From 423f10fb40a273cf3c9dc605b86469efa8c089a3 Mon Sep 17 00:00:00 2001
From: Mike Pall <mike>
Date: Tue, 1 Apr 2014 01:55:12 +0200
Subject: [PATCH] Fix alias analysis for table.len vs. table.clear.

---
 src/lj_opt_mem.c | 28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

diff --git a/src/lj_opt_mem.c b/src/lj_opt_mem.c
index 21c861bb..902392b5 100644
--- a/src/lj_opt_mem.c
+++ b/src/lj_opt_mem.c
@@ -309,6 +309,20 @@ int LJ_FASTCALL lj_opt_fwd_href_nokey(jit_State *J)
   return 1;  /* No conflict. Can fold to niltv. */
 }
 
+/* Check whether there's no aliasing table.clear. */
+static int fwd_aa_tab_clear(jit_State *J, IRRef lim, IRRef ta)
+{
+  IRRef ref = J->chain[IR_CALLS];
+  while (ref > lim) {
+    IRIns *calls = IR(ref);
+    if (calls->op2 == IRCALL_lj_tab_clear &&
+	(ta == calls->op1 || aa_table(J, ta, calls->op1) != ALIAS_NO))
+      return 0;  /* Conflict. */
+    ref = calls->prev;
+  }
+  return 1;  /* No conflict. Can safely FOLD/CSE. */
+}
+
 /* Check whether there's no aliasing NEWREF/table.clear for the left operand. */
 int LJ_FASTCALL lj_opt_fwd_tptr(jit_State *J, IRRef lim)
 {
@@ -320,15 +334,7 @@ int LJ_FASTCALL lj_opt_fwd_tptr(jit_State *J, IRRef lim)
       return 0;  /* Conflict. */
     ref = newref->prev;
   }
-  ref = J->chain[IR_CALLS];
-  while (ref > lim) {
-    IRIns *calls = IR(ref);
-    if (calls->op2 == IRCALL_lj_tab_clear &&
-	(ta == calls->op1 || aa_table(J, ta, calls->op1) != ALIAS_NO))
-      return 0;  /* Conflict. */
-    ref = calls->prev;
-  }
-  return 1;  /* No conflict. Can safely FOLD/CSE. */
+  return fwd_aa_tab_clear(J, lim, ta);
 }
 
 /* ASTORE/HSTORE elimination. */
@@ -862,6 +868,10 @@ TRef LJ_FASTCALL lj_opt_fwd_tab_len(jit_State *J)
     ref = store->prev;
   }
 
+  /* Search for aliasing table.clear. */
+  if (!fwd_aa_tab_clear(J, lim, tab))
+    return lj_ir_emit(J);
+
   /* Try to find a matching load. Below the conflicting store, if any. */
   return lj_opt_cselim(J, lim);
 }