Re: [PATCH] PR/17635

[Devang Patel <dpatel at apple dot com>]

On Sep 24, 2004, at 2:54 PM, Richard Henderson wrote:

> On Fri, Sep 24, 2004 at 09:49:34AM -0700, Devang Patel wrote:
> > 	* tree-if-conv.c (if_convertable_loop_p): Check for loop with zero
> > exits.
>
> It strikes me as likely that you're papering over some bug.
> I don't see any reason why a zero exit loop shouldn't be
> handled by your pass.

If-conversion pass does not allow loops with non-empty
basic blocks after exit edge. So latch is either empty or
bb with exit edge. However, in this case there is no exit
and latch is non-empty.

Alternatives
1) Reject such loops
2) Take care of non-empty latch in such cases and update
   loop structure after blocks are merged.

Following patch implements 2). Bootstrapped on powerpc-darwin
with -ftree-vectorize (to enable if-conversion during bootstrap).
DejaGNU run is in progress.

     * tree-if-conv.c (process_phi_nodes): Process latch block.
	(combine_blocks): Process latch block and update loop structure.

thoughts?
-
Devang

Index: tree-if-conv.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/tree-if-conv.c,v
retrieving revision 2.10
diff -Idpatel.pbxuser -c -3 -p -r2.10 tree-if-conv.c
*** tree-if-conv.c      28 Sep 2004 07:59:51 -0000      2.10
--- tree-if-conv.c      5 Oct 2004 16:03:18 -0000
*************** process_phi_nodes (struct loop *loop)
*** 815,821 ****
        basic_block true_bb = NULL;
        bb = ifc_bbs[i];

!       if (bb == loop->header || bb == loop->latch)
        continue;

        phi = phi_nodes (bb);
--- 815,821 ----
        basic_block true_bb = NULL;
        bb = ifc_bbs[i];

!       if (bb == loop->header)
        continue;

        phi = phi_nodes (bb);
*************** combine_blocks (struct loop *loop)
*** 863,871 ****

        bb = ifc_bbs[i];

-       if (bb == loop->latch)
-       continue;
-
        if (!exit_bb && bb_with_exit_edge_p (bb))
          exit_bb = bb;

--- 863,868 ----
*************** combine_blocks (struct loop *loop)
*** 891,896 ****
--- 888,896 ----
          continue;
        }

+       if (bb == loop->latch && empty_block_p (bb))
+       continue;
+
        /* It is time to remove this basic block.        First remove 
edges.  */
        while (EDGE_COUNT (bb->succs) > 0)
        ssa_remove_edge (EDGE_SUCC (bb, 0));
*************** combine_blocks (struct loop *loop)
*** 921,926 ****
--- 921,928 ----
        delete_from_dominance_info (CDI_POST_DOMINATORS, bb);

        /* Remove basic block.  */
+       if (bb == loop->latch)
+       loop->latch = merge_target_bb;
        remove_bb_from_loops (bb);
        expunge_block (bb);
      }
*************** combine_blocks (struct loop *loop)
*** 928,934 ****
    /* Now if possible, merge loop header and block with exit edge.
       This reduces number of basic blocks to 2. Auto vectorizer 
addresses
       loops with two nodes only.  FIXME: Use cleanup_tree_cfg().  */
!   if (exit_bb != loop->latch && empty_block_p (loop->latch))
      {
        if (can_merge_blocks_p (loop->header, exit_bb))
        {
--- 930,939 ----
    /* Now if possible, merge loop header and block with exit edge.
       This reduces number of basic blocks to 2. Auto vectorizer 
addresses
       loops with two nodes only.  FIXME: Use cleanup_tree_cfg().  */
!   if (exit_bb
!       && loop->header != loop->latch
!       && exit_bb != loop->latch
!       && empty_block_p (loop->latch))
      {
        if (can_merge_blocks_p (loop->header, exit_bb))
        {