src/lj_opt_dce.c

   1 /*
   2 ** DCE: Dead Code Elimination. Pre-LOOP only -- ASM already performs DCE.
   3 ** Copyright (C) 2005-2010 Mike Pall. See Copyright Notice in luajit.h
   4 */
   5
   6 #define lj_opt_dce_c
   7 #define LUA_CORE
   8
   9 #include "lj_obj.h"
  10
  11 #if LJ_HASJIT
  12
  13 #include "lj_ir.h"
  14 #include "lj_jit.h"
  15 #include "lj_iropt.h"
  16
  17 /* Some local macros to save typing. Undef'd at the end. */
  18 #define IR(ref)         (&J->cur.ir[(ref)])
  19
  20 /* Scan through all snapshots and mark all referenced instructions. */
  21 static void dce_marksnap(jit_State *J)
  22 {
  23   SnapNo i, nsnap = J->cur.nsnap;
  24   for (i = 0; i < nsnap; i++) {
  25     SnapShot *snap = &J->cur.snap[i];
  26     IRRef2 *map = &J->cur.snapmap[snap->mapofs];
  27     BCReg s, nslots = snap->nslots;
  28     for (s = 0; s < nslots; s++) {
  29       IRRef ref = snap_ref(map[s]);
  30       if (!irref_isk(ref))
  31         irt_setmark(IR(ref)->t);
  32     }
  33   }
  34 }
  35
  36 /* Backwards propagate marks. Replace unused instructions with NOPs. */
  37 static void dce_propagate(jit_State *J)
  38 {
  39   IRRef1 *pchain[IR__MAX];
  40   IRRef ins;
  41   uint32_t i;
  42   for (i = 0; i < IR__MAX; i++) pchain[i] = &J->chain[i];
  43   for (ins = J->cur.nins-1; ins >= REF_FIRST; ins--) {
  44     IRIns *ir = IR(ins);
  45     if (irt_ismarked(ir->t)) {
  46       irt_clearmark(ir->t);
  47       pchain[ir->o] = &ir->prev;
  48     } else if (!(irt_isguard(ir->t) || irm_sideeff(lj_ir_mode[ir->o]))) {
  49       *pchain[ir->o] = ir->prev;  /* Reroute original instruction chain. */
  50       *pchain[IR_NOP] = (IRRef1)ins;
  51       ir->t.irt = IRT_NIL;
  52       ir->o = IR_NOP;  /* Replace instruction with NOP. */
  53       ir->op1 = ir->op2 = 0;
  54       pchain[IR_NOP] = &ir->prev;
  55       continue;
  56     }
  57     if (!irref_isk(ir->op1)) irt_setmark(IR(ir->op1)->t);
  58     if (!irref_isk(ir->op2)) irt_setmark(IR(ir->op2)->t);
  59   }
  60   *pchain[IR_NOP] = 0;  /* Terminate NOP chain. */
  61 }
  62
  63 /* Dead Code Elimination.
  64 **
  65 ** First backpropagate marks for all used instructions. Then replace
  66 ** the unused ones with a NOP. Note that compressing the IR to eliminate
  67 ** the NOPs does not pay off.
  68 */
  69 void lj_opt_dce(jit_State *J)
  70 {
  71   if ((J->flags & JIT_F_OPT_DCE)) {
  72     dce_marksnap(J);
  73     dce_propagate(J);
  74   }
  75 }
  76
  77 #undef IR
  78
  79 #endif