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 issue 8404.

LGTM=khr, josharian
R=josharian, khr, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/112570044

1 files changed, 6 insertions, 6 deletions

diff --git a/src/cmd/gc/walk.c b/src/cmd/gc/walk.c
index f3886cf73..c251af660 100644
--- a/src/cmd/gc/walk.c
+++ b/src/cmd/gc/walk.c
@@ -2875,14 +2875,14 @@ sliceany(Node* n, NodeList **init)
 			lb = N;
 	}
 
-	// dynamic checks convert all bounds to unsigned to save us the bound < 0 comparison
-	// generate
-	//     if hb > bound || lb > hb { panicslice() }
+	// Checking src[lb:hb:cb] or src[lb:hb].
+	// if chk0 || chk1 || chk2 { panicslice() }
 	chk = N;
-	chk0 = N;
-	chk1 = N;
-	chk2 = N;
+	chk0 = N; // cap(src) < cb
+	chk1 = N; // cb < hb for src[lb:hb:cb]; cap(src) < hb for src[lb:hb]
+	chk2 = N; // hb < lb
 
+	// All comparisons are unsigned to avoid testing < 0.
 	bt = types[simtype[TUINT]];
 	if(cb != N && cb->type->width > 4)
 		bt = types[TUINT64];