[Bug tree-optimization/15524] [4.0 Regression] jump threading on trees is slow with switch statements with large # of cases

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2004-11-01 19:34 -------
Subject: Re:  [4.0 Regression] jump threading
 on trees is slow with switch statements with large # of cases

Hi Steven,

> OK, then can you see if this hack helps...?

+   /* Step 4: Update the case label vector.  */
+   TREE_VEC_LENGTH (label_vec) = n;
+   for (i = 0; i < n; ++i)
+     {
+       tree elt = TREE_VEC_ELT (label_vec, i);
+       e = case_to_edge[i];
+       CASE_LABEL (elt) = tree_block_label (e->dest);
+     }

Wow, this is barely safe.  You are assuming that the edge redirection
is done by changing e->dest and that cfg.c:redirect_edge_succ_nodup
does not remove e, which is actually the case because there is no
existing edge from the block containing SWITCH_EXPR to the new basic
block created by split_edge.

In any case, I agree this should work.  I'll be the second one to
admit this is gross. :-)

Kazu Hirata


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15524