cmd/6l, cmd/8l: emit no-ops to separate zero-stack funcs from nosplits.

The stack overflow checker in the linker uses the spadj field
to determine whether stack space will be large enough or not.
When spadj=0, the checker treats the function as a nosplit
and emits an error although the program is correct.

Also enable the stack checker in 8l.

Fixes #4316.

R=rsc, golang-dev
CC=golang-dev
https://golang.org/cl/6855088

src/cmd/6l/pass.c

@@ -604,6 +604,16 @@ dostkoff(void)
 			p->spadj = autoffset;
 			if(q != P)
 				q->pcond = p;
+		} else {
+			// zero-byte stack adjustment.
+			// Insert a fake non-zero adjustment so that stkcheck can
+			// recognize the end of the stack-splitting prolog.
+			p = appendp(p);
+			p->as = ANOP;
+			p->spadj = -PtrSize;
+			p = appendp(p);
+			p->as = ANOP;
+			p->spadj = PtrSize;
 		}
 		deltasp = autoffset;
 

src/cmd/8l/obj.c

@@ -308,6 +308,7 @@ main(int argc, char *argv[])
 	if(HEADTYPE == Hwindows)
 		dope();
 	dostkoff();
+	dostkcheck();
 	if(debug['p'])
 		if(debug['1'])
 			doprof1();

src/cmd/8l/pass.c

@@ -580,6 +580,16 @@ dostkoff(void)
 			p->spadj = autoffset;
 			if(q != P)
 				q->pcond = p;
+		} else {
+			// zero-byte stack adjustment.
+			// Insert a fake non-zero adjustment so that stkcheck can
+			// recognize the end of the stack-splitting prolog.
+			p = appendp(p);
+			p->as = ANOP;
+			p->spadj = -PtrSize;
+			p = appendp(p);
+			p->as = ANOP;
+			p->spadj = PtrSize;
 		}
 		deltasp = autoffset;
 		

test/fixedbugs/issue4316.go

+// run
+
+// Copyright 2012 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.
+
+// Issue 4316: the stack overflow check in the linker
+// is confused when it encounters a split-stack function
+// that needs 0 bytes of stack space.
+
+package main
+
+type Peano *Peano
+
+func makePeano(n int) *Peano {
+	if n == 0 {
+		return nil
+	}
+	p := Peano(makePeano(n - 1))
+	return &p
+}
+
+var countArg Peano
+var countResult int
+
+func countPeano() {
+	if countArg == nil {
+		countResult = 0
+		return
+	}
+	countArg = *countArg
+	countPeano()
+	countResult++
+}
+
+var s = "(())"
+var pT = 0
+
+func p() {
+	if pT >= len(s) {
+		return
+	}
+	if s[pT] == '(' {
+		pT += 1
+		p()
+		if pT < len(s) && s[pT] == ')' {
+			pT += 1
+		} else {
+			return
+		}
+		p()
+	}
+}
+
+func main() {
+	countArg = makePeano(4096)
+	countPeano()
+	if countResult != 4096 {
+		println("countResult =", countResult)
+		panic("countResult != 4096")
+	}
+
+	p()
+}