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annotate third_party/luajit/src/lj_opt_dce.c @ 204:e5aed6c36672

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[Notes] Added icons and updated styling a bit. Probalby usable now.
author MrJuneJune <me@mrjunejune.com>
date Sun, 15 Feb 2026 11:02:13 -0800
parents 94705b5986b3
children
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178
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1 /*
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2 ** DCE: Dead Code Elimination. Pre-LOOP only -- ASM already performs DCE.
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3 ** Copyright (C) 2005-2023 Mike Pall. See Copyright Notice in luajit.h
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4 */
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5
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6 #define lj_opt_dce_c
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7 #define LUA_CORE
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8
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9 #include "lj_obj.h"
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10
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11 #if LJ_HASJIT
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12
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13 #include "lj_ir.h"
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14 #include "lj_jit.h"
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15 #include "lj_iropt.h"
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16
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17 /* Some local macros to save typing. Undef'd at the end. */
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18 #define IR(ref) (&J->cur.ir[(ref)])
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19
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20 /* Scan through all snapshots and mark all referenced instructions. */
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21 static void dce_marksnap(jit_State *J)
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22 {
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23 SnapNo i, nsnap = J->cur.nsnap;
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24 for (i = 0; i < nsnap; i++) {
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25 SnapShot *snap = &J->cur.snap[i];
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26 SnapEntry *map = &J->cur.snapmap[snap->mapofs];
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27 MSize n, nent = snap->nent;
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28 for (n = 0; n < nent; n++) {
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29 IRRef ref = snap_ref(map[n]);
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30 if (ref >= REF_FIRST)
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31 irt_setmark(IR(ref)->t);
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32 }
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33 }
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34 }
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35
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36 /* Backwards propagate marks. Replace unused instructions with NOPs. */
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37 static void dce_propagate(jit_State *J)
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38 {
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39 IRRef1 *pchain[IR__MAX];
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40 IRRef ins;
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41 uint32_t i;
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42 for (i = 0; i < IR__MAX; i++) pchain[i] = &J->chain[i];
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43 for (ins = J->cur.nins-1; ins >= REF_FIRST; ins--) {
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44 IRIns *ir = IR(ins);
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45 if (irt_ismarked(ir->t)) {
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46 irt_clearmark(ir->t);
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47 pchain[ir->o] = &ir->prev;
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48 } else if (!ir_sideeff(ir)) {
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49 *pchain[ir->o] = ir->prev; /* Reroute original instruction chain. */
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50 lj_ir_nop(ir);
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51 continue;
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52 }
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53 if (ir->op1 >= REF_FIRST) irt_setmark(IR(ir->op1)->t);
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54 if (ir->op2 >= REF_FIRST) irt_setmark(IR(ir->op2)->t);
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55 }
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56 }
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57
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58 /* Dead Code Elimination.
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59 **
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60 ** First backpropagate marks for all used instructions. Then replace
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61 ** the unused ones with a NOP. Note that compressing the IR to eliminate
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62 ** the NOPs does not pay off.
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63 */
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64 void lj_opt_dce(jit_State *J)
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65 {
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66 if ((J->flags & JIT_F_OPT_DCE)) {
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67 dce_marksnap(J);
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68 dce_propagate(J);
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69 memset(J->bpropcache, 0, sizeof(J->bpropcache)); /* Invalidate cache. */
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70 }
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71 }
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72
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73 #undef IR
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74
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75 #endif