From 55b16959717084884fd4a0cbae6d19e3786c20c7 Mon Sep 17 00:00:00 2001
From: Mike Pall <mike>
Date: Tue, 8 Dec 2009 19:46:35 +0100
Subject: RELEASE LuaJIT-2.0.0-beta1

---
 src/lj_opt_dce.c | 79 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 79 insertions(+)
 create mode 100644 src/lj_opt_dce.c

(limited to 'src/lj_opt_dce.c')

diff --git a/src/lj_opt_dce.c b/src/lj_opt_dce.c
new file mode 100644
index 00000000..0cd60830
--- /dev/null
+++ b/src/lj_opt_dce.c
@@ -0,0 +1,79 @@
+/*
+** DCE: Dead Code Elimination. Pre-LOOP only -- ASM already performs DCE.
+** Copyright (C) 2005-2009 Mike Pall. See Copyright Notice in luajit.h
+*/
+
+#define lj_opt_dce_c
+#define LUA_CORE
+
+#include "lj_obj.h"
+
+#if LJ_HASJIT
+
+#include "lj_ir.h"
+#include "lj_jit.h"
+#include "lj_iropt.h"
+
+/* Some local macros to save typing. Undef'd at the end. */
+#define IR(ref)		(&J->cur.ir[(ref)])
+
+/* Scan through all snapshots and mark all referenced instructions. */
+static void dce_marksnap(jit_State *J)
+{
+  SnapNo i, nsnap = J->cur.nsnap;
+  for (i = 0; i < nsnap; i++) {
+    SnapShot *snap = &J->cur.snap[i];
+    IRRef2 *map = &J->cur.snapmap[snap->mapofs];
+    BCReg s, nslots = snap->nslots;
+    for (s = 0; s < nslots; s++) {
+      IRRef ref = snap_ref(map[s]);
+      if (!irref_isk(ref))
+	irt_setmark(IR(ref)->t);
+    }
+  }
+}
+
+/* Backwards propagate marks. Replace unused instructions with NOPs. */
+static void dce_propagate(jit_State *J)
+{
+  IRRef1 *pchain[IR__MAX];
+  IRRef ins;
+  uint32_t i;
+  for (i = 0; i < IR__MAX; i++) pchain[i] = &J->chain[i];
+  for (ins = J->cur.nins-1; ins >= REF_FIRST; ins--) {
+    IRIns *ir = IR(ins);
+    if (irt_ismarked(ir->t)) {
+      irt_clearmark(ir->t);
+      pchain[ir->o] = &ir->prev;
+    } else if (!(irt_isguard(ir->t) || irm_sideeff(lj_ir_mode[ir->o]))) {
+      *pchain[ir->o] = ir->prev;  /* Reroute original instruction chain. */
+      *pchain[IR_NOP] = (IRRef1)ins;
+      ir->t.irt = IRT_NIL;
+      ir->o = IR_NOP;  /* Replace instruction with NOP. */
+      ir->op1 = ir->op2 = 0;
+      pchain[IR_NOP] = &ir->prev;
+      continue;
+    }
+    if (!irref_isk(ir->op1)) irt_setmark(IR(ir->op1)->t);
+    if (!irref_isk(ir->op2)) irt_setmark(IR(ir->op2)->t);
+  }
+  *pchain[IR_NOP] = 0;  /* Terminate NOP chain. */
+}
+
+/* Dead Code Elimination.
+**
+** First backpropagate marks for all used instructions. Then replace
+** the unused ones with a NOP. Note that compressing the IR to eliminate
+** the NOPs does not pay off.
+*/
+void lj_opt_dce(jit_State *J)
+{
+  if ((J->flags & JIT_F_OPT_DCE)) {
+    dce_marksnap(J);
+    dce_propagate(J);
+  }
+}
+
+#undef IR
+
+#endif
-- 
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