[Bug middle-end/50251] [4.7 Regression] Revision 178353 caused many test failures

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50251

--- Comment #15 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-09-02 11:02:22 UTC ---
(In reply to comment #14)
> > but for some reason it doesn't trigger?
> 
> The bb containing the __builtin_stack_restore has 2 successors:
> ...
> <bb 6>:
>   D.2099_18 = MEM[(int *)&D.2129][5]{lb: 0 sz: 4};
>   __builtin_stack_restore (saved_stack.3_5);
>   if (D.2099_18 != 15)
>     goto <bb 7>;
>   else
>     goto <bb 8>;
> ...
> 
> This case doesn't fall into the cases described in the header comment.
> ...
> /* Try to optimize out __builtin_stack_restore.  Optimize it out
>    if there is another __builtin_stack_restore in the same basic
>    block and no calls or ASM_EXPRs are in between, or if this block's
>    only outgoing edge is to EXIT_BLOCK and there are no calls or
>    ASM_EXPRs after this __builtin_stack_restore.  */
> ...
> 
> It hits this piece of code in optimize_stack_restore and returns, so it doesn't
> reach second_stack_restore:
> ...
>   /* Allow one successor of the exit block, or zero successors.  */
>   switch (EDGE_COUNT (bb->succs))
>     {
>     case 0:
>       break;
>     case 1:
>       if (single_succ_edge (bb)->dest != EXIT_BLOCK_PTR)
>     return NULL_TREE;
>       break;
>     default:
>       return NULL_TREE;
>     }
>  second_stack_restore:
> ...
> 
> 
> For the stack-save-restore.c, if I declare p inside the function, cse creates a
> (set (sp) (sp)), which is eliminated.
> But for the 20010209-1.c example, that doesn't happen.

Ok, the code really tries to optimize the case of __builtin_stack_restore
being called right before exiting the function only.

That's because it doesn't look for alloca() calls inbetween the
stack-save/restore pair at all (just for ones following a restore).