gc: allow taking address of out parameters

Fixes #186.

R=ken2
CC=golang-dev
https://golang.org/cl/793041

doc/go_spec.html

@@ -5109,7 +5109,7 @@ The following minimal alignment properties are guaranteed:
 	<li><span class="alert">Method expressions are partially implemented.</span></li>
 	<li><span class="alert">Gccgo allows only one init() function per source file.</span></li>
 	<li><span class="alert">Deferred functions cannot access the surrounding function's result parameters.</span></li>
-	<li><span class="alert">Function results are not addressable.</span></li>
+	<li><span class="alert">Function results are not addressable in gccgo.</span></li>
 	<li><span class="alert">Recover is not implemented.</span></li>
 	<li><span class="alert">The implemented version of panic differs from its specification.</span></li>
 </ul>

src/cmd/5g/ggen.c

@@ -7,6 +7,8 @@
 #include "gg.h"
 #include "opt.h"
 
+static Prog *pret;
+
 void
 compile(Node *fn)
 {
@@ -65,6 +67,16 @@ compile(Node *fn)
 	afunclit(&ptxt->from);
 
 	genlist(curfn->enter);
+	
+	pret = nil;
+	if(hasdefer || curfn->exit) {
+		Prog *p1;
+
+		p1 = gjmp(nil);
+		pret = gjmp(nil);
+		patch(p1, pc);
+	}
+
 	genlist(curfn->nbody);
 	checklabels();
 	if(nerrors != 0)
@@ -73,6 +85,14 @@ compile(Node *fn)
 	if(curfn->type->outtuple != 0)
 		ginscall(throwreturn, 0);
 
+	if(pret)
+		patch(pret, pc);
+	ginit();
+	if(curfn->exit)
+		genlist(curfn->exit);
+	gclean();
+	if(nerrors != 0)
+		goto ret;
 	if(hasdefer)
 		ginscall(deferreturn, 0);
 	pc->as = ARET;	// overwrite AEND

src/cmd/6g/ggen.c

@@ -7,6 +7,8 @@
 #include "gg.h"
 #include "opt.h"
 
+static Prog *pret;
+
 void
 compile(Node *fn)
 {
@@ -66,6 +68,16 @@ compile(Node *fn)
 
 	ginit();
 	genlist(curfn->enter);
+
+	pret = nil;
+	if(hasdefer || curfn->exit) {
+		Prog *p1;
+
+		p1 = gjmp(nil);
+		pret = gjmp(nil);
+		patch(p1, pc);
+	}
+
 	genlist(curfn->nbody);
 	gclean();
 	checklabels();
@@ -75,6 +87,14 @@ compile(Node *fn)
 	if(curfn->type->outtuple != 0)
 		ginscall(throwreturn, 0);
 
+	if(pret)
+		patch(pret, pc);
+	ginit();
+	if(curfn->exit)
+		genlist(curfn->exit);
+	gclean();
+	if(nerrors != 0)
+		goto ret;
 	if(hasdefer)
 		ginscall(deferreturn, 0);
 	pc->as = ARET;	// overwrite AEND
@@ -325,8 +345,9 @@ void
 cgen_ret(Node *n)
 {
 	genlist(n->list);		// copy out args
-	if(hasdefer)
-		ginscall(deferreturn, 0);
+	if(hasdefer || curfn->exit)
+		gjmp(pret);
+	else
 		gins(ARET, N, N);
 }
 

src/cmd/8g/ggen.c

@@ -7,6 +7,8 @@
 #include "gg.h"
 #include "opt.h"
 
+static Prog *pret;
+
 void
 compile(Node *fn)
 {
@@ -66,6 +68,16 @@ compile(Node *fn)
 
 	ginit();
 	genlist(curfn->enter);
+
+	pret = nil;
+	if(hasdefer || curfn->exit) {
+		Prog *p1;
+
+		p1 = gjmp(nil);
+		pret = gjmp(nil);
+		patch(p1, pc);
+	}
+
 	genlist(curfn->nbody);
 	gclean();
 	checklabels();
@@ -75,6 +87,14 @@ compile(Node *fn)
 	if(curfn->type->outtuple != 0)
 		ginscall(throwreturn, 0);
 
+	if(pret)
+		patch(pret, pc);
+	ginit();
+	if(curfn->exit)
+		genlist(curfn->exit);
+	gclean();
+	if(nerrors != 0)
+		goto ret;
 	if(hasdefer)
 		ginscall(deferreturn, 0);
 	pc->as = ARET;	// overwrite AEND
@@ -362,8 +382,9 @@ void
 cgen_ret(Node *n)
 {
 	genlist(n->list);		// copy out args
-	if(hasdefer)
-		ginscall(deferreturn, 0);
+	if(pret)
+		gjmp(pret);
+	else
 		gins(ARET, N, N);
 }
 

src/cmd/gc/typecheck.c

@@ -1988,17 +1988,15 @@ addrescapes(Node *n)
 		if(n->noescape)
 			break;
 		switch(n->class) {
-		case PPARAMOUT:
-			yyerror("cannot take address of out parameter %s", n->sym->name);
-			break;
 		case PAUTO:
 		case PPARAM:
+		case PPARAMOUT:
 			// if func param, need separate temporary
 			// to hold heap pointer.
 			// the function type has already been checked
 			// (we're in the function body)
 			// so the param already has a valid xoffset.
-			if(n->class == PPARAM) {
+			if(n->class == PPARAM || n->class == PPARAMOUT) {
 				// expression to refer to stack copy
 				n->stackparam = nod(OPARAM, n, N);
 				n->stackparam->type = n->type;

src/cmd/gc/walk.c

@@ -2288,11 +2288,33 @@ paramstoheap(Type **argin)
 		if(v->alloc == nil)
 			v->alloc = callnew(v->type);
 		nn = list(nn, nod(OAS, v->heapaddr, v->alloc));
+		if((v->class & ~PHEAP) != PPARAMOUT)
 			nn = list(nn, nod(OAS, v, v->stackparam));
 	}
 	return nn;
 }
 
+/*
+ * walk through argout parameters copying back to stack
+ */
+NodeList*
+returnsfromheap(Type **argin)
+{
+	Type *t;
+	Iter savet;
+	Node *v;
+	NodeList *nn;
+
+	nn = nil;
+	for(t = structfirst(&savet, argin); t != T; t = structnext(&savet)) {
+		v = t->nname;
+		if(v == N || v->class != (PHEAP|PPARAMOUT))
+			continue;
+		nn = list(nn, nod(OAS, v->stackparam, v));
+	}
+	return nn;
+}
+
 /*
  * take care of migrating any function in/out args
  * between the stack and the heap.  adds code to
@@ -2305,7 +2327,9 @@ heapmoves(void)
 
 	nn = paramstoheap(getthis(curfn->type));
 	nn = concat(nn, paramstoheap(getinarg(curfn->type)));
+	nn = concat(nn, paramstoheap(getoutarg(curfn->type)));
 	curfn->enter = concat(curfn->enter, nn);
+	curfn->exit = returnsfromheap(getoutarg(curfn->type));
 }
 
 static Node*

test/escape.go

@@ -141,6 +141,24 @@ func for_escapes2(x int, y int) (*int, *int) {
 	return p[0], p[1]
 }
 
+func out_escapes(i int) (x int, p *int) {
+	x = i
+	p = &x;	// ERROR "address of out parameter"
+	return;
+}
+
+func out_escapes_2(i int) (x int, p *int) {
+	x = i
+	return x, &x;	// ERROR "address of out parameter"
+}
+
+func defer1(i int) (x int) {
+	c := make(chan int)
+	go func() { x = i; c <- 1 }()
+	<-c
+	return
+}
+
 func main() {
 	p, q := i_escapes(1), i_escapes(2);
 	chk(p, q, 1, "i_escapes");
@@ -169,6 +187,20 @@ func main() {
 	p, q = for_escapes2(103, 104);
 	chkalias(p, q, 103, "for_escapes2");
 
+	_, p = out_escapes(15)
+	_, q = out_escapes(16);
+	chk(p, q, 15, "out_escapes");
+
+	_, p = out_escapes_2(17)
+	_, q = out_escapes_2(18);
+	chk(p, q, 17, "out_escapes_2");
+
+	x := defer1(20)
+	if x != 20 {
+		println("defer failed", x)
+		bad = true
+	}
+
 	if bad {
 		panic("BUG: no escape");
 	}

test/escape1.go

-// errchk $G $D/$F.go
-
-// Copyright 2009 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package main
-
-func out_escapes() (x int, p *int) {
-	p = &x;	// ERROR "address of out parameter"
-	return;
-}
-
-func out_escapes_2() (x int, p *int) {
-	return 2, &x;	// ERROR "address of out parameter"
-}
-