experimental patch to limit GCSE so we don't produce partially dead code like crazy

diff --git a/ir/ir/iropt.c b/ir/ir/iropt.c
index f5aef5f..ba8b185 100644 (file)
--- a/ir/ir/iropt.c
+++ b/ir/ir/iropt.c
@@ -6316,10 +6316,23 @@ int identities_cmp(const void *elt, const void *key)
                /* for pinned nodes, the block inputs must be equal */
                if (get_irn_n(a, -1) != get_irn_n(b, -1))
                        return 1;
-       } else if (! get_opt_global_cse()) {
-               /* for block-local CSE both nodes must be in the same Block */
-               if (get_nodes_block(a) != get_nodes_block(b))
-                       return 1;
+       } else {
+               ir_node *block_a = get_nodes_block(a);
+               ir_node *block_b = get_nodes_block(b);
+               if (! get_opt_global_cse()) {
+                       /* for block-local CSE both nodes must be in the same Block */
+                       if (block_a != block_b)
+                               return 1;
+               } else {
+                       /* The optimistic approach would be to do nothing here.
+                        * However doing GCSE optimisatically produces alot of partially dead code which appears
+                        * to be worse in practice than the missed opportunities.
+                        * So we use a very conservative variant here and only CSE if 1 value dominates the
+                        * other. */
+                       if (!block_dominates(block_a, block_b)
+                           && !block_dominates(block_b, block_a))
+                           return 1;
+               }
        }
 
        /* compare a->in[0..ins] with b->in[0..ins] */