Problem report #5

Problem report #5

[Dan Nicolaescu]

ERROR
CID: 5
Checker: DEADCODE (help)
File: emacs/src/buffer.c
Function: recenter_overlay_lists
Description: Assigning "tail" to "prev"

3139 	     with some future change.  */

Event assignment: Assigning "0" to "prev"
Event const: After this line, the value of "prev" is equal to 0
Also see events: [dead_error_line][dead_error_condition][const][const][assignment]

3140 	  prev = NULL;

Event const: After this line, the value of "prev" is equal to 0
Event assignment: Assigning "tail" to "prev"
Also see events: [dead_error_line][dead_error_condition][assignment][const][assignment]

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))
3148 	#if 1
3149 		abort ();
3150 	#else
3151 		{
3152 		  /* Splice the cons cell TAIL out of overlays_before.  */
3153 		  if (!NILP (prev))
3154 		    XCDR (prev) = next;
3155 		  else
3156 		    buf->overlays_before = next;
3157 		  tail = prev;
3158 		  continue;
3159 		}
3160 	#endif
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)

Event dead_error_line: Cannot reach this line of code
Also see events: [dead_error_condition][assignment][const][const][assignment]

3173 		    prev->next = next;
3174 		  else
3175 		    buf->overlays_before = next;
3176 	
3177 		  /* Search thru overlays_after for where to put it.  */

Problem report #5

[Dan Nicolaescu]

CID: 5
Checker: DEADCODE (help)
File: emacs/src/buffer.c
Function: recenter_overlay_lists
Description: Assigning "tail" to "prev"

Event assignment: Assigning "0" to "prev"
Event const: After this line, the value of "prev" is equal to 0
Also see events: [dead_error_line][dead_error_condition][const][const][assignment]

3140 	  prev = NULL;

Event const: After this line, the value of "prev" is equal to 0
Event assignment: Assigning "tail" to "prev"
Also see events: [dead_error_line][dead_error_condition][assignment][const][assignment]

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))
3148 	#if 1
3149 		abort ();
3150 	#else
3151 		{
3152 		  /* Splice the cons cell TAIL out of overlays_before.  */
3153 		  if (!NILP (prev))
3154 		    XCDR (prev) = next;
3155 		  else
3156 		    buf->overlays_before = next;
3157 		  tail = prev;
3158 		  continue;
3159 		}
3160 	#endif
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)

Event dead_error_line: Cannot reach this line of code
Also see events: [dead_error_condition][assignment][const][const][assignment]

3173 		    prev->next = next;
3174 		  else
3175 		    buf->overlays_before = next;

Re: Problem report #5

[Stuart D. Herring]

> 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.

Davis

-- 
This product is sold by volume, not by mass.  If it appears too dense or
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shipping.

Re: Problem report #5

[Andreas Schwab]

"Stuart D. Herring" <herring@lanl.gov> 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, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
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