 	func->esc = EscFuncTagged;
 
-	// External functions must be assumed unsafe.
-	if(func->nbody == nil)
+	// External functions are assumed unsafe,
+	// unless //go:noescape is given before the declaration.
+	if(func->nbody == nil) {
+		if(func->noescape) {
+			for(t=getinargx(func->type)->type; t; t=t->down)
+				if(haspointers(t->type))
+					t->note = mktag(EscNone);
+		}
 		return;
+	}
 
 	savefn = curfn;
 	curfn = func;

src/cmd/gc/go.h

@@ -253,6 +253,7 @@ struct	Node
 	uchar	colas;		// OAS resulting from :=
 	uchar	diag;		// already printed error about this
 	uchar	esc;		// EscXXX
+	uchar	noescape;	// func arguments do not escape
 	uchar	funcdepth;
 	uchar	builtin;	// built-in name, like len or close
 	uchar	walkdef;
@@ -943,6 +944,7 @@ EXTERN	int	compiling_wrappers;
 EXTERN	int	pure_go;
 EXTERN	int	flag_race;
 EXTERN	int	flag_largemodel;
+EXTERN	int	noescape;
 
 EXTERN	int	nointerface;
 EXTERN	int	fieldtrack_enabled;

src/cmd/gc/go.y

@@ -1277,8 +1277,11 @@ xfndcl:
 		$$ = $2;
 		if($$ == N)
 			break;
+		if(noescape && $3 != nil)
+			yyerror("can only use //go:noescape with external func implementations");
 		$$->nbody = $3;
 		$$->endlineno = lineno;
+		$$->noescape = noescape;
 		funcbody($$);
 	}
 
@@ -1462,6 +1465,7 @@ xdcl_list:
 		if(nsyntaxerrors == 0)
 			testdclstack();
 		nointerface = 0;
+		noescape = 0;
 	}
 
 vardcl_list:

src/cmd/gc/lex.c

@@ -1436,7 +1436,7 @@ getlinepragma(void)
 	Hist *h;
 
 	c = getr();
-	if(c == 'g' && fieldtrack_enabled)
+	if(c == 'g')
 		goto go;
 	if(c != 'l')	
 		goto out;
@@ -1491,18 +1491,32 @@ getlinepragma(void)
 	goto out;
 
 go:
-	for(i=1; i<11; i++) {
-		c = getr();
-		if(c != "go:nointerface"[i])
-			goto out;
-	}
-	nointerface = 1;
+	cp = lexbuf;
+	ep = lexbuf+sizeof(lexbuf)-5;
+	*cp++ = 'g'; // already read
 	for(;;) {
 		c = getr();
-		if(c == EOF || c == '\n')
+		if(c == EOF || c >= Runeself)
+			goto out;
+		if(c == '\n')
 			break;
+		if(cp < ep)
+			*cp++ = c;
 	}
+	*cp = 0;
+	ep = strchr(lexbuf, ' ');
+	if(ep != nil)
+		*ep = 0;
 
+	if(strcmp(lexbuf, "go:nointerface") == 0 && fieldtrack_enabled) {
+		nointerface = 1;
+		goto out;
+	}
+	if(strcmp(lexbuf, "go:noescape") == 0) {
+		noescape = 1;
+		goto out;
+	}
+
 out:
 	return c;
 }