cmd/gc: fix infinite loop in nil check removal

Fixes #8076.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/93610043

diff --git a/src/cmd/gc/popt.c b/src/cmd/gc/popt.c
index d724637677f9abc3860990619fc459959a5c39bc..ea88b94dbeb928cbafd5553e02fe4120620083d6 100644 (file)
--- a/src/cmd/gc/popt.c
+++ b/src/cmd/gc/popt.c
@@ -956,7 +956,7 @@ nilwalkback(NilFlow *rcheck)
 static void
 nilwalkfwd(NilFlow *rcheck)
 {
-       NilFlow *r;
+       NilFlow *r, *last;
        Prog *p;
        ProgInfo info;
        
@@ -967,6 +967,7 @@ nilwalkfwd(NilFlow *rcheck)
        // avoid problems like:
        //      _ = *x // should panic
        //      for {} // no writes but infinite loop may be considered visible
+       last = nil;
        for(r = (NilFlow*)uniqs(&rcheck->f); r != nil; r = (NilFlow*)uniqs(&r->f)) {
                p = r->f.prog;
                proginfo(&info, p);
@@ -989,5 +990,12 @@ nilwalkfwd(NilFlow *rcheck)
                // Stop if memory write.
                if((info.flags & RightWrite) && !regtyp(&p->to))
                        return;
+               // Stop if we jump backward.
+               // This test is valid because all the NilFlow* are pointers into
+               // a single contiguous array. We will need to add an explicit
+               // numbering when the code is converted to Go.
+               if(last != nil && r <= last)
+                       return;
+               last = r;
        }
 }

diff --git a/test/fixedbugs/issue8076.go b/test/fixedbugs/issue8076.go
new file mode 100644 (file)
index 0000000..ad89067
--- /dev/null
+++ b/test/fixedbugs/issue8076.go
@@ -0,0 +1,17 @@
+// compile
+
+// Copyright 2014 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 8076. nilwalkfwd walked forward forever
+// on the instruction loop following the dereference.
+
+package main
+
+func main() {
+       _ = *(*int)(nil)
+L:
+       _ = 0
+       goto L
+}