preserve AX across stack jump so C routines return correct value when triggering morestack.

SVN=126935

src/runtime/rt0_amd64.s

diff --git a/src/runtime/rt0_amd64.s b/src/runtime/rt0_amd64.s
index d6dba02..71b1fd8 100644
--- a/src/runtime/rt0_amd64.s
+++ b/src/runtime/rt0_amd64.s
@@ -72,6 +72,25 @@
 	MOVL	$0, AX			// return 0
 	RET
 
+/*
+ * support for morestack
+ */
+
+// return point when leaving new stack.  save AX, jmp to oldstack to switch back
+TEXT retfromnewstack(SB), 7, $0
+	MOVQ	AX, 16(R14)	// save AX in m->cret
+	MOVQ	$oldstack(SB), AX
+	JMP	AX
+
+// gogo, returning 2nd arg instead of 1
+TEXT gogoret(SB), 7, $0
+	MOVQ	16(SP), AX			// return 2nd arg
+	MOVQ	8(SP), BX		// gobuf
+	MOVQ	0(BX), SP		// restore SP
+	MOVQ	8(BX), BX
+	MOVQ	BX, 0(SP)		// put PC on the stack
+	RET
+
 TEXT setspgoto(SB), 7, $0
 	MOVQ	8(SP), AX		// SP
 	MOVQ	16(SP), BX		// fn to call

src/runtime/rt2_amd64.c

diff --git a/src/runtime/rt2_amd64.c b/src/runtime/rt2_amd64.c
index dac1c46..a5f5d10 100644
--- a/src/runtime/rt2_amd64.c
+++ b/src/runtime/rt2_amd64.c
@@ -8,8 +8,6 @@
 
 static int8 spmark[] = "\xa7\xf1\xd9\x2a\x82\xc8\xd8\xfe";
 
-extern void morestack2();
-
 void
 traceback(uint8 *pc, uint8 *sp, void* r15)
 {
@@ -29,8 +27,8 @@
 	name = "panic";
 	for(;;){
 		callpc = pc;
-		if((uint8*)morestack2 == pc) {
-			// call site is morestack2(); pop to earlier stack block to get true caller
+		if((uint8*)retfromnewstack == pc) {
+			// call site is retfromnewstack(); pop to earlier stack block to get true caller
 			stktop = (Stktop*)g.stackbase;
 			g.stackbase = stktop->oldbase;
 			g.stackguard = stktop->oldguard;

src/runtime/runtime.c

diff --git a/src/runtime/runtime.c b/src/runtime/runtime.c
index 743e085..f8dfa95 100644
--- a/src/runtime/runtime.c
+++ b/src/runtime/runtime.c
@@ -698,15 +698,15 @@
 //	CALL	sys·morestack(SB)
 //
 
-int32 debug = 0;
-
 void
-morestack2(void)
+oldstack(void)
 {
 	Stktop *top;
 	uint32 siz2;
 	byte *sp;
-if(debug) prints("morestack2\n");
+if(debug) prints("oldstack m->cret = ");
+if(debug) sys·printpointer((void*)m->cret);
+if(debug) prints("\n");
 
 	top = (Stktop*)m->curg->stackbase;
 
@@ -723,16 +723,16 @@
 
 	m->morestack.SP = top->oldsp+8;
 	m->morestack.PC = (byte*)(*(uint64*)(top->oldsp+8));
-if(debug) prints("morestack2 sp=");
+if(debug) prints("oldstack sp=");
 if(debug) sys·printpointer(m->morestack.SP);
 if(debug) prints(" pc=");
 if(debug) sys·printpointer(m->morestack.PC);
 if(debug) prints("\n");
-	gogo(&m->morestack);
+	gogoret(&m->morestack, m->cret);
 }
 
 void
-morestack1(void)
+newstack(void)
 {
 	int32 siz1, siz2;
 	Stktop *top;
@@ -742,7 +742,7 @@
 	siz1 = m->morearg & 0xffffffffLL;
 	siz2 = (m->morearg>>32) & 0xffffLL;
 
-if(debug) prints("morestack1 siz1=");
+if(debug) prints("newstack siz1=");
 if(debug) sys·printint(siz1);
 if(debug) prints(" siz2=");
 if(debug) sys·printint(siz2);
@@ -778,7 +778,7 @@
 if(debug) prints("fn=");
 if(debug) sys·printpointer(fn);
 if(debug) prints("\n");
-	setspgoto(sp, fn, morestack2);
+	setspgoto(sp, fn, retfromnewstack);
 
 	*(int32*)345 = 123;
 }
@@ -793,7 +793,7 @@
 
 	g = m->g0;
 	m->moresp = (byte*)(&u-1);
-	setspgoto(m->sched.SP, morestack1, nil);
+	setspgoto(m->sched.SP, newstack, nil);
 
 	*(int32*)234 = 123;
 }

src/runtime/runtime.h

diff --git a/src/runtime/runtime.h b/src/runtime/runtime.h
index f7d31a1..3a2e3bd 100644
--- a/src/runtime/runtime.h
+++ b/src/runtime/runtime.h
@@ -82,6 +82,7 @@
 {
 	G*	g0;		// g0 w interrupt stack - must not move
 	uint64	morearg;	// arg to morestack - must not move
+	uint64	cret;	// return value from C - must not move
 	G*	curg;		// current running goroutine
 	Gobuf	sched;
 	Gobuf	morestack;
@@ -148,6 +149,8 @@
  */
 int32	gogo(Gobuf*);
 int32	gosave(Gobuf*);
+int32	gogoret(Gobuf*, uint64);
+void	retfromnewstack(void);
 void	setspgoto(byte*, void(*)(void), void(*)(void));
 void	FLUSH(void*);
 void*	getu(void);