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[tree-ssa] Splitting abnormal edges


On 13 Jun 2003, Andrew MacLeod wrote:
> 	* tree-cfg.c (bsi_commit_first_edge_insert): Only consider non-abnormal
> 	edges when determining whether an edge needs to be split.

I think I can explain why this didn't help my Java tests.

We ignore abnormal outgoing edges for consideration in edge splitting,
which is fine:

> !   for (e2 = src->succ; e2; e2 = e2->succ_next)
> !     if (!(e2->flags & EDGE_ABNORMAL))
> !       num_exit++;

though we later fall back on edge splitting for control-altering blocks:

      if (!is_ctrl_stmt (last) && !is_ctrl_altering_stmt (last))
        {
          bsi_insert_after (&bsi, stmt, BSI_NEW_STMT);
          return bsi;
        }

The trouble being that is_ctrl_altering_stmt often returns 1 for
statements we don't care about, i.e. certain CALL_EXPR and MODIFY_EXPR
nodes that merely introduce an abnormal edge.

For instance, the block in question has:

BLOCK       12
PRED:       11
SUCC:       16 13 (ab)
PARENT:     7
LOOP DEPTH: 0
NEXT BLOCK: 13
PREV BLOCK: 11
18  T.12 = T.11
18  parseInt.13 = (int (*<UPFNde00>) (struct java.lang.String *))parseInt
18  T.14 = parseInt.13 (T.12)


The quick patch below fixes this case but isn't quite right, since it
doesn't consider functions that longjmp or don't return.

Perhaps it would be cleaner if is_ctrl_altering_stmt() were split into two
functions, with something like stmt_may_throw_p().  Or have it
consider EDGE_FALLTHRU cases only.  Andrew, what do you think?

	* tree-cfg.c (bsi_commit_first_edge_insert): Don't use
	is_ctrl_altering_stmt when deciding not to split an edge.

Index: tree-cfg.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/Attic/tree-cfg.c,v
retrieving revision 1.1.4.108
diff -u -p -r1.1.4.108 tree-cfg.c
--- tree-cfg.c  14 Jun 2003 20:56:00 -0000      1.1.4.108
+++ tree-cfg.c  16 Jun 2003 04:34:07 -0000
@@ -3862,7 +3862,8 @@ bsi_commit_first_edge_insert (edge e, tr
          append this stmt to the basic block. This should mean the edge
is
         a fallthrough edge.  */

-      if (!is_ctrl_stmt (last) && !is_ctrl_altering_stmt (last))
+      if (!is_ctrl_stmt (last) && TREE_CODE (last) != GOTO_EXPR
+         && TREE_CODE (last) != RETURN_EXPR)
        {
          bsi_insert_after (&bsi, stmt, BSI_NEW_STMT);
          return bsi;



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