Tweaks to make libthread tell Valgrind about its stack limits,

so that Valgrind can detect stack overflow for me.

src/libthread/386.c

@@ -1,8 +1,23 @@
 #include "threadimpl.h"
+/*
+ * To use this you need some patches to Valgrind that
+ * let it help out with detecting stack overflow. 
+ */
+#define USEVALGRIND 0
+#ifdef USEVALGRIND
+#include <valgrind/memcheck.h>
+#endif
 
 static void
 launcher386(void (*f)(void *arg), void *arg)
 {
+	Proc *p;
+	Thread *t;
+
+	p = _threadgetproc();
+	t = p->thread;
+	_threadstacklimit(t->stk);
+
 	(*f)(arg);
 	threadexits(nil);
 }
@@ -19,3 +34,24 @@ _threadinitstack(Thread *t, void (*f)(void*), void *arg)
 	t->sched.sp = (ulong)tos - 8;		/* old PC and new PC */
 }
 
+void
+_threadinswitch(int enter)
+{
+	USED(enter);
+#ifdef USEVALGRIND
+	if(enter)
+		VALGRIND_SET_STACK_LIMIT(0, 0, 1);
+	else
+		VALGRIND_SET_STACK_LIMIT(0, 0, 0);
+#endif
+}
+
+void
+_threadstacklimit(void *addr)
+{
+	USED(addr);
+
+#ifdef USEVALGRIND
+	VALGRIND_SET_STACK_LIMIT(1, addr, 0);
+#endif
+}