Address compiler bug with `break`.

Using result from `break` expression could trigger code that would
work, yet contain invalid, dead code preventing good marshalling.

src/core/emit.c

@@ -26,6 +26,7 @@
 #include "emit.h"
 #include "vector.h"
 #include "regalloc.h"
+#include "util.h"
 #endif
 
 /* Get a register */
@@ -128,7 +129,8 @@ static void janetc_movenear(JanetCompiler *c,
                     ((uint32_t)(src.envindex) << 16) |
                     ((uint32_t)(dest) << 8) |
                     JOP_LOAD_UPVALUE);
-    } else if (src.index > 0xFF || src.index != dest) {
+    } else if (src.index != dest) {
+        janet_assert(src.index >= 0, "bad slot");
         janetc_emit(c,
                     ((uint32_t)(src.index) << 16) |
                     ((uint32_t)(dest) << 8) |
@@ -155,6 +157,7 @@ static void janetc_moveback(JanetCompiler *c,
                     ((uint32_t)(src) << 8) |
                     JOP_SET_UPVALUE);
     } else if (dest.index != src) {
+        janet_assert(dest.index >= 0, "bad slot");
         janetc_emit(c,
                     ((uint32_t)(dest.index) << 16) |
                     ((uint32_t)(src) << 8) |

src/core/specials.c

@@ -752,9 +752,8 @@ static JanetSlot janetc_break(JanetFopts opts, int32_t argn, const Janet *argv)
         if (!(scope->flags & JANET_SCOPE_WHILE) && argn) {
             /* Closure body with return argument */
             subopts.flags |= JANET_FOPTS_TAIL;
-            JanetSlot ret = janetc_value(subopts, argv[0]);
-            ret.flags |= JANET_SLOT_RETURNED;
-            return ret;
+            janetc_value(subopts, argv[0]);
+            return janetc_cslot(janet_wrap_nil());
         } else {
             /* while loop IIFE or no argument */
             if (argn) {
@@ -762,9 +761,7 @@ static JanetSlot janetc_break(JanetFopts opts, int32_t argn, const Janet *argv)
                 janetc_value(subopts, argv[0]);
             }
             janetc_emit(c, JOP_RETURN_NIL);
-            JanetSlot s = janetc_cslot(janet_wrap_nil());
-            s.flags |= JANET_SLOT_RETURNED;
-            return s;
+            return janetc_cslot(janet_wrap_nil());
         }
     } else {
         if (argn) {

test/suite-boot.janet

@@ -924,4 +924,25 @@
                        [:strict 3 4 "bar-oops"]])
         "maclintf 2")
 
+# Bad bytecode wrt. using result from break expression
+(defn bytecode-roundtrip
+  [f]
+  (assert-no-error "bytecode round-trip" (unmarshal (marshal f make-image-dict))))
+
+(defn case-1 [&] (def x (break 1)))
+(bytecode-roundtrip case-1)
+(defn foo [&])
+(defn case-2 [&]
+  (foo (break (foo)))
+  (foo))
+(bytecode-roundtrip case-2)
+(defn case-3 [&]
+  (def x (break (do (foo)))))
+(bytecode-roundtrip case-3)
+
+# Debug bytecode of these functions
+# (pp (disasm case-1))
+# (pp (disasm case-2))
+# (pp (disasm case-3))
+
 (end-suite)