runtime: let stack copier update Panic structs for us

It already is updating parts of them; we're just getting lucky
retraversing them and not finding much to do.
Change argp to a pointer so that it will be updated too.
Existing tests break if you apply the change to adjustpanics
without also updating the type of argp.

LGTM=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/139380043

src/runtime/panic.go

@@ -287,9 +287,9 @@ func gopanic(e interface{}) {
 		gp._defer = (*_defer)(noescape(unsafe.Pointer(&dabort)))
 		p._defer = d
 
-		p.argp = getargp(0)
+		p.argp = unsafe.Pointer(getargp(0))
 		reflectcall(unsafe.Pointer(d.fn), unsafe.Pointer(&d.args), uint32(d.siz), uint32(d.siz))
-		p.argp = 0
+		p.argp = nil
 
 		// reflectcall did not panic. Remove dabort.
 		if gp._defer != &dabort {
@@ -362,7 +362,7 @@ func gorecover(argp uintptr) interface{} {
 	// If they match, the caller is the one who can recover.
 	gp := getg()
 	p := gp._panic
-	if p != nil && !p.recovered && argp == p.argp {
+	if p != nil && !p.recovered && argp == uintptr(p.argp) {
 		p.recovered = true
 		return p.arg
 	}

src/runtime/runtime.h

@@ -653,7 +653,7 @@ struct Defer
  */
 struct Panic
 {
-	uintptr	argp;	// pointer to arguments of deferred call run during panic; cannot move - known to liblink
+	void*	argp;	// pointer to arguments of deferred call run during panic; cannot move - known to liblink
 	Eface	arg;		// argument to panic
 	Panic*	link;		// link to earlier panic
 	Defer*	defer;		// current executing defer

src/runtime/stack.c

@@ -712,6 +712,10 @@ adjustdefers(G *gp, AdjustInfo *adjinfo)
 			// get adjusted appropriately.
 			// This only happens for runtime.main and runtime.gopanic now,
 			// but a compiler optimization could do more of this.
+			// If such an optimization were introduced, Defer.argp should
+			// change to have pointer type so that it will be updated by
+			// the stack copying. Today both of those on-stack defers
+			// set argp = NoArgs, so no adjustment is necessary.
 			*dp = (Defer*)((byte*)d + adjinfo->delta);
 			continue;
 		}
@@ -751,17 +755,11 @@ adjustdefers(G *gp, AdjustInfo *adjinfo)
 static void
 adjustpanics(G *gp, AdjustInfo *adjinfo)
 {
-	Panic *p, **l;
-
-	// only the topmost panic is on the current stack
-	for(l = &gp->panic; (p = *l) != nil; ) {
-		if(adjinfo->oldstk <= (byte*)p && (byte*)p < adjinfo->oldbase)
-			*l = (Panic*)((byte*)p + adjinfo->delta);
-		l = &p->link;
-
-		if(adjinfo->oldstk <= (byte*)p->argp && (byte*)p->argp < adjinfo->oldbase)
-			p->argp += adjinfo->delta;
-	}
+	// Panic structs are all on the stack
+	// and are adjusted by stack copying.
+	// The only pointer we need to update is gp->panic, the head of the list.
+	if(adjinfo->oldstk <= (byte*)gp->panic && (byte*)gp->panic < adjinfo->oldbase)
+		gp->panic = (Panic*)((byte*)gp->panic + adjinfo->delta);
 }
 
 static void