cmd/gc, runtime: treat slices and strings like pointers in garbage collection

Before, a slice with cap=0 or a string with len=0 might have its
base pointer pointing beyond the actual slice/string data into
the next block. The collector had to ignore slices and strings with
cap=0 in order to avoid misinterpreting the base pointer.

Now, a slice with cap=0 or a string with len=0 still has a base
pointer pointing into the actual slice/string data, no matter what.
The collector can now always scan the pointer, which means
strings and slices are no longer special.

Fixes #8404.

LGTM=khr, josharian
R=josharian, khr, dvyukov
CC=golang-codereviews
https://golang.org/cl/112570044

1 files changed, 11 insertions, 7 deletions

diff --git a/test/slice3.go b/test/slice3.go
index 3cf34b57e7..857eaf3a09 100644
--- a/test/slice3.go
+++ b/test/slice3.go
@@ -19,10 +19,10 @@ var bout *bufio.Writer
 
 func main() {
 	bout = bufio.NewWriter(os.Stdout)
-	
+
 	fmt.Fprintf(bout, "%s", programTop)
 	fmt.Fprintf(bout, "func main() {\n")
-	
+
 	index := []string{
 		"0",
 		"1",
@@ -38,7 +38,7 @@ func main() {
 		"v10",
 		"v20",
 	}
-	
+
 	parse := func(s string) (n int, isconst bool) {
 		if s == "vminus1" {
 			return -1, false
@@ -69,7 +69,7 @@ func main() {
 						iconst && kconst && iv > kv,
 						iconst && base == "array" && iv > Cap,
 						jconst && base == "array" && jv > Cap,
-						kconst && base == "array" && kv > Cap:				
+						kconst && base == "array" && kv > Cap:
 						continue
 					}
 
@@ -82,7 +82,7 @@ func main() {
 						xlen = jv - iv
 						xcap = kv - iv
 					}
-					fmt.Fprintf(bout, "\tcheckSlice(%q, func() []byte { return %s }, %d, %d, %d)\n", expr, expr, xbase, xlen, xcap)									
+					fmt.Fprintf(bout, "\tcheckSlice(%q, func() []byte { return %s }, %d, %d, %d)\n", expr, expr, xbase, xlen, xcap)
 				}
 			}
 		}
@@ -147,9 +147,13 @@ func checkSlice(desc string, f func() []byte, xbase, xlen, xcap int) {
 		println(desc, "=", base, len, cap, "want panic")
 		return
 	}
-	if base != uintptr(xbase) || len != uintptr(xlen) || cap != uintptr(xcap) {
+	if cap != 0 && base != uintptr(xbase) || base >= 10 || len != uintptr(xlen) || cap != uintptr(xcap) {
 		notOK()
-		println(desc, "=", base, len, cap, "want", xbase, xlen, xcap)
+		if cap == 0 {
+			println(desc, "=", base, len, cap, "want", "0-9", xlen, xcap)
+		} else {
+			println(desc, "=", base, len, cap, "want", xbase, xlen, xcap)
+		}
 	}
 }