Add detection of missing usage deletions

So that we can find out where we have uses of values that are not in
the graph, but still listed in the DU chain. This means we didn't
properly delete the usage, and so might keep alive an instruction we
could otherwise delete.

src/spesh/usages.c

@@ -88,7 +88,22 @@ MVMuint32 MVM_spesh_usages_count(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpes
 #if MVM_SPESH_CHECK_DU
 /* Check the DU chains of a graph are well formed. */
 void MVM_spesh_usages_check(MVMThreadContext *tc, MVMSpeshGraph *g) {
-    MVMSpeshBB *cur_bb = g->entry;
+    MVMSpeshBB *cur_bb;
+
+    /* Clear the "did we see this in the graph" flags. */
+    MVMuint32 i, j;
+    for (i = 0; i < g->num_locals; i++) {
+        for (j = 0; j < g->fact_counts[i]; j++) {
+            MVMSpeshUseChainEntry *use_entry = g->facts[i][j].usage.users;
+            while (use_entry) {
+                use_entry->seen_in_graph = 0;
+                use_entry = use_entry->next;
+            }
+        }
+    }
+
+    /* Now walk the instruction graph. */
+    cur_bb = g->entry;
     while (cur_bb) {
         MVMSpeshIns *cur_ins = cur_bb->first_ins;
         while (cur_ins) {
@@ -102,7 +117,8 @@ void MVM_spesh_usages_check(MVMThreadContext *tc, MVMSpeshGraph *g) {
                     MVMSpeshUseChainEntry *use_entry = facts->usage.users;
                     MVMuint32 found = 0;
                     while (use_entry) {
-                        if (use_entry->user == cur_ins) {
+                        if (!use_entry->seen_in_graph && use_entry->user == cur_ins) {
+                            use_entry->seen_in_graph = 1;
                             found = 1;
                             break;
                         }
@@ -131,5 +147,23 @@ void MVM_spesh_usages_check(MVMThreadContext *tc, MVMSpeshGraph *g) {
         }
         cur_bb = cur_bb->linear_next;
     }
+
+    /* Check that the "did we see this in the graph" flags are all set, in
+     * order to spot things that not used in the graph, but still have
+     * usage marked. */
+    for (i = 0; i < g->num_locals; i++) {
+        for (j = 0; j < g->fact_counts[i]; j++) {
+            MVMSpeshUseChainEntry *use_entry = g->facts[i][j].usage.users;
+            while (use_entry) {
+                if (!use_entry->seen_in_graph) {
+                    MVMuint8 is_phi = use_entry->user->info->opcode == MVM_SSA_PHI;
+                    MVM_oops(tc, "Malformed DU chain: reading %s of %d(%d) not in graph\n%s",
+                        is_phi ? "PHI" : use_entry->user->info->name, i, j,
+                        MVM_spesh_dump(tc, g));
+                }
+                use_entry = use_entry->next;
+            }
+        }
+    }
 }
 #endif

src/spesh/usages.h

@@ -18,6 +18,9 @@ struct MVMSpeshUsages {
 struct MVMSpeshUseChainEntry {
     MVMSpeshIns *user;
     MVMSpeshUseChainEntry *next;
+#if MVM_SPESH_CHECK_DU
+    MVMuint8 seen_in_graph;
+#endif
 };
 
 void MVM_spesh_usages_add(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshFacts *facts, MVMSpeshIns *by);