Store deopt usage separate from normal usage

So that we can understand which things are kept alive for deopt and
which are being used for real. There's no behavior change from this,
it's simply making it clearer to see what lives only for the sake of
deopt in the spesh log. However, keeping this information separately
will allow us to do some further opts in the future.

src/spesh/dump.c

@@ -400,10 +400,12 @@ static void dump_facts(MVMThreadContext *tc, DumpStr *ds, MVMSpeshGraph *g) {
         for (j = 0; j < num_facts; j++) {
             MVMint32 usages = g->facts[i][j].usages;
             MVMint32 flags  = g->facts[i][j].flags;
+            MVMuint16 deopt_required = g->facts[i][j].deopt_required;
             if (i < 10) append(ds, " ");
             if (j < 10) append(ds, " ");
             if (flags || g->facts[i][j].dead_writer || g->facts[i][j].writer && g->facts[i][j].writer->info->opcode == MVM_SSA_PHI) {
-                appendf(ds, "    r%d(%d): usages=%d, flags=%-5d", i, j, usages, flags);
+                appendf(ds, "    r%d(%d): usages=%d%s, flags=%-5d", i, j, usages,
+                    deopt_required ? "+deopt" : "", flags);
                 if (flags & 1) {
                     append(ds, " KnTyp");
                 }

src/spesh/facts.c

@@ -477,7 +477,9 @@ static void add_bb_facts(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshBB *bb,
             if ((is_phi && i > 0)
                 || (!is_phi && (ins->info->operands[i] & MVM_operand_rw_mask) == MVM_operand_read_reg)) {
                 MVMSpeshFacts *facts = &(g->facts[ins->operands[i].reg.orig][ins->operands[i].reg.i]);
-                facts->usages += facts->deopt_idx == cur_deopt_idx ? 1 : 2;
+                facts->usages++;
+                if (facts->deopt_idx != cur_deopt_idx)
+                    facts->deopt_required = 1;
             }
 
             /* Writes need the current deopt index and the writing instruction
@@ -747,3 +749,10 @@ void MVM_spesh_facts_discover(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshPl
     add_bb_facts(tc, g, g->entry, p, -1);
     tweak_block_handler_usage(tc, g);
 }
+
+/* Checks if an operand is in use, either by having a non-zero usage count or
+ * being required for deopt. */
+MVMint32 MVM_spesh_facts_is_used(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshOperand operand) {
+   MVMSpeshFacts *facts = MVM_spesh_get_facts(tc, g, operand);
+   return facts->usages > 0 || facts->deopt_required;
+}

src/spesh/facts.h

@@ -31,8 +31,11 @@ struct MVMSpeshFacts {
     /* The log guard the facts depend on, if any. */
     MVMuint32 log_guard;
 
+    /* Does the instruction need to be preserved for the sake of deopt? */
+    MVMuint16 deopt_required;
+
     /* Has the instruction that wrote this value been deleted? */
-    MVMuint32 dead_writer;
+    MVMuint16 dead_writer;
 };
 
 /* Various fact flags. */
@@ -57,3 +60,4 @@ void MVM_spesh_facts_depend(MVMThreadContext *tc, MVMSpeshGraph *g,
     MVMSpeshFacts *target, MVMSpeshFacts *source);
 void MVM_spesh_facts_object_facts(MVMThreadContext *tc, MVMSpeshGraph *g,
     MVMSpeshOperand tgt, MVMObject *obj);
+MVMint32 MVM_spesh_facts_is_used(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshOperand operand);

src/spesh/optimize.c

@@ -2416,8 +2416,7 @@ static void eliminate_dead_ins(MVMThreadContext *tc, MVMSpeshGraph *g) {
                 while (ins) {
                     MVMSpeshIns *prev = ins->prev;
                     if (ins->info->opcode == MVM_SSA_PHI) {
-                        MVMSpeshFacts *facts = get_facts_direct(tc, g, ins->operands[0]);
-                        if (facts->usages == 0) {
+                        if (!MVM_spesh_facts_is_used(tc, g, ins->operands[0])) {
                             /* Remove this phi. */
                             MVM_spesh_manipulate_delete_ins(tc, g, bb, ins);
                             death = 1;
@@ -2426,8 +2425,7 @@ static void eliminate_dead_ins(MVMThreadContext *tc, MVMSpeshGraph *g) {
                     else if (ins->info->pure) {
                         /* Sanity check to make sure it's a write reg as first operand. */
                         if ((ins->info->operands[0] & MVM_operand_rw_mask) == MVM_operand_write_reg) {
-                            MVMSpeshFacts *facts = get_facts_direct(tc, g, ins->operands[0]);
-                            if (facts->usages == 0) {
+                            if (!MVM_spesh_facts_is_used(tc, g, ins->operands[0])) {
                                 /* Remove this instruction. */
                                 MVM_spesh_manipulate_delete_ins(tc, g, bb, ins);
                                 death = 1;
@@ -2512,7 +2510,7 @@ static void try_eliminate_set(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshBB
             ins->prev->operands[0].reg.orig == ins->operands[1].reg.orig &&
             ins->prev->operands[0].reg.i == ins->operands[1].reg.i) {
         MVMSpeshFacts *elim_facts = get_facts_direct(tc, g, ins->operands[1]);
-        if (elim_facts->usages == 1 && within_inline(tc, g, bb, ins->operands[0])) {
+        if (elim_facts->usages == 1 && !elim_facts->deopt_required && within_inline(tc, g, bb, ins->operands[0])) {
             ins->prev->operands[0].reg = ins->operands[0].reg;
             get_facts_direct(tc, g, ins->prev->operands[0])->writer = ins->prev;
             MVM_spesh_manipulate_delete_ins(tc, g, bb, ins);