Stop pointer parameters leaking value-pool slots

Create structure parameters before allocating their representation, then keep pointer references in the call variable's inline slot. Add an 8,000-call regression and document the remaining by-value structure escape limitation.

Co-authored-by: andrew <andrew@aklabs.net>
Co-authored-by: OpenAI Codex (GPT-5) <noreply@openai.com>

docs/14-architecture.md

@@ -380,8 +380,10 @@ no free, so anything drawn from it is spent for the life of the run — and scop
 returns a variable's *slot* without returning its storage. A scalar therefore does not
 draw from it at all: `akbasic_variable_init()` points a one-element non-`@` variable at
 its own `inlinevalue`, which is what makes a local, a `FOR` counter and a `DEF` parameter
-free. Arrays and structures still spend, deliberately, because a pointer into a record is
+free. A pointer parameter is also free: it owns only its one-slot reference, so its
-allowed to outlive the scope that DIMmed it.
+`inlinevalue` dies with the call while the target remains in the caller's storage. Arrays,
+structures and by-value structure parameters still spend, deliberately, because a pointer
+into a record is allowed to outlive the scope that DIMmed it.
 [Chapter 13](13-differences.md) states the same budget from a BASIC programmer's side.
 
 The per-environment three are reset at the top of every line, which is what makes

docs/16-structures.md

@@ -266,6 +266,13 @@ The function saw 99; the caller still has 5. To change a caller's record on purp
 a pointer — `DEF POKEIT(P@ AS PTR TO CRATE)` — and reach through it with `->`. Neither is
 a special rule: both fall out of the parameter being assigned like any other variable.
 
+A pointer parameter's own reference is kept in the call variable's inline slot, so repeated
+calls do not spend the value pool. Its target remains the caller's structure. A by-value
+structure parameter is different: its copied slots remain in the value pool because
+`POINT Q@ AT B@` may retain a pointer to that copy after the function returns. Until escape
+analysis can distinguish that case, **do not call a by-value structure-parameter function in
+a loop**; bind a host global and rebind it per instance instead.
+
 ## What is checked, and what is not
 
 A field name is checked against the set the type declared, and the refusal lists the

include/akbasic/environment.h

@@ -246,6 +246,16 @@ akerr_ErrorContext AKERR_NOIGNORE *akbasic_environment_collect_subscripts(akbasi
  */
 akerr_ErrorContext AKERR_NOIGNORE *akbasic_environment_create(akbasic_Environment *obj, const char *varname, akbasic_Variable **dest);
 
+/**
+ * @brief Create a variable slot without allocating value storage.
+ *
+ * Used when the caller knows the variable's representation before its first
+ * initialization, such as a structure parameter. The caller must initialize
+ * the variable before evaluating it.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *akbasic_environment_create_empty(akbasic_Environment *obj, const char *varname,
+										akbasic_Variable **dest);
+
 /**
  * @brief Resolve a label to the line number it marks.
  * @param obj Scope to search; the parent chain is walked.

src/environment.c

@@ -306,7 +306,8 @@ akerr_ErrorContext *akbasic_environment_get(akbasic_Environment *obj, const char
     SUCCEED_RETURN(errctx);
 }
 
-akerr_ErrorContext *akbasic_environment_create(akbasic_Environment *obj, const char *varname, akbasic_Variable **dest)
+static akerr_ErrorContext *environment_create_named(akbasic_Environment *obj, const char *varname,
+							akbasic_Variable **dest, bool initialize)
 {
     PREPARE_ERROR(errctx);
     akbasic_Variable *variable = NULL;
@@ -340,12 +341,31 @@ akerr_ErrorContext *akbasic_environment_create(akbasic_Environment *obj, const c
     PASS(errctx, aksl_strcpy(variable->name, sizeof(variable->name), varname));
     variable->valuetype = AKBASIC_TYPE_UNDEFINED;
     variable->mutable_ = true;
+    if ( initialize ) {
 	PASS(errctx, akbasic_variable_init(variable, &obj->runtime->valuepool, sizes, 1));
+    }
     PASS(errctx, akbasic_symtab_set(&obj->variables, varname, variable, 0));
     *dest = variable;
     SUCCEED_RETURN(errctx);
 }
 
+akerr_ErrorContext *akbasic_environment_create(akbasic_Environment *obj, const char *varname, akbasic_Variable **dest)
+{
+    PREPARE_ERROR(errctx);
+
+    PASS(errctx, environment_create_named(obj, varname, dest, true));
+    SUCCEED_RETURN(errctx);
+}
+
+akerr_ErrorContext *akbasic_environment_create_empty(akbasic_Environment *obj, const char *varname,
+							akbasic_Variable **dest)
+{
+    PREPARE_ERROR(errctx);
+
+    PASS(errctx, environment_create_named(obj, varname, dest, false));
+    SUCCEED_RETURN(errctx);
+}
+
 /*
  * Evaluate an lvalue's subscript list, if it has one, into `subscripts`. A bare
  * identifier yields the single subscript {0}, which is how a scalar is addressed

src/runtime.c

@@ -989,12 +989,13 @@ static akerr_ErrorContext *bind_structure_parameter(akbasic_Runtime *obj, akbasi
 		     obj->structtypes.types[typeindex].name,
 		     obj->structtypes.types[argvalue->structtype].name);
 
-    PASS(errctx, akbasic_environment_create(callenv, param->identifier, &variable));
+    PASS(errctx, akbasic_environment_create_empty(callenv, param->identifier, &variable));
     sizes[0] = (ispointer ? 1 : obj->structtypes.types[typeindex].slotcount);
+    /* A pointer parameter's own reference cannot escape its call scope. */
+    variable->ispointer = ispointer;
     PASS(errctx, akbasic_variable_init(variable, &obj->valuepool, sizes, 1));
     variable->valuetype = AKBASIC_TYPE_STRUCT;
     variable->structtype = typeindex;
-    variable->ispointer = ispointer;
 
     if ( ispointer ) {
 	PASS(errctx, akbasic_value_clone(argvalue, &variable->values[0]));

src/variable.c

@@ -99,18 +99,21 @@ akerr_ErrorContext *akbasic_variable_init(akbasic_Variable *obj, akbasic_ValuePo
      * the run was over, which a game loop reaches in half a minute. TODO.md
      * section 6 item 30 has the whole reduction.
      *
-     * **A `@` name is the one exclusion**, and it is the whole of it. A
+     * **A `@` name is the one exclusion**, except for a one-slot pointer
-     * structure or a pointer to one keeps pool storage because a pointer may
+     * parameter. A structure or a pointer variable keeps pool storage because
-     * outlive the scope that DIMmed it -- docs/16-structures.md says nothing is
+     * a pointer may outlive the scope that DIMmed it -- docs/16-structures.md
-     * reclaimed and akbasic_runtime_prev_environment() relies on it. The suffix
+     * says nothing is reclaimed and akbasic_runtime_prev_environment() relies
-     * is the right test rather than `structtype`, which the DIM path sets
+     * on it. A pointer parameter owns only its reference slot; the target is
+     * owned by the caller, so bind_structure_parameter() marks it before this
+     * call and lets that slot use inline storage. The suffix is the right test
+     * for every other case rather than `structtype`, which the DIM path sets
      * *after* calling this.
      *
      * Otherwise: reuse the existing slice when it is already big enough, which
      * makes a re-DIM to the same or a smaller size free; growing takes fresh
      * slots and abandons the old ones, as documented on akbasic_ValuePool.
      */
-    if ( totalsize == 1 && lastchar != '@' ) {
+    if ( totalsize == 1 && (lastchar != '@' || obj->ispointer) ) {
 	obj->values = &obj->inlinevalue;
     } else if ( obj->values == NULL || obj->valuecount < (int)totalsize ) {
 	PASS(errctx, akbasic_valuepool_take(pool, (int)totalsize, &obj->values));

tests/user_functions.c

@@ -183,6 +183,33 @@ static void test_structure_parameters(void)
     harness_stop();
 }
 
+/**
+ * @brief Pointer parameters do not consume value-pool slots per call.
+ *
+ * The pointer's parameter variable owns only one reference to the caller's
+ * record, so that reference can live in the variable's inline slot. The target
+ * remains in the caller's storage. One pointer parameter and 8,000 calls make
+ * one leaked slot per call fail against the 4,096-slot value pool.
+ */
+static void test_pointer_parameters_do_not_leak_value_slots(void)
+{
+    TEST_REQUIRE_OK(run_program_bounded("10 TYPE CRATE\n"
+					"20   W#\n"
+					"30 END TYPE\n"
+					"40 DIM A@ AS CRATE\n"
+					"50 DIM P@ AS PTR TO CRATE\n"
+					"60 POINT P@ AT A@\n"
+					"70 DEF POKEIT(P@ AS PTR TO CRATE)\n"
+					"80 P@->W# = P@->W# + 1\n"
+					"90 RETURN P@->W#\n"
+					"100 FOR I# = 1 TO 8000\n"
+					"110 R# = POKEIT(P@)\n"
+					"120 NEXT I#\n"
+					"130 PRINT A@.W#\n", 1000000));
+    TEST_REQUIRE_STR(HARNESS_OUTPUT, "8000\n");
+    harness_stop();
+}
+
 /**
  * @brief A structure parameter must name its type, and the type is checked.
  *
@@ -347,6 +374,7 @@ int main(void)
     test_runaway_recursion_is_diagnosed();
     test_single_expression_form();
     test_structure_parameters();
+    test_pointer_parameters_do_not_leak_value_slots();
     test_structure_parameter_types_are_checked();
     test_call_scopes_are_reclaimed();
     test_calls_do_not_leak_value_slots();