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src/lj_opt_dce.c - luajit-2.0-src

Functions defined

Macros defined

Source code

  1. /*
  2. ** DCE: Dead Code Elimination. Pre-LOOP only -- ASM already performs DCE.
  3. ** Copyright (C) 2005-2015 Mike Pall. See Copyright Notice in luajit.h
  4. */

  6. ‌#define lj_opt_dce_c
  7. ‌#define LUA_CORE

  9. #include "lj_obj.h"

  11. #if LJ_HASJIT

  13. #include "lj_ir.h"
  14. #include "lj_jit.h"
  15. #include "lj_iropt.h"

  17. /* Some local macros to save typing. Undef'd at the end. */
  18. ‌#define IR(ref)                (&J->cur.ir[(ref)])

  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.     SnapEntry *map = &J->cur.snapmap[snap->mapofs];
  27.     MSize n, nent = snap->nent;
  28.     for (n = 0; n < nent; n++) {
  29.       IRRef ref = snap_ref(map[n]);
  30.       if (ref >= REF_FIRST)
  31.         irt_setmark(IR(ref)->t);
  32.     }
  33.   }
  34. }

  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 (!ir_sideeff(ir)) {
  49.       *pchain[ir->o] = ir->prev;  /* Reroute original instruction chain. */
  50.       ir->t.irt = IRT_NIL;
  51.       ir->o = IR_NOP;  /* Replace instruction with NOP. */
  52.       ir->op1 = ir->op2 = 0;
  53.       ir->prev = 0;
  54.       continue;
  55.     }
  56.     if (ir->op1 >= REF_FIRST) irt_setmark(IR(ir->op1)->t);
  57.     if (ir->op2 >= REF_FIRST) irt_setmark(IR(ir->op2)->t);
  58.   }
  59. }

  61. /* Dead Code Elimination.
  62. **
  63. ** First backpropagate marks for all used instructions. Then replace
  64. ** the unused ones with a NOP. Note that compressing the IR to eliminate
  65. ** the NOPs does not pay off.
  66. */
  67. ‌void lj_opt_dce(jit_State *J)
  68. {
  69.   if ((J->flags & JIT_F_OPT_DCE)) {
  70.     dce_marksnap(J);
  71.     dce_propagate(J);
  72.     memset(J->bpropcache, 0, sizeof(J->bpropcache));  /* Invalidate cache. */
  73.   }
  74. }

  76. ‌#undef IR

  78. #endif

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