[PATCH v4 04/31] tcg: Remove branch-to-next regardless of reference count

[Richard Henderson]

Just because the label reference count is more than 1 does
not mean we cannot remove a branch-to-next.  By doing this
first, the label reference count may drop to 0, and then
the label itself gets removed as before.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 tcg/tcg.c | 33 +++++++++++++++++----------------
 1 file changed, 17 insertions(+), 16 deletions(-)

diff --git a/tcg/tcg.c b/tcg/tcg.c
index 06209e6160..0992fb4f31 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -2638,7 +2638,7 @@ TCGOp *tcg_op_insert_after(TCGContext *s, TCGOp *old_op,
 /* Reachable analysis : remove unreachable code.  */
 static void reachable_code_pass(TCGContext *s)
 {
-    TCGOp *op, *op_next;
+    TCGOp *op, *op_next, *op_prev;
     bool dead = false;
 
     QTAILQ_FOREACH_SAFE(op, &s->ops, link, op_next) {
@@ -2648,6 +2648,22 @@ static void reachable_code_pass(TCGContext *s)
         switch (op->opc) {
         case INDEX_op_set_label:
             label = arg_label(op->args[0]);
+
+            /*
+             * Optimization can fold conditional branches to unconditional.
+             * If we find a label which is preceded by an unconditional
+             * branch to next, remove the branch.  We couldn't do this when
+             * processing the branch because any dead code between the branch
+             * and label had not yet been removed.
+             */
+            op_prev = QTAILQ_PREV(op, link);
+            if (op_prev->opc == INDEX_op_br &&
+                label == arg_label(op_prev->args[0])) {
+                tcg_op_remove(s, op_prev);
+                /* Fall through means insns become live again.  */
+                dead = false;
+            }
+
             if (label->refs == 0) {
                 /*
                  * While there is an occasional backward branch, virtually
@@ -2661,21 +2677,6 @@ static void reachable_code_pass(TCGContext *s)
                 /* Once we see a label, insns become live again.  */
                 dead = false;
                 remove = false;
-
-                /*
-                 * Optimization can fold conditional branches to unconditional.
-                 * If we find a label with one reference which is preceded by
-                 * an unconditional branch to it, remove both.  This needed to
-                 * wait until the dead code in between them was removed.
-                 */
-                if (label->refs == 1) {
-                    TCGOp *op_prev = QTAILQ_PREV(op, link);
-                    if (op_prev->opc == INDEX_op_br &&
-                        label == arg_label(op_prev->args[0])) {
-                        tcg_op_remove(s, op_prev);
-                        remove = true;
-                    }
-                }
             }
             break;
 
-- 
2.34.1