The writer's side gets the probe, the reader's side gets the scan

flan_dev_reg_dead was reaching for the containment scan, and it is on the free
path: a dev build would have paid a 4096-entry sweep per free. A free hands back
the base address the allocator gave out, which is what the slot is keyed on, so
the question there is equality and never containment. Only free-all needs the
scan, and that runs once a frame.

The table is allocated when it is armed, not declared. A fixed array was a
quarter of a megabyte of BSS in a shipped game for a table that build never
writes; now a release build carries a null pointer and the not-taken branch.

The pointer arm binds its subject to a slot before naming it three times — the
slice arm's rule, and its reason: an inspect with a path reaches a leaf through
a bounds check, and three of those to render one pointer is the walk paying for
its own shape.

dev-ptr.flan shows both halves on a stopped stack. It was read by hand; the
test_dev.ml case that would drive it is another lane's file, and NEXT.md says so.

lib/render.ml

@@ -158,13 +158,23 @@ let rec render c depth (e : Tast.expr) : Tast.expr list =
       (match c.ptrs with
        | None -> [ lit "<ptr>" ]
        | Some pt ->
+         (* Into a slot first, the slice arm's rule and for its reason: this
+            arm names the pointer three times — asked about, followed, and
+            mourned — and the expression it came from may be a call. An
+            [inspect] with a path reaches a leaf through [flan_vec_at], which
+            is a bounds check and a transfer guard; three of those to render
+            one pointer would be the walk paying for its own shape. *)
+         let pv = c.alloc e.Tast.ty in
+         let p () = { Tast.e = Tast.Local pv; ty = e.Tast.ty; loc } in
          let inner =
            do_ ([ lit "<ptr " ]
-                @ render c (depth + 1) { Tast.e = Tast.Deref e; ty = t; loc }
+                @ render c (depth + 1) { Tast.e = Tast.Deref (p ()); ty = t; loc }
                 @ [ lit ">" ])
          in
-         let gone = do_ [ lit "<ptr"; pt.epitaph e; lit ">" ] in
-         [ unit_ (Tast.If (pt.live e, inner, gone)) ])
+         let gone = do_ [ lit "<ptr"; pt.epitaph (p ()); lit ">" ] in
+         [ unit_
+             (Tast.Let ([ (pv, e) ],
+                        [ unit_ (Tast.If (pt.live (p ()), inner, gone)) ])) ])
     (* Opaque on purpose, and for the same reason: its contents are the
        runtime's, its address is not stable across runs, and printing either
        would make an acceptance test's output depend on the heap. *)

runtime/flan_dev.c

@@ -862,7 +862,12 @@ typedef struct {
   int64_t     died;     /* when it was released, or 0 while it is live */
 } flan_reg_entry;
 
-static flan_reg_entry flan_reg[FLAN_REG_CAP];
+/* Allocated by flan_dev_reg_enable and null until then, which is the whole of
+ * what a release build carries: a null pointer, a zero flag, and the load and
+ * not-taken branch each of the hooks below begins with. A fixed array here
+ * instead would be a quarter of a megabyte of BSS in a shipped game, for a
+ * table nothing in that build ever writes. */
+static flan_reg_entry *flan_reg;
 static int64_t        flan_reg_used;   /* live + dead slots in use */
 static int64_t        flan_reg_seq;    /* a monotonic clock, in events */
 static int            flan_reg_on;     /* only a dev build turns this on */
@@ -874,7 +879,15 @@ static int            flan_reg_full;   /* something found no slot */
  * second version of the allocator gated on a build flag is worse than a
  * branch. That is a real cost and not zero; BUILT.md says so rather than
  * repeating the claim that a release build carries nothing. */
-void flan_dev_reg_enable(void) { flan_reg_on = 1; }
+void flan_dev_reg_enable(void) {
+  if (flan_reg_on) return;
+  flan_reg = (flan_reg_entry *)calloc(FLAN_REG_CAP, sizeof *flan_reg);
+  /* A registry that could not be made is not worth dying over; the flag stays
+     off and every question about an address answers "never heard of it",
+     which is what a release build answers too. */
+  if (flan_reg == NULL) return;
+  flan_reg_on = 1;
+}
 
 int flan_dev_reg_enabled(void) { return flan_reg_on; }
 
@@ -889,10 +902,15 @@ static size_t flan_reg_slot(uintptr_t a) {
  * fills: in a long-running program the dead are the bulk of it, and losing
  * them is much cheaper than losing the live half. */
 static void flan_reg_compact(void) {
-  static flan_reg_entry old[FLAN_REG_CAP];   /* static: 160KB is not stack */
+  size_t bytes = FLAN_REG_CAP * sizeof(flan_reg_entry);
+  flan_reg_entry *old = (flan_reg_entry *)malloc(bytes);
   int64_t i;
-  memcpy(old, flan_reg, sizeof old);
-  memset(flan_reg, 0, sizeof flan_reg);
+  /* Borrowed rather than kept: a second permanent copy would double what a
+     dev build holds for a rearrangement that happens rarely. If it cannot be
+     had, the table simply stays as it is and says it is full. */
+  if (old == NULL) { flan_reg_full = 1; return; }
+  memcpy(old, flan_reg, bytes);
+  memset(flan_reg, 0, bytes);
   flan_reg_used = 0;
   for (i = 0; i < FLAN_REG_CAP; i++) {
     size_t s;
@@ -908,6 +926,7 @@ static void flan_reg_compact(void) {
       }
     }
   }
+  free(old);
 }
 
 /* One note per allocation. [base] replaces whatever was recorded there, live
@@ -962,12 +981,27 @@ static flan_reg_entry *flan_reg_find(uintptr_t a) {
 }
 
 /* One block dies. The heap allocator's free calls this, and so does a resize,
- * for the block it moved away from. */
+ * for the block it moved away from.
+ *
+ * The probe, not the scan — and the difference matters because this is on the
+ * *writer's* side. A free hands back the base address the allocator gave out,
+ * which is what the slot is keyed on, so the question here is equality and
+ * never containment. Reaching for flan_reg_find would put a 4096-entry sweep
+ * on every free in a dev build, which is the cost the note was careful not to
+ * have. */
 void flan_dev_reg_dead(void *base) {
-  flan_reg_entry *e;
-  if (!flan_reg_on) return;
-  e = flan_reg_find((uintptr_t)base);
-  if (e != NULL && e->died == 0) e->died = ++flan_reg_seq;
+  uintptr_t a = (uintptr_t)base;
+  size_t s;
+  int64_t probe;
+  if (!flan_reg_on || a == 0) return;
+  s = flan_reg_slot(a);
+  for (probe = 0; probe < FLAN_REG_CAP; probe++) {
+    size_t j = (s + (size_t)probe) & (FLAN_REG_CAP - 1);
+    if (flan_reg[j].base == 0) return;      /* never noted; nothing to mark */
+    if (flan_reg[j].base != a) continue;
+    if (flan_reg[j].died == 0) flan_reg[j].died = ++flan_reg_seq;
+    return;
+  }
 }
 
 /* Every block inside [base, base+bytes) dies — which is an arena's free-all,
@@ -1040,6 +1074,7 @@ int32_t flan_dev_reg_emit(const void *p) {
  * test, and for the breakdown-by-type listing that is not built yet. */
 int64_t flan_dev_reg_count(int32_t live_only) {
   int64_t i, n = 0;
+  if (!flan_reg_on) return 0;
   for (i = 0; i < FLAN_REG_CAP; i++) {
     if (flan_reg[i].base == 0) continue;
     if (live_only && flan_reg[i].died != 0) continue;

test/programs/dev-ptr.flan

@@ -0,0 +1,46 @@
+;;;; A stopped stack holding two pointers into Vec storage, one of which is
+;;;; already dead. The allocation registry is what lets the inspector tell
+;;;; them apart, and this is the program that shows it:
+;;;;
+;;;;   ("live" "(Ptr Enemy)" "<ptr (Enemy {.hp 41 .x 2})>")
+;;;;   ("dead" "(Ptr Enemy)" "<ptr dead: was Enemy, freed at step 15>")
+;;;;
+;;;; Both lines are what `(:op "locals" :frame 1)` answers with today, and
+;;;; both were read off a running session by hand. **No test drives this
+;;;; program yet**: the case belongs beside the other `locals` and `inspect`
+;;;; cases in test_dev.ml, which is another lane's file. NEXT.md says so
+;;;; rather than letting the verification read as automated.
+;;;;
+;;;; Why a Vec rather than a struct on the stack: a stack address is not in
+;;;; the registry by design — the shadow stack already answers for a local by
+;;;; name — so a pointer to one renders <ptr>, which is neither half of what
+;;;; this is demonstrating.
+(import agent "vendor:agent")
+
+(defstruct Boom [why i32])
+(defstruct Enemy [hp i32  x i32])
+
+(defn deeper [] i64
+  (restart-case
+    (do (error (Boom {.why 7})) 1)
+    (carry-on [] 5)))
+
+(defn outer [] i64
+  (let [v (vec-new Enemy)
+        w (vec-new Enemy)]
+    (push v (Enemy {.hp 41 .x 2}))
+    (push w (Enemy {.hp 7 .x 9}))
+    (let [live (addr (at v 0))
+          dead (addr (at w 0))]
+      (free w)
+      (deeper))))
+
+(defvar ticks i64)
+
+(defn main [] i32
+  (agent/start "/tmp/flan-ptr-fallback.sock")
+  (print (outer)) (println "")
+  (dotimes [i 4000]
+    (agent/wait 5)
+    (set ticks (+ ticks 1)))
+  0)