[PATCH 2/2] cse: place common expressions in the Lowest Common Dominator

[Luc Van Oostenryck <luc.vanoostenryck@gmail.com>]

Currently, during CSE, common expressions are eliminated if either:
1) they belong to the same Basic Block;
2) one of the expressions dominates the other (so, one of the BB they
   belong dominates the other);
3) they have a single parent which is the same for both.

The third case can be extended to another suitable ancestor:
any common dominator would be OK, which is exactly what is done in
this patch.

Note: This patch is interesting because it allows significantly more
      common expressions to be eliminated. However, it has the annoying
      disadvantage of making the 'context imbalance' problem slightly
      worse. So, it's not intended to be merged as-is.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
---
 cse.c | 38 ++++++++++----------------------------
 1 file changed, 10 insertions(+), 28 deletions(-)

diff --git a/cse.c b/cse.c
index 1e58a973ecf6..c33babf2c6c8 100644
--- a/cse.c
+++ b/cse.c
@@ -279,20 +279,6 @@ static struct instruction * cse_one_instruction(struct instruction *insn, struct
 	return def;
 }
 
-static struct basic_block *trivial_common_parent(struct basic_block *bb1, struct basic_block *bb2)
-{
-	struct basic_block *parent;
-
-	if (bb_list_size(bb1->parents) != 1)
-		return NULL;
-	parent = first_basic_block(bb1->parents);
-	if (bb_list_size(bb2->parents) != 1)
-		return NULL;
-	if (first_basic_block(bb2->parents) != parent)
-		return NULL;
-	return parent;
-}
-
 static inline void remove_instruction(struct instruction_list **list, struct instruction *insn, int count)
 {
 	delete_ptr_list_entry((struct ptr_list **)list, insn, count);
@@ -318,7 +304,7 @@ static struct instruction * try_to_cse(struct entrypoint *ep, struct instruction
 	b2 = i2->bb;
 
 	/*
-	 * Currently we only handle the uninteresting degenerate case where
+	 * First try the uninteresting degenerate case where
 	 * the CSE is inside one basic-block.
 	 */
 	if (b1 == b2) {
@@ -332,22 +318,18 @@ static struct instruction * try_to_cse(struct entrypoint *ep, struct instruction
 		warning(b1->pos, "Whaa? unable to find CSE instructions");
 		return i1;
 	}
-	if (domtree_dominates(b1, b2))
-		return cse_one_instruction(i2, i1);
 
-	if (domtree_dominates(b2, b1))
+	/* Then try the case where one of the BB already dominate the other. */
+	common = bb_common_dominator(b1, b2);
+	if (common == b1)
+		return cse_one_instruction(i2, i1);
+	if (common == b2)
 		return cse_one_instruction(i1, i2);
 
-	/* No direct dominance - but we could try to find a common ancestor.. */
-	common = trivial_common_parent(b1, b2);
-	if (common) {
-		i1 = cse_one_instruction(i2, i1);
-		remove_instruction(&b1->insns, i1, 1);
-		add_instruction_to_end(i1, common);
-	} else {
-		i1 = i2;
-	}
-
+	/* No direct dominance - use the common dominator. */
+	cse_one_instruction(i2, i1);
+	remove_instruction(&b1->insns, i1, 1);
+	add_instruction_to_end(i1, common);
 	return i1;
 }
 
-- 
2.29.2