add stub routines stackalloc() and stackfree().

run oldstack on g0's stack, just like newstack does,
so that oldstack can free the old stack.

R=r
DELTA=53  (44 added, 0 deleted, 9 changed)
OCL=20404
CL=20433

src/runtime/Makefile

@@ -25,6 +25,7 @@ LIBOFILES=\
 	print.$O\
 	rune.$O\
 	proc.$O\
+	stack.$O\
 	string.$O\
 	symtab.$O\
 	sys_file.$O\

src/runtime/proc.c

@@ -567,6 +567,7 @@ oldstack(void)
 	Stktop *top;
 	uint32 siz2;
 	byte *sp;
+	uint64 oldsp, oldpc, oldbase, oldguard;
 
 // printf("oldstack m->cret=%p\n", m->cret);
 
@@ -581,15 +582,36 @@ oldstack(void)
 		mcpy(top->oldsp+16, sp, siz2);
 	}
 
-	// call  no more functions after this point - stackguard disagrees with SP
-	m->curg->stackbase = top->oldbase;
-	m->curg->stackguard = top->oldguard;
-	m->morestack.SP = top->oldsp+8;
-	m->morestack.PC = (byte*)(*(uint64*)(top->oldsp+8));
+	oldsp = (uint64)top->oldsp + 8;
+	oldpc = *(uint64*)(top->oldsp + 8);
+	oldbase = (uint64)top->oldbase;
+	oldguard = (uint64)top->oldguard;
 
+	stackfree((byte*)m->curg->stackguard - 512 - 160);
+
+	m->curg->stackbase = (byte*)oldbase;
+	m->curg->stackguard = (byte*)oldguard;
+	m->morestack.SP = (byte*)oldsp;
+	m->morestack.PC = (byte*)oldpc;
+
+	// These two lines must happen in sequence;
+	// once g has been changed, must switch to g's stack
+	// before calling any non-assembly functions.
+	// TODO(rsc): Perhaps make the new g a parameter
+	// to gogoret and setspgoto, so that g is never
+	// explicitly assigned to without also setting
+	// the stack pointer.
+	g = m->curg;
 	gogoret(&m->morestack, m->cret);
 }
 
+void
+lessstack(void)
+{
+	g = m->g0;
+	setspgoto(m->sched.SP, oldstack, nil);
+}
+
 void
 newstack(void)
 {
@@ -611,7 +633,7 @@ newstack(void)
 
 	if(siz1 < 4096)
 		siz1 = 4096;
-	stk = mal(siz1 + 1024);
+	stk = stackalloc(siz1 + 1024);
 	stk += 512;
 
 	top = (Stktop*)(stk+siz1-sizeof(*top));
@@ -658,3 +680,4 @@ sys·morestack(uint64 u)
 
 	*(int32*)234 = 123;	// never return
 }
+

src/runtime/rt0_amd64.s

@@ -89,10 +89,10 @@ TEXT gosave(SB), 7, $0
  * support for morestack
  */
 
-// return point when leaving new stack.  save AX, jmp to oldstack to switch back
+// return point when leaving new stack.  save AX, jmp to lessstack to switch back
 TEXT retfromnewstack(SB), 7, $0
 	MOVQ	AX, 16(R14)	// save AX in m->cret
-	MOVQ	$oldstack(SB), AX
+	MOVQ	$lessstack(SB), AX
 	JMP	AX
 
 // gogo, returning 2nd arg instead of 1

src/runtime/runtime.h

@@ -277,6 +277,8 @@ G*	malg(int32);
 void	minit(void);
 Func*	findfunc(uint64);
 int32	funcline(Func*, uint64);
+void*	stackalloc(uint32);
+void	stackfree(void*);
 
 /*
  * mutual exclusion locks.  in the uncontended case,

src/runtime/stack.c

@@ -0,0 +1,19 @@
+// 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.
+
+#include "runtime.h"
+
+// Stubs for stack management.
+// In a separate file so they can be overridden during testing of gc.
+
+void*
+stackalloc(uint32 n)
+{
+	return mal(n);
+}
+
+void
+stackfree(void*)
+{
+}