Simplify GCtrace * reference embedding for trace stitching.

This is now possible due to the immovable IR.
Contributed by Peter Cawley.

src/lj_asm.c

@@ -1933,6 +1933,11 @@ static void asm_tail_link(ASMState *as)
   }
   emit_addptr(as, RID_BASE, 8*(int32_t)baseslot);
 
+  if (as->J->ktrace) {  /* Patch ktrace slot with the final GCtrace pointer. */
+    setgcref(IR(as->J->ktrace)->gcr, obj2gco(as->J->curfinal));
+    IR(as->J->ktrace)->o = IR_KGC;
+  }
+
   /* Sync the interpreter state with the on-trace state. */
   asm_stack_restore(as, snap);
 

src/lj_ffrecord.c

@@ -118,9 +118,8 @@ static void recff_stitch(jit_State *J)
   /* Ditto for the IR. */
   memmove(&J->base[1], &J->base[-1], sizeof(TRef)*(J->maxslot+1));
   J->base[0] = lj_ir_kptr(J, contptr(cont)) | TREF_CONT;
-  J->ktracep = lj_ir_k64_reserve(J);
-  lua_assert(irt_toitype_(IRT_P64) == LJ_TTRACE);
-  J->base[-1] = emitir(IRT(IR_XLOAD, IRT_P64), lj_ir_kptr(J, &J->ktracep->gcr), 0);
+  J->base[-1] = lj_ir_ktrace(J);
+  J->ktrace = tref_ref(J->base[-1]);
   J->base += 2;
   J->baseslot += 2;
   J->framedepth++;

src/lj_ir.c

@@ -348,6 +348,18 @@ TRef lj_ir_kgc(jit_State *J, GCobj *o, IRType t)
   return TREF(ref, t);
 }
 
+/* Allocate GCtrace constant placeholder (no interning). */
+TRef lj_ir_ktrace(jit_State *J)
+{
+  IRRef ref = ir_nextk(J);
+  IRIns *ir = IR(ref);
+  lua_assert(irt_toitype_(IRT_P64) == LJ_TTRACE);
+  ir->t.irt = IRT_P64;
+  ir->o = IR_KNULL;  /* Not IR_KGC yet, but same size. */
+  ir->prev = 0;
+  return TREF(ref, IRT_P64);
+}
+
 /* Intern 32 bit pointer constant. */
 TRef lj_ir_kptr_(jit_State *J, IROp op, void *ptr)
 {

src/lj_iropt.h

@@ -51,6 +51,7 @@ LJ_FUNC TRef lj_ir_kgc(jit_State *J, GCobj *o, IRType t);
 LJ_FUNC TRef lj_ir_kptr_(jit_State *J, IROp op, void *ptr);
 LJ_FUNC TRef lj_ir_knull(jit_State *J, IRType t);
 LJ_FUNC TRef lj_ir_kslot(jit_State *J, TRef key, IRRef slot);
+LJ_FUNC TRef lj_ir_ktrace(jit_State *J);
 
 #if LJ_64
 #define lj_ir_kintp(J, k)	lj_ir_kint64(J, (uint64_t)(k))

src/lj_jit.h

@@ -416,7 +416,7 @@ typedef struct jit_State {
   GCRef *trace;		/* Array of traces. */
   TraceNo freetrace;	/* Start of scan for next free trace. */
   MSize sizetrace;	/* Size of trace array. */
-  TValue *ktracep;	/* Pointer to K64Array slot with GCtrace pointer. */
+  IRRef1 ktrace;	/* Reference to KGC with GCtrace. */
 
   IRRef1 chain[IR__MAX];  /* IR instruction skip-list chain anchors. */
   TRef slot[LJ_MAX_JSLOTS+LJ_STACK_EXTRA];  /* Stack slot map. */

src/lj_trace.c

@@ -436,7 +436,7 @@ static void trace_start(jit_State *J)
   J->postproc = LJ_POST_NONE;
   lj_resetsplit(J);
   J->retryrec = 0;
-  J->ktracep = NULL;
+  J->ktrace = 0;
   setgcref(J->cur.startpt, obj2gco(J->pt));
 
   L = J->L;
@@ -512,9 +512,6 @@ static void trace_stop(jit_State *J)
   lj_mcode_commit(J, J->cur.mcode);
   J->postproc = LJ_POST_NONE;
   trace_save(J, T);
-  if (J->ktracep) {  /* Patch K64Array slot with the final GCtrace pointer. */
-    setgcV(J->L, J->ktracep, obj2gco(T), LJ_TTRACE);
-  }
 
   L = J->L;
   lj_vmevent_send(L, TRACE,