tcg: Add reachable_code_pass

Delete trivially dead code that follows unconditional branches and
noreturn helpers.  These can occur either via optimization or via
the structure of a target's translator following an exception.

Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tcg/tcg.c

@@ -2239,6 +2239,81 @@ TCGOp *tcg_op_insert_after(TCGContext *s, TCGOp *old_op, TCGOpcode opc)
     return new_op;
 }
 
+/* Reachable analysis : remove unreachable code.  */
+static void reachable_code_pass(TCGContext *s)
+{
+    TCGOp *op, *op_next;
+    bool dead = false;
+
+    QTAILQ_FOREACH_SAFE(op, &s->ops, link, op_next) {
+        bool remove = dead;
+        TCGLabel *label;
+        int call_flags;
+
+        switch (op->opc) {
+        case INDEX_op_set_label:
+            label = arg_label(op->args[0]);
+            if (label->refs == 0) {
+                /*
+                 * While there is an occasional backward branch, virtually
+                 * all branches generated by the translators are forward.
+                 * Which means that generally we will have already removed
+                 * all references to the label that will be, and there is
+                 * little to be gained by iterating.
+                 */
+                remove = true;
+            } else {
+                /* 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, TCGOpHead, link);
+                    if (op_prev->opc == INDEX_op_br &&
+                        label == arg_label(op_prev->args[0])) {
+                        tcg_op_remove(s, op_prev);
+                        remove = true;
+                    }
+                }
+            }
+            break;
+
+        case INDEX_op_br:
+        case INDEX_op_exit_tb:
+        case INDEX_op_goto_ptr:
+            /* Unconditional branches; everything following is dead.  */
+            dead = true;
+            break;
+
+        case INDEX_op_call:
+            /* Notice noreturn helper calls, raising exceptions.  */
+            call_flags = op->args[TCGOP_CALLO(op) + TCGOP_CALLI(op) + 1];
+            if (call_flags & TCG_CALL_NO_RETURN) {
+                dead = true;
+            }
+            break;
+
+        case INDEX_op_insn_start:
+            /* Never remove -- we need to keep these for unwind.  */
+            remove = false;
+            break;
+
+        default:
+            break;
+        }
+
+        if (remove) {
+            tcg_op_remove(s, op);
+        }
+    }
+}
+
 #define TS_DEAD  1
 #define TS_MEM   2
 
@@ -3515,6 +3590,7 @@ int tcg_gen_code(TCGContext *s, TranslationBlock *tb)
     atomic_set(&prof->la_time, prof->la_time - profile_getclock());
 #endif
 
+    reachable_code_pass(s);
     liveness_pass_1(s);
 
     if (s->nb_indirects > 0) {