cmd/gc: do simple bounds checking of constant indices/slices in typecheck.

This should make the compiler emit errors specific to the bounds checking instead of overflow errors on the underlying types.

Updates #4232.

R=rsc
CC=golang-dev
https://golang.org/cl/6783054

diff --git a/src/cmd/gc/typecheck.c b/src/cmd/gc/typecheck.c
index cebc2ff8a9262b66f062312486e2aedad54c5dca..6f3449eec3cb34a4a5d294cba7dcfbd4a7ee884a 100644 (file)
--- a/src/cmd/gc/typecheck.c
+++ b/src/cmd/gc/typecheck.c
@@ -817,9 +817,18 @@ reswitch:
 
                case TARRAY:
                        defaultlit(&n->right, T);
-                       if(n->right->type != T && !isint[n->right->type->etype])
-                               yyerror("non-integer array index %N", n->right);
                        n->type = t->type;
+                       if(n->right->type != T && !isint[n->right->type->etype]) {
+                               yyerror("non-integer array index %N", n->right);
+                               break;
+                       }
+                       if(n->right->op == OLITERAL) {
+                               if(mpgetfix(n->right->val.u.xval) < 0) {
+                                       why = isfixedarray(t) ? "array" : "slice";
+                                       yyerror("invalid %s index %N (index must be non-negative)", why, n->right);
+                               } else if(isfixedarray(t) && t->bound > 0 && mpgetfix(n->right->val.u.xval) >= t->bound)
+                                       yyerror("invalid array index %N (out of bounds for %d-element array)", n->right, t->bound);
+                       }
                        break;
 
                case TMAP:
@@ -912,6 +921,8 @@ reswitch:
                                yyerror("invalid slice index %N (type %T)", n->right->left, t);
                                goto error;
                        }
+                       if(n->right->left->op == OLITERAL && mpgetfix(n->right->left->val.u.xval) < 0)
+                               yyerror("invalid slice index %N (index must be non-negative)", n->right->left);
                }
                if(n->right->right != N) {
                        if((t = n->right->right->type) == T)
@@ -920,6 +931,8 @@ reswitch:
                                yyerror("invalid slice index %N (type %T)", n->right->right, t);
                                goto error;
                        }
+                       if(n->right->right->op == OLITERAL && mpgetfix(n->right->right->val.u.xval) < 0)
+                               yyerror("invalid slice index %N (index must be non-negative)", n->right->right);
                }
                l = n->left;
                if((t = l->type) == T)