code review 112570044: cmd/gc, runtime: treat slices and strings like pointers...

src/cmd/gc/walk.c

 // Copyright 2009 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.
 
 #include        <u.h>
 #include        <libc.h>
 #include        "go.h"
 #include        "../ld/textflag.h"
 
 static  Node*   walkprint(Node*, NodeList**, int);
@@ skipping 2857 matching lines @@
         if(isconst(lb, CTINT)) {
                 lbv = mpgetfix(lb->val.u.xval);
                 if(lbv < 0 || lbv > bv) {
                         yyerror("slice index out of bounds");
                         lbv = -1;
                 }
                 if(lbv == 0)
                         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];
         if(hb != N && hb->type->width > 4)
                 bt = types[TUINT64];
         if(lb != N && lb->type->width > 4)
                 bt = types[TUINT64];
 
         bound = cheapexpr(conv(bound, bt), init);
 
@@ skipping 848 matching lines @@
            !candiscardlist(n->ninit) ||
            !candiscardlist(n->nbody) ||
            !candiscardlist(n->nelse) ||
            !candiscardlist(n->list) ||
            !candiscardlist(n->rlist)) {
                 return 0;
         }
 ········
         return 1;
 }