rc: a const interface or bound method made over a field read holds a count

tslang/lib/TypeScript/MLIRGenImpl.h

@@ -1701,22 +1701,28 @@ class MLIRGenImpl
             return varOp && varOp->hasAttr(OWNED_LOCAL_ATTR_NAME);
         }
 
-        // An interface or a bound method made over a value read out of such a local: `v` seen
-        // through `I` in `const i: I = v`, or `this` of `const m = v.get`.
-        static bool viewsLocalSlot(mlir::Value value)
+        // The value an interface or a bound method is made over: `v` seen through `I` in
+        // `const i: I = v`, or `this` of `const m = v.get`.
+        static mlir::Value viewedValue(mlir::Value value)
         {
             auto *definingOp = value ? value.getDefiningOp() : nullptr;
             if (auto newInterfaceOp = dyn_cast_or_null<mlir_ts::NewInterfaceOp>(definingOp))
             {
-                return readsLocalSlot(newInterfaceOp.getThisVal());
+                return newInterfaceOp.getThisVal();
             }
 
             if (isa_and_nonnull<mlir_ts::ThisSymbolRefOp, mlir_ts::ThisVirtualSymbolRefOp, mlir_ts::CreateBoundFunctionOp>(definingOp))
             {
-                return readsLocalSlot(definingOp->getOperand(0));
+                return definingOp->getOperand(0);
             }
 
-            return false;
+            return mlir::Value();
+        }
+
+        // An interface or a bound method made over a value read out of such a local.
+        static bool viewsLocalSlot(mlir::Value value)
+        {
+            return readsLocalSlot(viewedValue(value));
         }
 
         // A value read out of a field, through any casts: what `h.c` is in `const c = h.c`.
@@ -1881,8 +1887,10 @@ class MLIRGenImpl
             // it holds the block as `b` would.
             //
             // Under rc, so does a const read out of a field (`const c = h.c`) or an element (`const c =
-            // a[0]`, `const [c] = a`): folded, `h.c = d` or `a[0] = d` freed what `c` still read. Under
-            // own the folded read is a borrow, and a use after the overwrite is an error there.
+            // a[0]`, `const [c] = a`): folded, `h.c = d` or `a[0] = d` freed what `c` still read. So
+            // does an interface or a bound method made over a field read (`const i: I = h.v`,
+            // `const m = h.v.get`), which holds the block as `c` would. Under own the folded read is a
+            // borrow, and a use after the overwrite is an error there.
             //
             // And a const record whose fields own blocks (`const o = { s: p.child }`): the literal
             // hands its fields over unretained - its receiver is the one that retains them, as a
@@ -1901,7 +1909,8 @@ class MLIRGenImpl
                 || MLIRTypeHelper::isConstTupleHoldingArray(type)
                 || ((isa<mlir_ts::ArrayType>(type) || MLIRTypeHelper::isSharedHandleType(type)
                      || ((readsLocalSlot(initial) || viewsLocalSlot(initial)) && mth.ownsHeapMemory(location, type))
-                     || (compileOptions.isRefCounted() && (readsField(initial) || readsElement(initial))
+                     || (compileOptions.isRefCounted()
+                         && (readsField(initial) || readsElement(initial) || readsField(viewedValue(initial)))
                          && mth.ownsHeapMemory(location, type))
                      || (readsLiteralSlot(mth, location, initial) && isa<mlir_ts::TupleType>(type)
                          && mth.ownsHeapMemory(location, type))

tslang/test/tester/CMakeLists.txt

@@ -390,6 +390,7 @@ tslang_add_test(NAME test-compile-00-parse-number-range COMMAND test-runner "${P
 tslang_add_test(NAME test-compile-00-global-argument-held COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00global_argument_held.ts")
 tslang_add_test(NAME test-compile-00-narrowed-assign-other-member COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00narrowed_assign_other_member.ts")
 tslang_add_test(NAME test-compile-00-bound-method-count COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00bound_method_count.ts")
+tslang_add_test(NAME test-compile-00-const-view-of-field COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00const_view_of_field.ts")
 tslang_add_test(NAME test-compile-00-const-record-owned-fields COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00const_record_owned_fields.ts")
 tslang_add_test(NAME test-compile-00-generator-temporaries COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00generator_temporaries.ts")
 tslang_add_test(NAME test-compile-00-array-length-valid COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00array_length_valid.ts")
@@ -963,6 +964,7 @@ tslang_add_test(NAME test-jit-00-parse-number-range COMMAND test-runner -jit "${
 tslang_add_test(NAME test-jit-00-global-argument-held COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00global_argument_held.ts")
 tslang_add_test(NAME test-jit-00-narrowed-assign-other-member COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00narrowed_assign_other_member.ts")
 tslang_add_test(NAME test-jit-00-bound-method-count COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00bound_method_count.ts")
+tslang_add_test(NAME test-jit-00-const-view-of-field COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00const_view_of_field.ts")
 tslang_add_test(NAME test-jit-00-const-record-owned-fields COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00const_record_owned_fields.ts")
 tslang_add_test(NAME test-jit-00-generator-temporaries COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00generator_temporaries.ts")
 tslang_add_test(NAME test-jit-00-array-length-valid COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00array_length_valid.ts")
@@ -1926,6 +1928,7 @@ set(TSLANG_CORPUS
     00global_argument_held.ts
     00narrowed_assign_other_member.ts
     00bound_method_count.ts
+    00const_view_of_field.ts
     00const_record_owned_fields.ts
     00generator_temporaries.ts
     00array_length_valid.ts

tslang/test/tester/tests/00const_view_of_field.ts

@@ -0,0 +1,63 @@
+// A `const` interface or bound method made over a field read keeps the object alive when the field
+// is given a new value, as a `let` does. Folded, the const was the view itself and held no
+// reference of its own: under rc `h.v = mkv(..)` freed the object `m()` and `i.get()` then ran on.
+// Each call follows allocations of the same size, so a freed object is reused.
+interface I {
+    get(): number;
+}
+
+class W {
+    constructor(public x: number) {}
+}
+
+class V {
+    items: number[] = [1, 2, 3];
+    constructor(public w: W) {}
+
+    get() {
+        return this.w.x;
+    }
+}
+
+class H {
+    v: V = mkv(0);
+}
+
+function mkv(n: number) {
+    return new V(new W(n));
+}
+
+function churn(): number {
+    let s: number = 0;
+    const a: V[] = [];
+    for (let i = 0; i < 64; i++) a.push(mkv(1000 + i));
+    for (const e of a) s += e.w.x;
+    return s;
+}
+
+function boundMethod(n: number) {
+    const h = new H();
+    h.v = mkv(n);
+    const m = h.v.get;
+    h.v = mkv(n + 1);
+    churn();
+    return m() == n && h.v.w.x == n + 1;
+}
+
+function interfaceView(n: number) {
+    const h = new H();
+    h.v = mkv(n);
+    const i: I = h.v;
+    h.v = mkv(n + 1);
+    churn();
+    return i.get() == n && h.v.w.x == n + 1;
+}
+
+function main() {
+    for (let n = 0; n < 20; n++) {
+        assert(boundMethod(n), "a const bound method over a field");
+        assert(interfaceView(n), "a const interface over a field");
+    }
+
+    print("done.");
+}