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/*
** DCE: Dead Code Elimination. Pre-LOOP only -- ASM already performs DCE.
** Copyright (C) 2005-2022 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];
SnapEntry *map = &J->cur.snapmap[snap->mapofs];
MSize n, nent = snap->nent;
for (n = 0; n < nent; n++) {
IRRef ref = snap_ref(map[n]);
if (ref >= REF_FIRST)
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 (!ir_sideeff(ir)) {
*pchain[ir->o] = ir->prev; /* Reroute original instruction chain. */
lj_ir_nop(ir);
continue;
}
if (ir->op1 >= REF_FIRST) irt_setmark(IR(ir->op1)->t);
if (ir->op2 >= REF_FIRST) irt_setmark(IR(ir->op2)->t);
}
}
/* 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);
memset(J->bpropcache, 0, sizeof(J->bpropcache)); /* Invalidate cache. */
}
}
#undef IR
#endif
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