Re: Problem report #5

[Andreas Schwab]

"Stuart D. Herring" <address@hidden> writes:

>> 3140           prev = NULL;
>> 3141           for (tail = buf->overlays_before; tail; prev = tail, tail = 
>> next)
>> 3142             {
>> 3143               next = tail->next;
>> 3144               XSETMISC (overlay, tail);
>> 3145
>> 3146               /* If the overlay is not valid, get rid of it.  */
>> 3147               if (!OVERLAY_VALID (overlay))
>> [...]
>> 3161
>> 3162               beg = OVERLAY_START (overlay);
>> 3163               end = OVERLAY_END (overlay);
>> 3164
>> 3165               if (OVERLAY_POSITION (end) > pos)
>> 3166                 {
>> 3167                   /* OVERLAY needs to be moved.  */
>> 3168                   int where = OVERLAY_POSITION (beg);
>> 3169                   struct Lisp_Overlay *other, *other_prev;
>> 3170
>> 3171                   /* Splice the cons cell TAIL out of overlays_before.  
>> */
>>
>> Event dead_error_condition: On this path, the condition "prev != 0" could
>> not be true
>> Also see events: [dead_error_line][assignment][const][const][assignment]
>>
>> 3172                   if (prev)
>
> It's true that tail is never 0 within the loop (loop test), and that on
> every iteration but the first prev has been assigned tail, which wasn't 0
> at the time of assignment.  But on the first iteration, this test is
> guaranteed to pass!  No bug.

The point is that prev can never get non-NULL because later prev (while
still being NULL) is assigned to tail, or else the loop is exited right
away in the false arm of the conditional in line 3165.  But the comment
before the loop explains why it is written this way:

  /* We don't strictly need prev in this loop; it should always be nil.
     But we use it for symmetry and in case that should cease to be true
     with some future change.  */

So this is on purpose.

Andreas.

-- 
Andreas Schwab, SuSE Labs, address@hidden
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
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