From ff0a1f3f4eedf8e43eeb5cc3d2f8d051b648e0c0 Mon Sep 17 00:00:00 2001
From: Mike Pall <mike>
Date: Wed, 15 Aug 2012 16:17:34 +0200
Subject: [PATCH] FOLD x / 2^k ==> x * 2^-k.

---
 src/lj_opt_fold.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/lj_opt_fold.c b/src/lj_opt_fold.c
index 74cea785..461fa621 100644
--- a/src/lj_opt_fold.c
+++ b/src/lj_opt_fold.c
@@ -816,6 +816,15 @@ LJFOLDF(simplify_nummuldiv_k)
     fins->o = IR_ADD;
     fins->op2 = fins->op1;
     return RETRYFOLD;
+  } else if (fins->o == IR_DIV) {  /* x / 2^k ==> x * 2^-k */
+    uint64_t u = ir_knum(fright)->u64;
+    if ((u & U64x(000fffff,ffffffff)) == 0 &&
+	(uint32_t)(u = ((u >> 52) & 0x7ff)) - 1 < 0x7fd) {
+      u = (u & (uint64_t)1 << 63) | ((uint64_t)(0x7fe - (uint32_t)u) << 52);
+      fins->o = IR_MUL;  /* Multiply by exact reciprocal. */
+      fins->op2 = lj_ir_knum_u64(J, u);
+      return RETRYFOLD;
+    }
   }
   return NEXTFOLD;
 }