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