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libc: compress directly recursive functions while profiling

When a function calls itself, the execution slot of its child is now
just added to its own time.  This makes conceptual sense and also
reduces a big cause of depth inflation.

--- a/sys/src/libc/port/profile.c

+++ b/sys/src/libc/port/profile.c

@@ -19,6 +19,7 @@

 	long	pc;

 	long	count;

 	vlong	time;

+	uint	rec;

 };

 #pragma profile off

@@ -44,6 +45,11 @@

 	pp = _tos->prof.pp;

 	if(pp == 0 || (_tos->prof.pid && _tos->pid != _tos->prof.pid))

 		return _restore(arg, ret);

+	if(pc == pp->pc){

+		pp->rec++;

+		p = pp;

+		goto out;

+	}

 	for(p=pp->down; p; p=p->link)

 		if(p->pc == pc)

 			goto out;

@@ -113,7 +119,10 @@

 		p->time = p->time + _tos->clock;

 		break;

 	}

-	_tos->prof.pp = p->old;

+	if(p->rec)

+		p->rec--;

+	else

+		_tos->prof.pp = p->old;

 	return _restore(arg, ret);

 }