Sketch out a new set of usage-handling functions

So far we've kept a count of instructions. Going forward, however, we
will build and maintain a chain of the using instructions; together
with knowing the writing instruction, this will give us a define-use
chain.

build/Makefile.in

@@ -192,6 +192,7 @@ OBJECTS = src/core/callsite@obj@ \
           src/spesh/candidate@obj@ \
           src/spesh/manipulate@obj@ \
           src/spesh/args@obj@ \
+          src/spesh/usages@obj@ \
           src/spesh/facts@obj@ \
           src/spesh/optimize@obj@ \
           src/spesh/dead_bb_elimination@obj@ \
@@ -354,6 +355,7 @@ HEADERS = src/moar.h \
           src/spesh/candidate.h \
           src/spesh/manipulate.h \
           src/spesh/args.h \
+          src/spesh/usages.h \
           src/spesh/facts.h \
           src/spesh/optimize.h \
           src/spesh/dead_bb_elimination.h \

src/moar.h

@@ -149,6 +149,7 @@ MVM_PUBLIC const MVMint32 MVM_jit_support(void);
 #include "spesh/candidate.h"
 #include "spesh/manipulate.h"
 #include "spesh/args.h"
+#include "spesh/usages.h"
 #include "spesh/facts.h"
 #include "spesh/optimize.h"
 #include "spesh/dead_bb_elimination.h"

src/spesh/facts.h

@@ -1,4 +1,4 @@
-/* Facts we might have about a local. */
+/* Facts we might have about a particular SSA version of a register. */
 struct MVMSpeshFacts {
     /* Flags indicating things we know. */
     MVMint32 flags;
@@ -24,6 +24,10 @@ struct MVMSpeshFacts {
      * this is unique). */
     MVMSpeshIns *writer;
 
+    /* Usages of this version of the register; thanks to SSA form, this taken
+     * together with writer form a define-use chain. */
+    MVMSpeshUsages usage;
+
     /* The deoptimization index in effect at the point of declaration, or -1
      * if none yet. */
     MVMint32 deopt_idx;

src/spesh/usages.c

@@ -0,0 +1,68 @@
+#include "moar.h"
+
+/* Adds a usage of an SSA value. */
+void MVM_spesh_usages_add(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshFacts *facts, MVMSpeshIns *by) {
+    MVMSpeshUseChainEntry *entry = MVM_spesh_alloc(tc, g, sizeof(MVMSpeshUseChainEntry));
+    entry->user = by;
+    entry->next = facts->usage.users;
+    facts->usage.users = entry;
+}
+void MVM_spesh_usages_add_by_reg(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshOperand used, MVMSpeshIns *by) {
+    MVM_spesh_usages_add(tc, g, MVM_spesh_get_facts(tc, g, used), by);
+}
+
+/* Removes a usage of an SSA value. */
+void MVM_spesh_usages_delete(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshFacts *facts, MVMSpeshIns *by) {
+    MVMSpeshUseChainEntry *cur = facts->usage.users;
+    MVMSpeshUseChainEntry *prev = NULL;
+    while (cur) {
+        if (cur->user == by) {
+            if (prev)
+                prev->next = cur->next;
+            else
+                facts->usage.users = cur->next;
+            return;
+        }
+        prev = cur;
+        cur = cur->next;
+    }
+    MVM_oops(tc, "Spesh: instruction missing from define-use chain");
+}
+void MVM_spesh_usages_delete_by_reg(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshOperand used, MVMSpeshIns *by) {
+    MVM_spesh_usages_delete(tc, g, MVM_spesh_get_facts(tc, g, used), by);
+}
+
+/* Marks that an SSA value is required for deopt purposes. */
+void MVM_spesh_usages_add_for_deopt(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshFacts *facts) {
+    facts->usage.deopt_required = 1;
+}
+
+/* Checks if the value is used, either by another instruction in the graph or
+ * by being needed for deopt. */
+MVMuint32 MVM_spesh_usages_is_used(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshOperand check) {
+    MVMSpeshFacts *facts = MVM_spesh_get_facts(tc, g, check);
+    return facts->usage.deopt_required || facts->usage.users;
+}
+
+/* Checks if the value is used due to being required for deopt. */
+MVMuint32 MVM_spesh_usages_is_used_by_deopt(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshOperand check) {
+    MVMSpeshFacts *facts = MVM_spesh_get_facts(tc, g, check);
+    return facts->usage.deopt_required;
+}
+
+/* Checks if there is precisely one known non-deopt user of the value. */
+MVMuint32 MVM_spesh_usages_used_once(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshOperand check) {
+    MVMSpeshFacts *facts = MVM_spesh_get_facts(tc, g, check);
+    return !facts->usage.deopt_required && facts->usage.users && !facts->usage.users->next;
+}
+
+/* Gets the count of usages, excluding use for deopt purposes. */
+MVMuint32 MVM_spesh_usages_count(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshOperand check) {
+    MVMuint32 count = 0;
+    MVMSpeshUseChainEntry *cur = MVM_spesh_get_facts(tc, g, check)->usage.users;
+    while (cur) {
+        count++;
+        cur = cur->next;
+    }
+    return count;
+}

src/spesh/usages.h

@@ -0,0 +1,24 @@
+/* Usage information, which is held per SSA written register. */
+struct MVMSpeshUsages {
+    /* The use chain entries. */
+    MVMSpeshUseChainEntry *users;
+
+    /* Does the instruction need to be preserved for the sake of deopt? */
+    MVMuint16 deopt_required;
+};
+
+/* Linked list of using instructions. */
+struct MVMSpeshUseChainEntry {
+    MVMSpeshIns *user;
+    MVMSpeshUseChainEntry *next;
+};
+
+void MVM_spesh_usages_add(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshFacts *facts, MVMSpeshIns *by);
+void MVM_spesh_usages_add_by_reg(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshOperand used, MVMSpeshIns *by);
+void MVM_spesh_usages_delete(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshFacts *facts, MVMSpeshIns *by);
+void MVM_spesh_usages_delete_by_reg(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshOperand used, MVMSpeshIns *by);
+void MVM_spesh_usages_add_for_deopt(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshFacts *facts);
+MVMuint32 MVM_spesh_usages_is_used(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshOperand check);
+MVMuint32 MVM_spesh_usages_is_used_by_deopt(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshOperand check);
+MVMuint32 MVM_spesh_usages_used_once(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshOperand check);
+MVMuint32 MVM_spesh_usages_count(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshOperand check);

src/types.h

@@ -198,6 +198,8 @@ typedef struct MVMSpeshPlan MVMSpeshPlan;
 typedef struct MVMSpeshPlanned MVMSpeshPlanned;
 typedef struct MVMSpeshArgGuard MVMSpeshArgGuard;
 typedef struct MVMSpeshArgGuardNode MVMSpeshArgGuardNode;
+typedef struct MVMSpeshUsages MVMSpeshUsages;
+typedef struct MVMSpeshUseChainEntry MVMSpeshUseChainEntry;
 typedef struct MVMSTable MVMSTable;
 typedef struct MVMStaticFrame MVMStaticFrame;
 typedef struct MVMStaticFrameBody MVMStaticFrameBody;