[tree-ssa]: PRE updates and note on bug

[Daniel Berlin]

On Tuesday, July 22, 2003, at 2:27 PM, Andrew MacLeod wrote:

> On Tue, 2003-07-22 at 14:19, Daniel Berlin wrote:
>>
>>> As I said, it could be the reverse iterators.
>>> If the end of the bb is an empty statement, bsi = bsi_last (bb);
>>> bsi_prev (bsi), doesn't work.
>>
>> As an FYI, the reason it doesn't work is because of bsi_last:
>>
>>   /* If the last stmt pointer isn't something a BSI can represent 
>> (ie, an
>>       empty statement node), then find the last stmt the slow way.  */
>>    if (b.tp == NULL)
>>      {
>>
>>
>> This condition is wrong for the comment.
>> If BSI's can't represent empty statement nodes, it needs to check
>> at least
>> 1. IS_EMPTY_STMT (*b.tp)
>> and I believe
>> 2. If *b.tp == COMPOUND_EXPR, IS_EMPTY_STMT (the first operand).
>>
> b = bsi_init (bb->end_tree_p, bb);
>
> The call to bsi_init() above it ought to be setting b.tp to NULL if it
> can't be represented by a BSI node. (like an empty stmt node of a
> BIND_EXPR or whetever).

Well, it's not in some cases, particularly, empty statement node cases. 
:)
I presume this is because bsi_init only looks *forwards* for the next 
non-empty statement, when for bsi_last, it needs to look *backwards*.



I was getting an empty stmt for end of bb using last_stmt and 
last_stmt_ptr, but if i replace it with:

       {
         block_stmt_iterator bsi2;
         block_stmt_iterator bsi3;
         for (bsi2 = bsi3 = bsi_start (bb);
        !bsi_end_p (bsi2);
        bsi_next (&bsi2))
           bsi3 = bsi2;
         endtree = bsi_stmt (bsi3);
         endtreep = bsi_stmt_ptr (bsi3);
       }

(which is the code that bsi_last falls back on), it works fine and 
gives the correct statement.

> The NULL condition means it cant be
> represented...
>
> Andrew
>