From c1bb553964ddfe547c74932ad5ecef14c622147f Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Wed, 21 Oct 2026 08:30:09 -0500
Subject: [PATCH 1/4] Give ProgramLifetime class instances a 32-bit header and
 packed fields

---
 .../compiler/src/backend/llvm/classes.test.ts |  72 ++++-
 packages/compiler/src/backend/llvm/classes.ts | 256 +++++++++++++++---
 packages/compiler/src/backend/llvm/emitter.ts |  33 ++-
 packages/compiler/src/backend/llvm/publish.ts |  82 ++++--
 packages/runtime/src/scr_message.c            |   2 +-
 packages/runtime/src/scr_publish.c            |  18 +-
 packages/runtime/src/scr_rc.h                 |  22 ++
 .../4635-contract-program-lifetime-layouts.ts | 224 +++++++++++++++
 ...-contract-program-lifetime-layouts.ts.json |   1 +
 9 files changed, 625 insertions(+), 85 deletions(-)
 create mode 100644 tests/corpus/4635-contract-program-lifetime-layouts.ts
 create mode 100644 tests/harness/corpus-costs/4635-contract-program-lifetime-layouts.ts.json

diff --git a/packages/compiler/src/backend/llvm/classes.test.ts b/packages/compiler/src/backend/llvm/classes.test.ts
index 8a2a4659a..3c2b0095c 100644
--- a/packages/compiler/src/backend/llvm/classes.test.ts
+++ b/packages/compiler/src/backend/llvm/classes.test.ts
@@ -7,6 +7,7 @@ import {
   mangleClassReleaseDirect,
   mangleClassRelease,
   mangleClassRetain,
+  mangleClassStruct,
   mangleClassTrace,
   mangleRecordRelease,
 } from "../mangle.js";
@@ -129,18 +130,19 @@ test("ProgramLifetime hierarchies are never freed and take no reference counts",
     expect(body(mangleClassRetain(cls)), cls).toMatch(/entry:\n {2}ret ptr %o\n}/);
     expect(body(mangleClassRelease(cls)), cls).toMatch(/entry:\n {2}ret void\n}/);
     // new: no collector header (no descriptor, no scr_cyc_alloc) and the
-    // program-lifetime count 2^31 under the class id (Node's preorder number
-    // 0 + 1, Leaf's 1 + 1), not SIZE_MAX (which means published).
+    // 32-bit header: the tag 0x70000000 and the class id (Node's preorder
+    // number 0 + 1, Leaf's 1 + 1), without a count.
     const nw = body(mangleClassNew(cls));
     const id = cls === "Node" ? 1 : 2;
-    expect(nw, cls).toContain(`store i64 ${id * 2 ** 32 + 2 ** 31}, ptr %o.vt`);
-    expect(nw, cls).not.toContain("store i64 -1, ptr %o");
+    expect(nw, cls).toContain(`store i32 ${0x70000000 + id}, ptr %o.vt`);
+    expect(nw, cls).not.toContain("store i64");
     expect(nw, cls).not.toContain("scr_cyc_alloc");
     expect(llvm, cls).not.toContain(`@${mangleClassTrace(cls)}.cyctype`);
     // The RC audit (sanitize builds) learns that the program keeps
     // program-lifetime objects; other builds emit nothing for it.
     expect(nw, cls).toContain("call void @scr_obj_lifetime_note(ptr %o)");
   }
+  expect(llvm).toContain(`%${mangleClassStruct("Leaf")} = type { i32, ptr }`);
   const plain = emitLlvmModule(mod, { pointerBits: 64, objectAudit: false });
   expect(plain).not.toContain("scr_obj_lifetime_note");
   // The mortal Owner keeps its header, counts and possible-root step.
@@ -149,6 +151,44 @@ test("ProgramLifetime hierarchies are never freed and take no reference counts",
   expect(body(mangleClassRelease("Owner"))).toContain("load i32, ptr %bufp");
 });
 
+test("ProgramLifetime classes pack their fields after a 32-bit header", () => {
+  const node = { kind: "object" as const, className: "Node" };
+  const leaf = { kind: "object" as const, className: "Leaf" };
+  const BOOL = { kind: "bool" as const };
+  const nodeFields = [
+    { name: "parent", type: node },
+    { name: "flag", type: BOOL },
+    { name: "weight", type: F64 },
+  ];
+  const mod: IrModule = {
+    irVersion: 17,
+    sourceFile: loc.file,
+    entry: "main",
+    functions: [{ name: "main", params: [], returnType: VOID, locals: [], body: [], loc }],
+    classes: [
+      { name: "Node", fields: nodeFields, methods: [], loc },
+      {
+        name: "Leaf",
+        base: "Node",
+        fields: [...nodeFields, { name: "next", type: leaf }, { name: "mark", type: BOOL }],
+        methods: [],
+        loc,
+      },
+    ],
+    contracts: { programLifetime: ["Node"] },
+  };
+  const llvm = emitLlvmModule(mod, { pointerBits: 64 });
+  // Fields go widest first, each at the first offset that fits it. Node's
+  // weight (an int32 slot: nothing stores a fraction) takes the word the
+  // header shares (24 bytes, not 32); Leaf keeps Node's offsets and puts its
+  // bool next to Node's (32 bytes, not 48).
+  expect(llvm).toContain(`%${mangleClassStruct("Node")} = type { i32, i32, ptr, i8 }`);
+  expect(llvm).toContain(`%${mangleClassStruct("Leaf")} = type { i32, i32, ptr, i8, i8, ptr }`);
+  // Without the contract, the layout is the declared one.
+  const mortal = emitLlvmModule({ ...mod, contracts: {} }, { pointerBits: 64 });
+  expect(mortal).toContain(`%${mangleClassStruct("Leaf")} = type { i64, ptr, i8, i32, ptr, i8 }`);
+});
+
 test("published ProgramLifetime instances use the published mark every other object uses", () => {
   const node = { kind: "object" as const, className: "Node" };
   const leaf = { kind: "object" as const, className: "Leaf" };
@@ -212,19 +252,25 @@ test("published ProgramLifetime instances use the published mark every other obj
     contracts: { programLifetime: ["Node"] },
   };
   const llvm = emitLlvmModule(mod, { pointerBits: 64 });
-  // The walker marks each instance like a mortal object (SIZE_MAX, logged so
-  // a refusal restores the program-lifetime count); untraced, so it never
-  // leaves a collector buffer.
+  // The walker marks each instance by the top bit of its 32-bit header,
+  // logging the word so a refusal restores it; untraced, so it never leaves
+  // a collector buffer.
   expect(llvm).toContain("call void @scr_publish(ptr");
-  // Kind 4 (an object) | 0x100 (the rc word keeps a class id: the mark
-  // writes only the count).
-  expect(llvm).toMatch(/@scr_pub_mark\(ptr %ctx, ptr %o, i1 false, i32 260\)/);
-  // The frozen-object guard of the store tests the count against UINT32_MAX:
-  // an unpublished instance (2^31) stays writable.
+  // Kind 4 (an object) | 0x200 (the 32-bit header: the mark sets its top
+  // bit).
+  expect(llvm).toMatch(/@scr_pub_mark\(ptr %ctx, ptr %o, i1 false, i32 516\)/);
+  const plain = emitLlvmModule(mod, { pointerBits: 64, objectAudit: false });
+  expect(plain).toContain("%mpub = or i32 %mrc, -2147483648");
+  // Pushing a reference skips one published before: a set top bit.
+  expect(llvm).toMatch(/%immortal0 = icmp slt i32 %rc0, 0/);
+  // The walker dispatches on the class id in the header's low 28 bits.
+  expect(llvm).toContain("%id = and i32 %idw, 268435455");
+  // The frozen-object guard of the store tests the top bit: an unpublished
+  // instance stays writable.
   const store =
     llvm.split(/^define /m).find((fn) => fn.includes("call void @scr_throw_published_field(")) ??
     "";
-  expect(store).toMatch(/icmp eq i32 %\S+, -1/);
+  expect(store).toMatch(/icmp slt i32 %\S+, 0/);
   expect(store).toContain("call void @scr_publish(ptr");
   // 32-bit targets use 2^31.
   const llvm32 = emitLlvmModule(mod, { pointerBits: 32 });
diff --git a/packages/compiler/src/backend/llvm/classes.ts b/packages/compiler/src/backend/llvm/classes.ts
index 5ad29430d..061bd06a2 100644
--- a/packages/compiler/src/backend/llvm/classes.ts
+++ b/packages/compiler/src/backend/llvm/classes.ts
@@ -32,6 +32,7 @@ import {
 } from "../mangle.js";
 import { cycTypeDescriptor, emitObjectAlloc, emitObjectFree } from "./alloc.js";
 import { llvmCommentText } from "./common.js";
+import { INLINE_UNION_FIELD } from "./inline-unions.js";
 import { NullableRefFields } from "./nullable-fields.js";
 import type { Int32Slots } from "../../ir/int32-slots.js";
 import {
@@ -88,6 +89,15 @@ export class LlClassMeta {
    * bits, and the hierarchy's vtables are one array indexed by the id
    * (mangleVtArray). */
   idInRc = false;
+  /** A ProgramLifetime class with the 32-bit header (assignCompactLayouts):
+   * the i32 at offset 0 is compactHeaderWord, with the class id in place of
+   * the rc word's high half, and the fields are packed after it. */
+  compact = false;
+  /** Compact classes: the struct's elements by offset (the header, field
+   * -1, then the fields by their index in def.fields). */
+  compactElems: { offset: number; size: number; align: number; type: string; field: number }[] = [];
+  /** Compact classes: each field's GEP index, by its index in def.fields. */
+  compactSlots: number[] = [];
 
   constructor(readonly def: IrClassDef) {
     this.root = this;
@@ -371,6 +381,10 @@ export function classSlotWordIndex(meta: LlClassMeta): number {
 /** The GEP index where a class's own fields start: after the header words
  * and the slot word. */
 export function classFieldBase(meta: LlClassMeta): number {
+  if (meta.compact)
+    throw new InternalCompilerError(
+      `llvm emitter bug: compact class ${meta.def.name} packs its fields`,
+    );
   return headerEnd(meta) + (meta.threadSlot ? 1 : 0);
 }
 
@@ -387,7 +401,12 @@ export function classFieldIndex(meta: LlClassMeta, field: string): { index: numb
     throw new InternalCompilerError(
       `llvm emitter bug: unknown field ${field} on class ${meta.def.name}`,
     );
-  return { index: classFieldBase(meta) + idx, type: meta.def.fields[idx]!.type };
+  return { index: classFieldSlot(meta, idx), type: meta.def.fields[idx]!.type };
+}
+
+/** The GEP index of the field at `idx` in `meta.def.fields`. */
+export function classFieldSlot(meta: LlClassMeta, idx: number): number {
+  return meta.compact ? meta.compactSlots[idx]! : classFieldBase(meta) + idx;
 }
 
 /** Lines that release the ThreadSlots values of the object `%o` before it
@@ -472,8 +491,7 @@ export function classPreLines(
 ): string[] {
   if (meta.idInRc)
     return [
-      `  ${pre}.idp = getelementptr inbounds i8, ptr ${obj}, i64 ${CLASS_ID_OFFSET}`,
-      `  ${pre}.id = load i32, ptr ${pre}.idp`,
+      ...classIdLoadLines(meta, obj, `${pre}.id`, ""),
       `  ${pre}.n = add i32 ${pre}.id, -1 ; the class id is the preorder number + 1`,
       `  ${pre} = zext i32 ${pre}.n to ${sizeType}`,
     ];
@@ -499,14 +517,33 @@ export function classIdVtLines(
   const vtt = `%${mangleVtStruct(root.def.name)}`;
   const n = root.post - root.pre + 1;
   return [
-    `  ${t}idp = getelementptr inbounds i8, ptr ${obj}, i64 ${CLASS_ID_OFFSET}`,
-    `  ${t}id = load i32, ptr ${t}idp${group}`,
+    ...classIdLoadLines(meta, `${obj}`, `${t}id`, group),
     `  ${t}ix = sub i32 ${t}id, ${root.pre + 1}`,
     `  ${t}ix64 = zext i32 ${t}ix to i64`,
     `  ${vt} = getelementptr inbounds [${n} x ${vtt}], ptr @${mangleVtArray(root.def.name)}, i64 0, i64 ${t}ix64`,
   ];
 }
 
+/** Lines that load the class id of the LlClassMeta.idInRc instance `obj`
+ * into the i32 `id`: the high half of the rc word, or the low 28 bits of a
+ * compact instance's header word (compactHeaderWord). */
+export function classIdLoadLines(
+  meta: LlClassMeta,
+  obj: string,
+  id: string,
+  group: string,
+): string[] {
+  if (meta.compact)
+    return [
+      `  ${id}.w = load i32, ptr ${obj}${group}`,
+      `  ${id} = and i32 ${id}.w, ${COMPACT_ID_MASK}`,
+    ];
+  return [
+    `  ${id}p = getelementptr inbounds i8, ptr ${obj}, i64 ${CLASS_ID_OFFSET}`,
+    `  ${id} = load i32, ptr ${id}p${group}`,
+  ];
+}
+
 /** The byte offset of the class id in the rc word of an instance that
  * keeps it there (LlClassMeta.idInRc): the high half of the little-endian
  * 64-bit word. The count is the i32 at offset 0. */
@@ -522,6 +559,124 @@ export function classIdRcWord(meta: LlClassMeta, lifetime: boolean): string {
   return (BigInt(meta.pre + 1) * 2n ** 32n + (lifetime ? 2n ** 31n : 1n)).toString();
 }
 
+/** The 32-bit header of a compact class instance (LlClassMeta.compact):
+ * COMPACT_HEADER_TAG, the class id (classIdRcWord's preorder number + 1, 0
+ * outside a hierarchy) in the low 28 bits, and COMPACT_PUBLISHED once
+ * publication marks the object. No other object's count reaches
+ * 0x70000000, so the runtime reads a word of 0xf0000000 and up as published
+ * for every object (scr_rc.h SCR_RC_PUBLISHED). */
+export const COMPACT_HEADER_TAG = 0x70000000;
+export const COMPACT_PUBLISHED = 0x80000000;
+export const COMPACT_ID_MASK = 0x0fffffff;
+
+/** The header word a compact instance is created with. */
+export function compactHeaderWord(meta: LlClassMeta): number {
+  return COMPACT_HEADER_TAG | (meta.hierarchy ? meta.pre + 1 : 0);
+}
+
+/** Byte size and alignment of a class field's LLVM storage type. */
+function fieldStorage(type: string): { size: number; align: number } {
+  switch (type) {
+    case "i8":
+      return { size: 1, align: 1 };
+    case "i32":
+      return { size: 4, align: 4 };
+    case "ptr":
+    case "double":
+      return { size: 8, align: 8 };
+    case INLINE_UNION_FIELD:
+      return { size: 16, align: 8 };
+    default:
+      throw new InternalCompilerError(`llvm emitter bug: class field storage ${type}`);
+  }
+}
+
+/** The LLVM storage type of each of a class's flattened fields. */
+export function classFieldTypes(host: ShapeHost, cls: IrClassDef, int32?: Int32Slots): string[] {
+  // Whole-program int32 fields (int32-slots.ts) store the exact i32.
+  return cls.fields.map((f) =>
+    int32?.isField(cls.name, f.name)
+      ? "i32"
+      : llFieldType(f.type, host.inlineUnions.classField(cls.name, f.name)),
+  );
+}
+
+/** Gives the classes `compact` admits (whole hierarchies: the test is the
+ * root's) the 32-bit header and packed fields. A class keeps its base's
+ * field offsets, and places each of its own fields, widest first, at the
+ * first offset after the header that fits it: in a hole another field's
+ * alignment left, in the base's tail padding, or at the end. So 32-bit and
+ * smaller fields share 8-byte words. The struct lists the header and the
+ * fields by offset, which LLVM's layout of the type reproduces (checked). */
+export function assignCompactLayouts(
+  metaMap: ReadonlyMap<string, LlClassMeta>,
+  compact: (root: LlClassMeta) => boolean,
+  fieldTypes: (cls: IrClassDef) => string[],
+): void {
+  const byPre = [...metaMap.values()].sort((a, b) => a.pre - b.pre);
+  for (const meta of byPre) {
+    if (meta.def.runtime === true || !compact(meta.root)) continue;
+    const base = meta.base;
+    const inherited = base?.compactElems ?? [
+      { offset: 0, size: 4, align: 4, type: "i32", field: -1 },
+    ];
+    const baseCount = base?.def.fields.length ?? 0;
+    if (base !== null && (!base.compact || !sameFieldPrefix(base.def, meta.def)))
+      throw new InternalCompilerError(
+        `llvm emitter bug: compact class ${meta.def.name} does not extend its base's layout`,
+      );
+    const elems = inherited.map((e) => ({ ...e }));
+    const types = fieldTypes(meta.def);
+    const own = types
+      .map((type, field) => ({ type, field, ...fieldStorage(type) }))
+      .slice(baseCount)
+      .sort((a, b) => b.size - a.size || a.field - b.field);
+    for (const f of own) {
+      const sorted = [...elems].sort((a, b) => a.offset - b.offset);
+      let at = -1;
+      let cursor = 0;
+      for (const e of sorted) {
+        const start = alignTo(cursor, f.align);
+        if (start + f.size <= e.offset) {
+          at = start;
+          break;
+        }
+        cursor = Math.max(cursor, e.offset + e.size);
+      }
+      if (at < 0) at = alignTo(cursor, f.align);
+      elems.push({ offset: at, size: f.size, align: f.align, type: f.type, field: f.field });
+    }
+    elems.sort((a, b) => a.offset - b.offset);
+    // LLVM places each element at the next offset its alignment allows.
+    let end = 0;
+    for (const e of elems) {
+      if (alignTo(end, e.align) !== e.offset)
+        throw new InternalCompilerError(
+          `llvm emitter bug: compact layout of ${meta.def.name} is not LLVM's`,
+        );
+      end = e.offset + e.size;
+    }
+    const slots = new Array<number>(types.length).fill(-1);
+    elems.forEach((e, index) => {
+      if (e.field >= 0) slots[e.field] = index;
+    });
+    meta.compact = true;
+    meta.compactElems = elems;
+    meta.compactSlots = slots;
+  }
+}
+
+function alignTo(n: number, align: number): number {
+  return Math.ceil(n / align) * align;
+}
+
+function sameFieldPrefix(base: IrClassDef, cls: IrClassDef): boolean {
+  return (
+    base.fields.length <= cls.fields.length &&
+    base.fields.every((f, i) => cls.fields[i]!.name === f.name)
+  );
+}
+
 function undefFieldInits(
   host: ClassHost,
   meta: LlClassMeta,
@@ -589,12 +744,17 @@ export function emitClassShapes(
 
   for (const cls of emitted) {
     const meta = metaMap.get(cls.name)!;
-    // Whole-program int32 fields (int32-slots.ts) store the exact i32.
-    const fieldTys = cls.fields.map((f) =>
-      int32?.isField(cls.name, f.name)
-        ? "i32"
-        : llFieldType(f.type, host.inlineUnions.classField(cls.name, f.name)),
-    );
+    if (meta.compact) {
+      typeDefs.push(
+        `%${mangleClassStruct(cls.name)} = type { ${meta.compactElems.map((e) => e.type).join(", ")} } ` +
+          `; class ${cls.name} (32-bit header) { ${meta.compactElems
+            .slice(1)
+            .map((e) => llvmCommentText(cls.fields[e.field]!.name))
+            .join("; ")} }`,
+      );
+      continue;
+    }
+    const fieldTys = classFieldTypes(host, cls, int32);
     // Subclasses of a runtime base embed its remaining prefix (the state
     // and display-name slots) between the vtable word and the field list;
     // stream subclasses add the stream-state pointer — upcasts to
@@ -718,7 +878,7 @@ export function emitClassShapes(
     const traced = host.tracedShapes.has(`object:${cls.name}`);
     const runtimePrefix = runtimePrefixOf(meta);
     const isStreamRooted = streamRooted(meta);
-    const fieldIndex = (i: number): number => classFieldBase(meta) + i;
+    const fieldIndex = (i: number): number => classFieldSlot(meta, i);
     // Teardown, trace and gcFree see each slot's storage type: a nullable
     // class field holds the instance pointer (NULL for the unit arm), and
     // every class retain/release/visit entry point is NULL-tolerant.
@@ -851,30 +1011,41 @@ export function emitClassShapes(
         ``,
       );
       // Direct release (the vtable entry): the class's own teardown. The
-      // caller already handled NULL/immortal.
-      const reld: string[] = [
-        `define internal void @${mangleClassReleaseDirect(cls.name)}(ptr %o) ${FN_ATTRS} { ; direct release ${cls.name}`,
-        `entry:`,
-        `  %rc = load ${count}, ptr %o`,
-        `  %n = sub ${count} %rc, 1`,
-        `  store ${count} %n, ptr %o`,
-        `  %dead = icmp eq ${count} %n, 0`,
-        `  br i1 %dead, label %free, label %${traced ? "root" : "done"}`,
-        `free:`,
-      ];
-      if (traced) reld.push(...cycleDeadLines(host, "teardown"));
-      if (bounded) {
-        const destroy = `${mangleClassReleaseDirect(cls.name)}_destroy`;
-        const destroyBody: string[] = [];
-        teardown(destroyBody);
-        defs.push(...destroyDefinitions(destroy, destroyBody, chained), ``);
-        host.declare(`declare void @scr_rc_destroy(ptr, ptr)`);
-        reld.push(`  call void @scr_rc_destroy(ptr %o, ptr @${destroy})`);
-      } else teardown(reld);
-      reld.push(`  br label %done`);
-      if (traced) reld.push(`root:`, ...cycleRootLines(host, "done"));
-      reld.push(`done:`, `  ret void`, `}`, ``);
-      defs.push(...reld);
+      // caller already handled NULL/immortal. A compact instance has no
+      // count: nothing releases it.
+      if (meta.compact) {
+        defs.push(
+          `define internal void @${mangleClassReleaseDirect(cls.name)}(ptr %o) ${FN_ATTRS} { ; direct release ${cls.name} (immortal)`,
+          `entry:`,
+          `  ret void`,
+          `}`,
+          ``,
+        );
+      } else {
+        const reld: string[] = [
+          `define internal void @${mangleClassReleaseDirect(cls.name)}(ptr %o) ${FN_ATTRS} { ; direct release ${cls.name}`,
+          `entry:`,
+          `  %rc = load ${count}, ptr %o`,
+          `  %n = sub ${count} %rc, 1`,
+          `  store ${count} %n, ptr %o`,
+          `  %dead = icmp eq ${count} %n, 0`,
+          `  br i1 %dead, label %free, label %${traced ? "root" : "done"}`,
+          `free:`,
+        ];
+        if (traced) reld.push(...cycleDeadLines(host, "teardown"));
+        if (bounded) {
+          const destroy = `${mangleClassReleaseDirect(cls.name)}_destroy`;
+          const destroyBody: string[] = [];
+          teardown(destroyBody);
+          defs.push(...destroyDefinitions(destroy, destroyBody, chained), ``);
+          host.declare(`declare void @scr_rc_destroy(ptr, ptr)`);
+          reld.push(`  call void @scr_rc_destroy(ptr %o, ptr @${destroy})`);
+        } else teardown(reld);
+        reld.push(`  br label %done`);
+        if (traced) reld.push(`root:`, ...cycleRootLines(host, "done"));
+        reld.push(`done:`, `  ret void`, `}`, ``);
+        defs.push(...reld);
+      }
     } else {
       const freeBody: string[] = [];
       teardown(freeBody);
@@ -930,7 +1101,18 @@ export function emitClassShapes(
           meta.hierarchy && !meta.idInRc ? 16 : 8,
         ),
       );
-    if (meta.idInRc) {
+    if (meta.compact && !meta.hierarchy)
+      nw.push(`  store i32 ${compactHeaderWord(meta)}, ptr %o ; no count (compactHeaderWord)`);
+    else if (meta.compact) {
+      // The header word: the class id and no count (compactHeaderWord).
+      // Only publication writes it, setting COMPACT_PUBLISHED, which the
+      // class id reads mask off, so they keep !invariant.group.
+      host.declare(`declare ptr @llvm.launder.invariant.group.p0(ptr)`);
+      nw.push(
+        `  %o.vt = call ptr @llvm.launder.invariant.group.p0(ptr %o)`,
+        `  store i32 ${compactHeaderWord(meta)}, ptr %o.vt ; class ${llvmCommentText(cls.name)} is ${meta.pre}`,
+      );
+    } else if (meta.idInRc) {
       // The class id in the high half of the rc word, the count in the low
       // half (classIdRcWord). It never changes while the object lives:
       // retains, releases and the runtime write only the low half.
diff --git a/packages/compiler/src/backend/llvm/emitter.ts b/packages/compiler/src/backend/llvm/emitter.ts
index 18cee0501..0382efc17 100644
--- a/packages/compiler/src/backend/llvm/emitter.ts
+++ b/packages/compiler/src/backend/llvm/emitter.ts
@@ -264,7 +264,9 @@ import {
   emitLibCall,
 } from "./lib-dispatch.js";
 import {
+  assignCompactLayouts,
   buildClassGraph,
+  classFieldTypes,
   classMembershipIntervals,
   classFieldIndex,
   classPreLines,
@@ -1492,6 +1494,15 @@ export class LlEmitter {
         classDefinitions: (cls, sizeOf) => regions.classDefs(cls, sizeOf),
       };
     }
+    // ProgramLifetime hierarchies count nothing: their instances get the
+    // 32-bit header and packed fields (classes.ts LlClassMeta.compact),
+    // except where regions count them or a slot word follows the header.
+    if (this.sizeType === "i64" && mod.lifetimeRegions !== true)
+      assignCompactLayouts(
+        this.classMeta,
+        (root) => programLifetimeClass(this.shapeHost, root) && !root.threadSlot,
+        (cls) => classFieldTypes(this.shapeHost, cls, this.int32Slots),
+      );
     for (const [name, rec] of RUNTIME_ERROR_CLASSES) {
       const meta = this.classMeta.get(name);
       if (!meta) break; // hand-written IR without the builtin defs: no stamps
@@ -5114,6 +5125,7 @@ export class LlEmitter {
           post: meta.post,
           runtimeRooted: meta.root.def.runtime === true,
           idInRc: meta.idInRc,
+          compact: meta.compact,
           // Inline scalar-union fields are plain bits: nothing to push.
           fields: meta.def.fields.flatMap((f) =>
             this.inlineUnions.classField(className, f.name)
@@ -5154,15 +5166,21 @@ export class LlEmitter {
 
   /** Branches on "`obj` is published" (rc == SIZE_MAX, or a count of
    * UINT32_MAX in the low half for `count` i32: classes.ts
-   * LlClassMeta.idInRc); leaves the builder in the cold block and answers
-   * the continuation label. */
-  private emitPublishedTest(obj: string, count: "i32" | "i64" = this.sizeType): string {
+   * LlClassMeta.idInRc, or the top bit of a compact header word for
+   * "compact": classes.ts COMPACT_PUBLISHED); leaves the builder in the cold
+   * block and answers the continuation label. */
+  private emitPublishedTest(obj: string, count: "i32" | "i64" | "compact" = this.sizeType): string {
     const B = this.B;
     const rc = B.tmp(),
       frozen = B.tmp(),
       expected = B.tmp();
-    B.line(`${rc} = load ${count}, ptr ${obj}`);
-    B.line(`${frozen} = icmp eq ${count} ${rc}, -1`);
+    if (count === "compact") {
+      B.line(`${rc} = load i32, ptr ${obj}`);
+      B.line(`${frozen} = icmp slt i32 ${rc}, 0`);
+    } else {
+      B.line(`${rc} = load ${count}, ptr ${obj}`);
+      B.line(`${frozen} = icmp eq ${count} ${rc}, -1`);
+    }
     this.declare(`declare i1 @llvm.expect.i1(i1, i1)`);
     B.line(`${expected} = call i1 @llvm.expect.i1(i1 ${frozen}, i1 false)`);
     const cold = B.newLabel("pub.frozen"),
@@ -5203,7 +5221,10 @@ export class LlEmitter {
     if (className !== null && !published.classGuarded(className)) return;
     const B = this.B;
     const meta = className === null ? undefined : this.classMeta.get(className);
-    const ok = this.emitPublishedTest(obj, meta?.idInRc === true ? "i32" : this.sizeType);
+    const ok = this.emitPublishedTest(
+      obj,
+      meta?.compact === true ? "compact" : meta?.idInRc === true ? "i32" : this.sizeType,
+    );
     let owner: string;
     if (meta === undefined) owner = this.cstr("Object");
     else if (!meta.hierarchy) owner = this.cstr(meta.def.jsName ?? meta.def.name);
diff --git a/packages/compiler/src/backend/llvm/publish.ts b/packages/compiler/src/backend/llvm/publish.ts
index 15221bbc3..ebe9b1a64 100644
--- a/packages/compiler/src/backend/llvm/publish.ts
+++ b/packages/compiler/src/backend/llvm/publish.ts
@@ -25,6 +25,7 @@ import type { IrClassDef, IrRecordShape, IrType, IrUnionDef, SrcLoc } from "../.
 import { mapOf, STRING, typeKey } from "../../ir/ir.js";
 import { mangleClassStruct, mangleRecordStruct } from "../mangle.js";
 import { FN_ATTRS } from "./shapes.js";
+import { COMPACT_ID_MASK } from "./classes.js";
 import { shapeTagFn } from "./nullable-unions.js";
 import type { InlineUnionFields } from "./inline-unions.js";
 import { LlvmUnsupportedError } from "./unsupported.js";
@@ -66,6 +67,8 @@ export interface PublishClassInfo {
   /** Instances keep their class id in the rc word (classes.ts
    * LlClassMeta.idInRc). */
   idInRc: boolean;
+  /** Instances have the 32-bit header (classes.ts LlClassMeta.compact). */
+  compact: boolean;
   /** The class's own and inherited fields: GEP index and storage type. */
   fields: { name: string; index: number; type: IrType }[];
 }
@@ -76,6 +79,11 @@ const PUB_KIND_UNION = 5;
 /** SCR_PUB_CLASS_ID, or'ed into a kind: the object keeps its class id
  * in the high half of its rc word (classes.ts LlClassMeta.idInRc). */
 const PUB_CLASS_ID = 0x100;
+/** SCR_PUB_COMPACT, or'ed into a kind: the object has the 32-bit header
+ * (classes.ts compactHeaderWord), whose top bit is the mark. */
+const PUB_COMPACT = 0x200;
+/** COMPACT_PUBLISHED as an i32 constant. */
+const COMPACT_MARK = "-2147483648";
 /** SCR_PUB_LOG_COUNT (scr_publish.c), in a log entry's `buf`: `rc` is the
  * count of the low half. */
 const PUB_LOG_COUNT = "-2147483648";
@@ -223,7 +231,11 @@ export class PublishWalkers {
         // hold time-value bits that no walker may push.
         if (def.arms.some((arm) => scalarRefusal(arm)))
           throw new LlvmUnsupportedError(`publish of ${typeKey(t)}`, loc);
-        const live = arms.flatMap((a) => (a.fn === null ? [] : [{ tag: a.tag, fn: a.fn }]));
+        const live = arms.flatMap((a) =>
+          a.fn === null
+            ? []
+            : [{ tag: a.tag, fn: a.fn, compact: this.compactRef(def.arms[a.tag]!) }],
+        );
         if (this.host.shapeUnion(t)) {
           // The value is the record itself (or a unit encoding): no box to
           // mark; the arm's walker marks and walks the record.
@@ -256,7 +268,7 @@ export class PublishWalkers {
           for (const a of live)
             lines.push(
               `arm${a.tag}:`,
-              ...this.push("%slot", a.fn, `u${a.tag}`),
+              ...this.push("%slot", a.fn, `u${a.tag}`, a.compact),
               `  br label %done`,
             );
         }
@@ -316,18 +328,25 @@ export class PublishWalkers {
     const lines = [
       head,
       `entry:`,
-      ...(info.idInRc
+      ...(info.compact
         ? [
-            `  %idp = getelementptr inbounds i8, ptr %o, i64 4 ; the class id (classes.ts classIdRcWord)`,
-            `  %id = load i32, ptr %idp`,
+            `  %idw = load i32, ptr %o ; the class id (classes.ts compactHeaderWord)`,
+            `  %id = and i32 %idw, ${COMPACT_ID_MASK}`,
             `  %idpre = add i32 %id, -1`,
             `  %pre = zext i32 %idpre to ${this.host.sizeType}`,
           ]
-        : [
-            `  %vtp = getelementptr inbounds ptr, ptr %o, i64 1`,
-            `  %vt = load ptr, ptr %vtp`,
-            `  %pre = load ${this.host.sizeType}, ptr %vt`,
-          ]),
+        : info.idInRc
+          ? [
+              `  %idp = getelementptr inbounds i8, ptr %o, i64 4 ; the class id (classes.ts classIdRcWord)`,
+              `  %id = load i32, ptr %idp`,
+              `  %idpre = add i32 %id, -1`,
+              `  %pre = zext i32 %idpre to ${this.host.sizeType}`,
+            ]
+          : [
+              `  %vtp = getelementptr inbounds ptr, ptr %o, i64 1`,
+              `  %vt = load ptr, ptr %vtp`,
+              `  %pre = load ${this.host.sizeType}, ptr %vt`,
+            ]),
       `  switch ${this.host.sizeType} %pre, label %other [ ${cases.map((c) => `${this.host.sizeType} ${c.pre}, label %c${c.pre}`).join(" ")} ]`,
     ];
     for (const c of cases)
@@ -363,6 +382,7 @@ export class PublishWalkers {
       path,
       loc,
       info.idInRc,
+      info.compact,
     );
   }
 
@@ -374,23 +394,26 @@ export class PublishWalkers {
    * targets), and an object with a collector header leaves its candidate
    * buffer through scr_cyc_pub_take. Anything else calls scr_pub_mark, as
    * does every mark of an RC-audit build. */
-  private markLines(cyc: boolean, kind: number, classId = false): string[] {
+  private markLines(cyc: boolean, kind: number, classId = false, compact = false): string[] {
+    const flags = compact ? PUB_COMPACT : classId ? PUB_CLASS_ID : 0;
     const call = (name: string): string[] => [
-      `  %${name} = call zeroext i1 @scr_pub_mark(ptr %ctx, ptr %o, i1 ${cyc ? "true" : "false"}, i32 ${classId ? kind | PUB_CLASS_ID : kind})`,
+      `  %${name} = call zeroext i1 @scr_pub_mark(ptr %ctx, ptr %o, i1 ${cyc ? "true" : "false"}, i32 ${kind | flags})`,
       `  br i1 %${name}, label %walk, label %done`,
     ];
     if (this.host.rcAudit === true) return call("m");
     const S = this.host.sizeType;
     // An object that keeps its class id in the rc word is marked and
     // logged through its count, the low half.
-    const C = classId ? "i32" : S;
+    // A compact object's header word is logged whole (the restore writes
+    // the low 32 bits back) and marked by its top bit.
+    const C = classId || compact ? "i32" : S;
     const field = (name: string, slot: number): string[] => [
       `  %${name}p = getelementptr inbounds ptr, ptr %ctx, i32 ${slot}`,
       `  %${name} = load ${S === "i64" ? "i64" : "i32"}, ptr %${name}p`,
     ];
     const lines = [
       `  %mrc = load ${C}, ptr %o`,
-      `  %mimm = icmp eq ${C} %mrc, -1`,
+      compact ? `  %mimm = icmp slt i32 %mrc, 0` : `  %mimm = icmp eq ${C} %mrc, -1`,
       `  br i1 %mimm, label %done, label %mtest`,
       `mtest:`,
       `  %mrefp = getelementptr inbounds ptr, ptr %ctx, i32 6`,
@@ -408,7 +431,7 @@ export class PublishWalkers {
       ok = "%mok2";
     }
     lines.push(`  br i1 ${ok}, label %mfast, label %mslow`, `mslow:`, ...call("ms"), `mfast:`);
-    let buf = classId ? String(PUB_LOG_COUNT) : "0";
+    let buf = classId || compact ? String(PUB_LOG_COUNT) : "0";
     if (cyc) {
       // ScrCycHdr.buf, the word before the object: its top bit while the
       // object is a buffered candidate.
@@ -443,7 +466,9 @@ export class PublishWalkers {
       `  store i32 ${buf}, ptr %mebuf`,
       `  %mlogged1 = add ${S} %mlogged, 1`,
       `  store ${S} %mlogged1, ptr %mloggedp`,
-      `  store ${C} -1, ptr %o`,
+      ...(compact
+        ? [`  %mpub = or i32 %mrc, ${COMPACT_MARK}`, `  store i32 %mpub, ptr %o`]
+        : [`  store ${C} -1, ptr %o`]),
       `  br label %walk`,
     );
     return lines;
@@ -453,15 +478,18 @@ export class PublishWalkers {
    * immortal references (the parent links of a tree, values published
    * before) are the ones scr_pub_push skips: they are tested here, without
    * the call. The lines end in a fresh block labelled `pushed${id}`. */
-  private push(v: string, fn: string, id: string): string[] {
+  private push(v: string, fn: string, id: string, compact = false): string[] {
     return [
       `  %null${id} = icmp eq ptr ${v}, null`,
       `  br i1 %null${id}, label %pushed${id}, label %live${id}`,
       `live${id}:`,
       // The count (the low half of a 64-bit rc word) is UINT32_MAX for an
-      // immortal object of every kind (scr_rc.h SCR_RC_COUNT).
+      // immortal object of every kind (scr_rc.h SCR_RC_COUNT), and a
+      // compact object's header word has its top bit set once published.
       `  %rc${id} = load i32, ptr ${v}`,
-      `  %immortal${id} = icmp eq i32 %rc${id}, -1`,
+      compact
+        ? `  %immortal${id} = icmp slt i32 %rc${id}, 0`
+        : `  %immortal${id} = icmp eq i32 %rc${id}, -1`,
       `  br i1 %immortal${id}, label %pushed${id}, label %push${id}`,
       `push${id}:`,
       ...this.pushLines(v, fn, id),
@@ -497,6 +525,12 @@ export class PublishWalkers {
     ];
   }
 
+  /** Whether references of type `t` are compact class instances. */
+  private compactRef(t: IrType): boolean {
+    const arm = this.host.nullableArm(t) ?? t;
+    return arm.kind === "object" && this.host.classInfo(arm.className).compact;
+  }
+
   private objectBody(
     head: string,
     struct: string,
@@ -505,8 +539,14 @@ export class PublishWalkers {
     path: string,
     loc: SrcLoc,
     classId = false,
+    compact = false,
   ): string[] {
-    const lines = [head, `entry:`, ...this.markLines(traced, PUB_KIND_OBJECT, classId), `walk:`];
+    const lines = [
+      head,
+      `entry:`,
+      ...this.markLines(traced, PUB_KIND_OBJECT, classId, compact),
+      `walk:`,
+    ];
     let n = 0;
     for (const member of members) {
       const fn = this.walker(member.type, `${path}.${member.name}`, loc);
@@ -519,7 +559,7 @@ export class PublishWalkers {
       lines.push(
         `  %f${n} = getelementptr inbounds %${struct}, ptr %o, i64 0, i32 ${member.index} ; ${llvmComment(member.name)}`,
         `  %v${n} = load ptr, ptr %f${n}`,
-        ...this.push(`%v${n}`, fn, `${n}`),
+        ...this.push(`%v${n}`, fn, `${n}`, this.compactRef(member.type)),
       );
       n++;
     }
diff --git a/packages/runtime/src/scr_message.c b/packages/runtime/src/scr_message.c
index 750dcc97d..cd0e92740 100644
--- a/packages/runtime/src/scr_message.c
+++ b/packages/runtime/src/scr_message.c
@@ -282,7 +282,7 @@ static size_t scr_message_value(ScrMessageEncoder *encoder, const ScrDyn *input)
   /* Published values (immortal) travel by pointer: a typed value as a
    * capsule over the same object, a dyn graph as itself. */
   if (input->kind == SCR_DYN_TYPED_REF && input->v.typed_ref.ptr &&
-      SCR_RC_COUNT(input->v.typed_ref.ptr) == SCR_RC_IMMORTAL) {
+      SCR_RC_PUBLISHED(input->v.typed_ref.ptr)) {
     scr_sa_thread_shares();
     encoder->visits[id].view = (ScrDyn *)input;
     ScrMessageNode *node = &encoder->message->nodes[id];
diff --git a/packages/runtime/src/scr_publish.c b/packages/runtime/src/scr_publish.c
index 0cbb02c8f..218f94d51 100644
--- a/packages/runtime/src/scr_publish.c
+++ b/packages/runtime/src/scr_publish.c
@@ -42,7 +42,8 @@ typedef struct {
 } ScrPubLog;
 
 /* In ScrPubLog.buf: the object keeps its class id in the high half of
- * its rc word (SCR_PUB_CLASS_ID), and `rc` is the count of the low half. */
+ * its rc word (SCR_PUB_CLASS_ID), and `rc` is the count of the low half,
+ * or it is compact (SCR_PUB_COMPACT), and `rc` is its header word. */
 #define SCR_PUB_LOG_COUNT UINT32_C(0x80000000)
 
 #define SCR_PUB_DYN ((uintptr_t)1)
@@ -108,7 +109,7 @@ static void *scr_pub_grow(void *p, size_t *cap, size_t size, void *inline_buf) {
 void scr_pub_str_fn(void *v, ScrPub *ctx);
 
 void scr_pub_push(ScrPub *ctx, void *v, ScrPubFn fn) {
-  if (!v || SCR_RC_COUNT(v) == SCR_RC_IMMORTAL || ctx->refusal) return;
+  if (!v || SCR_RC_PUBLISHED(v) || ctx->refusal) return;
   if (fn == scr_pub_str_fn) { /* a leaf: mark it now */
     scr_pub_str_fn(v, ctx);
     return;
@@ -125,9 +126,11 @@ void scr_pub_push(ScrPub *ctx, void *v, ScrPubFn fn) {
  * touch the objects again. */
 bool scr_pub_mark(ScrPub *ctx, void *obj, bool cyc, int kind) {
   size_t *rc = obj;
-  if (SCR_RC_COUNT(obj) == SCR_RC_IMMORTAL || ctx->refusal) return false;
-  bool class_id = (kind & SCR_PUB_CLASS_ID) != 0;
-  kind &= ~SCR_PUB_CLASS_ID;
+  if (SCR_RC_PUBLISHED(obj) || ctx->refusal) return false;
+  bool compact = (kind & SCR_PUB_COMPACT) != 0;
+  /* Both log the low 32 bits: a count, or a compact header word. */
+  bool class_id = compact || (kind & SCR_PUB_CLASS_ID) != 0;
+  kind &= ~(SCR_PUB_CLASS_ID | SCR_PUB_COMPACT);
   if (ctx->logged == ctx->log_cap)
     ctx->log = scr_pub_grow(ctx->log, &ctx->log_cap, sizeof *ctx->log, ctx->log_inline);
 #ifdef SCR_RC_AUDIT
@@ -156,7 +159,8 @@ bool scr_pub_mark(ScrPub *ctx, void *obj, bool cyc, int kind) {
   }
 #endif
   *e = (ScrPubLog){(uintptr_t)obj, logged, buf};
-  if (class_id) SCR_RC_COUNT(obj) = SCR_RC_IMMORTAL;
+  if (compact) SCR_RC_COUNT(obj) |= SCR_RC_COMPACT_PUBLISHED;
+  else if (class_id) SCR_RC_COUNT(obj) = SCR_RC_IMMORTAL;
   else *rc = SIZE_MAX;
   return true;
 }
@@ -382,7 +386,7 @@ static bool scr_pub_keep_ok(size_t bytes) {
 }
 
 void scr_publish(void *root, ScrPubFn fn) {
-  if (!root || SCR_RC_COUNT(root) == SCR_RC_IMMORTAL) return;
+  if (!root || SCR_RC_PUBLISHED(root)) return;
   ScrPub ctx;
   if (scr_pub_keep.stack) {
     ctx.stack = scr_pub_keep.stack;
diff --git a/packages/runtime/src/scr_rc.h b/packages/runtime/src/scr_rc.h
index 889202975..9ce6e385d 100644
--- a/packages/runtime/src/scr_rc.h
+++ b/packages/runtime/src/scr_rc.h
@@ -27,6 +27,24 @@ typedef size_t ScrRcCount;
 #define SCR_RC_IMMORTAL ((ScrRcCount)-1)
 #define SCR_RC_COUNT(obj) (*(ScrRcCount *)(obj))
 
+/* Compact class instances (64-bit targets; llvm/classes.ts
+ * LlClassMeta.compact): the instances of a ProgramLifetime hierarchy count
+ * nothing, so their header is one 32-bit word instead of the rc word, and
+ * their fields start at offset 4. The word is SCR_RC_COMPACT_TAG, the class
+ * id (0 outside a hierarchy) in the low 28 bits, and SCR_RC_COMPACT_PUBLISHED
+ * once publication marks it. No count reaches the tag, so a word read through
+ * SCR_RC_COUNT tells the two apart, and a count or word of
+ * SCR_RC_PUBLISHED_MIN and up is published for every object. Only emitted
+ * code holds compact instances in typed slots; the runtime reaches them
+ * through their emitted adapters, publication walkers (SCR_PUB_COMPACT) and
+ * the published test. */
+#define SCR_RC_COMPACT_TAG UINT32_C(0x70000000)
+#define SCR_RC_COMPACT_PUBLISHED UINT32_C(0x80000000)
+#define SCR_RC_COMPACT_ID_MASK UINT32_C(0x0fffffff)
+#define SCR_RC_PUBLISHED_MIN ((ScrRcCount)UINT32_C(0xf0000000))
+/* Whether `obj`, an object of any kind, is published (immortal). */
+#define SCR_RC_PUBLISHED(obj) (SCR_RC_COUNT(obj) >= SCR_RC_PUBLISHED_MIN)
+
 /* The class id in the high half of `obj`'s rc word, or 0 for an object
  * without one: the high half of any other object's word is 0, or 0xffffffff
  * (immortal) or 0x80000000 (program-lifetime), and a class id is never one of
@@ -636,6 +654,10 @@ enum {
    * the high half (SCR_RC_COUNT). The mark, and a refusal's restore, write
    * only the count. */
   SCR_PUB_CLASS_ID = 0x100,
+  /* Or'ed into a kind: a compact class instance (SCR_RC_COMPACT_TAG). The
+   * mark sets SCR_RC_COMPACT_PUBLISHED; a refusal's restore writes the
+   * logged word back. */
+  SCR_PUB_COMPACT = 0x200,
 };
 void scr_publish(void *root, ScrPubFn fn);
 void scr_pub_push(ScrPub *ctx, void *v, ScrPubFn fn);
diff --git a/tests/corpus/4635-contract-program-lifetime-layouts.ts b/tests/corpus/4635-contract-program-lifetime-layouts.ts
new file mode 100644
index 000000000..b62b7fa1a
--- /dev/null
+++ b/tests/corpus/4635-contract-program-lifetime-layouts.ts
@@ -0,0 +1,224 @@
+import { publish } from "@scriptc/threads";
+import type { ProgramLifetime } from "@scriptc/types";
+
+// ProgramLifetime classes whose fields mix widths: booleans, small integers,
+// numbers, strings, references and optional values, declared in an order
+// that leaves alignment holes, with subclasses that add fields of each width.
+// Field reads and writes, virtual dispatch, instanceof, boxing into unknown,
+// thrown instances and publication behave the same before and after the
+// objects are published.
+
+abstract class Node implements ProgramLifetime {
+  readonly kind: number;
+  flagged = false;
+  readonly pos: number;
+  end: number;
+  parent: Node | undefined = undefined;
+  constructor(kind: number, pos: number, end: number) {
+    this.kind = kind;
+    this.pos = pos;
+    this.end = end;
+  }
+  abstract label(): string;
+  width(): number {
+    return this.end - this.pos;
+  }
+}
+
+class Ident extends Node {
+  readonly text: string;
+  id = 0;
+  escaped = false;
+  constructor(pos: number, text: string) {
+    super(1, pos, pos + text.length);
+    this.text = text;
+  }
+  label(): string {
+    return `Ident(${this.text}#${this.id}${this.escaped ? "!" : ""})`;
+  }
+}
+
+class Literal extends Node {
+  readonly value: number;
+  negative: boolean;
+  radix = 10;
+  constructor(pos: number, value: number) {
+    super(2, pos, pos + String(value).length);
+    this.value = value;
+    this.negative = value < 0;
+  }
+  label(): string {
+    return `Literal(${this.value.toString(this.radix)}${this.negative ? "-" : ""})`;
+  }
+}
+
+class Binary extends Node {
+  readonly left: Node;
+  readonly right: Node;
+  operator: string;
+  precedence = 0;
+  parenthesized = false;
+  constructor(left: Node, operator: string, right: Node) {
+    super(3, left.pos, right.end);
+    this.left = left;
+    this.right = right;
+    this.operator = operator;
+    left.parent = this;
+    right.parent = this;
+  }
+  label(): string {
+    const inner = `${this.left.label()} ${this.operator} ${this.right.label()}`;
+    return this.parenthesized ? `(${inner})` : inner;
+  }
+}
+
+class Tagged extends Ident {
+  tag: string | undefined = undefined;
+  weight = 0.5;
+  marked = false;
+  label(): string {
+    return `Tagged(${super.label()}, ${this.tag ?? "-"}, ${this.weight}, ${this.marked})`;
+  }
+}
+
+class Flow implements ProgramLifetime {
+  flags: number;
+  node: Node | undefined;
+  antecedent: Flow | undefined;
+  reachable = true;
+  constructor(flags: number, node: Node | undefined, antecedent: Flow | undefined) {
+    this.flags = flags;
+    this.node = node;
+    this.antecedent = antecedent;
+  }
+}
+
+function describe(n: Node): string {
+  const parent = n.parent === undefined ? "-" : String(n.parent.kind);
+  return `${n.label()} k${n.kind} ${n.pos}-${n.end} w${n.width()} ${n.flagged ? "F" : "f"} up ${parent}`;
+}
+
+function classify(value: unknown): string {
+  if (value instanceof Tagged) return `tagged ${value.tag ?? "-"}`;
+  if (value instanceof Ident) return `ident ${value.text}`;
+  if (value instanceof Literal) return `literal ${value.value}`;
+  if (value instanceof Binary) return `binary ${value.operator}`;
+  if (value instanceof Flow) return `flow ${value.flags}`;
+  return typeof value;
+}
+
+function isNode(value: unknown): boolean {
+  return value instanceof Node;
+}
+
+function isLiteral(value: unknown): boolean {
+  return value instanceof Literal;
+}
+
+function walk(n: Node, out: string[]): void {
+  out.push(describe(n));
+  if (n instanceof Binary) {
+    walk(n.left, out);
+    walk(n.right, out);
+  }
+}
+
+function build(prefix: string): { root: Binary; flows: Flow[]; idents: Ident[] } {
+  const a = new Ident(0, `${prefix}a`);
+  a.id = 7;
+  const b = new Tagged(4, `${prefix}b`);
+  b.tag = "t";
+  b.weight = 1.25;
+  b.marked = true;
+  b.escaped = true;
+  const lit = new Literal(8, -255);
+  lit.radix = 16;
+  const sum = new Binary(a, "+", b);
+  sum.precedence = 12;
+  sum.parenthesized = true;
+  const root = new Binary(sum, "*", lit);
+  root.precedence = 13;
+  root.flagged = true;
+  const flows: Flow[] = [];
+  let prev: Flow | undefined = undefined;
+  for (const n of [a, b, lit, sum, root]) {
+    prev = new Flow(flows.length * 3 + 1, n, prev);
+    flows.push(prev);
+  }
+  flows[2]!.reachable = false;
+  return { root, flows, idents: [a, b] };
+}
+
+function report(side: string, g: { root: Binary; flows: Flow[]; idents: Ident[] }): void {
+  const out: string[] = [];
+  walk(g.root, out);
+  for (const line of out) console.log(side, line);
+  for (const f of g.flows) {
+    const node = f.node === undefined ? "-" : classify(f.node);
+    const before = f.antecedent === undefined ? "-" : String(f.antecedent.flags);
+    console.log(side, "flow", f.flags, f.reachable, node, before);
+  }
+  const boxed: unknown[] = [g.root, g.root.left, g.idents[1], g.flows[0], 3, "s"];
+  console.log(side, boxed.map(classify).join(", "));
+}
+
+function attempt(what: string, write: () => void): void {
+  try {
+    write();
+    console.log(what, "wrote");
+  } catch (e) {
+    console.log(what, (e as Error).name, (e as Error).message);
+  }
+}
+
+const g = build("x");
+report("before", g);
+
+// Writes to fields that share an 8-byte word with their neighbours.
+g.idents[0]!.id = 9;
+g.idents[0]!.escaped = true;
+const tagged = g.idents[1] as Tagged;
+tagged.marked = false;
+tagged.weight = -2;
+g.root.flagged = false;
+g.flows[1]!.flags = 40;
+report("written", g);
+
+// A thrown instance keeps its class.
+try {
+  throw g.idents[1];
+} catch (e) {
+  console.log("caught", classify(e), isNode(e), isLiteral(e));
+}
+
+// Instances as map keys and set members.
+const seen = new Map<Node, number>();
+const visited = new Set<Flow>();
+for (const f of g.flows) {
+  if (f.node !== undefined) seen.set(f.node, (seen.get(f.node) ?? 0) + f.flags);
+  visited.add(f);
+}
+console.log("keys", [...seen.entries()].map(([n, v]) => `${n.kind}:${v}`).join(" "), visited.size);
+
+const published = publish(g);
+console.log("same", published === g, publish(g.root) === g.root);
+report("published", published);
+attempt("ident id", () => {
+  published.idents[0]!.id = 1;
+});
+attempt("tagged flag", () => {
+  (published.idents[1] as Tagged).marked = true;
+});
+attempt("root flag", () => {
+  published.root.flagged = true;
+});
+attempt("flow reachable", () => {
+  published.flows[2]!.reachable = true;
+});
+report("after writes", published);
+
+// New instances next to published ones stay writable.
+const fresh = new Binary(new Ident(16, "y"), "-", new Literal(20, 3));
+fresh.precedence = 11;
+fresh.right.flagged = true;
+console.log("fresh", describe(fresh), describe(fresh.right), classify(fresh.left), classify(published.root));
diff --git a/tests/harness/corpus-costs/4635-contract-program-lifetime-layouts.ts.json b/tests/harness/corpus-costs/4635-contract-program-lifetime-layouts.ts.json
new file mode 100644
index 000000000..2b790bed2
--- /dev/null
+++ b/tests/harness/corpus-costs/4635-contract-program-lifetime-layouts.ts.json
@@ -0,0 +1 @@
+{ "plain": 4944, "san": 7176 }

From d78bd871f68f7e038c056cb71298d8a73c5de1a1 Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Wed, 21 Oct 2026 09:16:06 -0500
Subject: [PATCH 2/4] Clear recycled blocks for compact instances and keep
 TypeScript 7 parity

---
 packages/compiler/src/backend/llvm/alloc.ts   |  7 ++-
 .../compiler/src/backend/llvm/classes.test.ts | 25 ++++++++
 packages/compiler/src/backend/llvm/classes.ts |  4 +-
 ...-contract-program-lifetime-layouts.ts.json |  6 ++
 .../4635-contract-program-lifetime-layouts.ts | 61 +++++++++++++++++++
 ...-contract-program-lifetime-layouts.ts.json |  2 +-
 6 files changed, 100 insertions(+), 5 deletions(-)
 create mode 100644 packages/compiler/test/ts7/baselines/order-parity/tests/corpus/4635-contract-program-lifetime-layouts.ts.json

diff --git a/packages/compiler/src/backend/llvm/alloc.ts b/packages/compiler/src/backend/llvm/alloc.ts
index c1fc5998d..c28a63e62 100644
--- a/packages/compiler/src/backend/llvm/alloc.ts
+++ b/packages/compiler/src/backend/llvm/alloc.ts
@@ -269,8 +269,9 @@ export function cycTypeDescriptor(name: string, trace: string, free: string, kin
  * expression) in `%o`, aborting on OOM, with the RC-audit allocation note
  * on the out-of-line path. With `cyc`, the object carries a collector
  * header (scr_cyc_alloc's contract). `stored` is the number of leading
- * object bytes the caller stores unconditionally right after (at least the
- * refcount word), which a recycled block need not clear. Labels use the
+ * object bytes the caller stores unconditionally right after (the refcount
+ * word, or the 4-byte header of a compact class instance), which a recycled
+ * block need not clear; it is a multiple of 4 and at most `size`. Labels use the
  * `sa.` prefix; the lines end in an open block.
  */
 export function emitObjectAlloc(
@@ -350,7 +351,7 @@ function allocLines(
     `  store ptr %sa.next, ptr %sa.fp`,
     `  %sa.z = getelementptr inbounds i8, ptr %sa.b, i64 ${keep}`,
     `  %sa.zn = sub i64 %sa.n, ${keep}`,
-    `  call void @llvm.memset.p0.i64(ptr align 8 %sa.z, i8 0, i64 %sa.zn, i1 false)`,
+    `  call void @llvm.memset.p0.i64(ptr align ${keep % 8 === 0 ? 8 : 4} %sa.z, i8 0, i64 %sa.zn, i1 false)`,
     `  br label %sa.fast`,
     `sa.bump: ; fresh reservation memory is already zero`,
     `  %sa.c1 = add i64 %sa.c, 1`,
diff --git a/packages/compiler/src/backend/llvm/classes.test.ts b/packages/compiler/src/backend/llvm/classes.test.ts
index 3c2b0095c..2f554b458 100644
--- a/packages/compiler/src/backend/llvm/classes.test.ts
+++ b/packages/compiler/src/backend/llvm/classes.test.ts
@@ -189,6 +189,31 @@ test("ProgramLifetime classes pack their fields after a 32-bit header", () => {
   expect(mortal).toContain(`%${mangleClassStruct("Leaf")} = type { i64, ptr, i8, i32, ptr, i8 }`);
 });
 
+test("a recycled block is cleared from the end of a ProgramLifetime instance's 32-bit header", () => {
+  const BOOL = { kind: "bool" as const };
+  const mod: IrModule = {
+    irVersion: 17,
+    sourceFile: loc.file,
+    entry: "main",
+    functions: [{ name: "main", params: [], returnType: VOID, locals: [], body: [], loc }],
+    classes: [
+      { name: "Marker", fields: [], methods: [], loc },
+      { name: "Toggle", fields: [{ name: "on", type: BOOL }], methods: [], loc },
+    ],
+    contracts: { programLifetime: ["Marker", "Toggle"] },
+  };
+  const llvm = emitLlvmModule(mod, { pointerBits: 64 });
+  for (const name of ["Marker", "Toggle"]) {
+    const start = llvm.indexOf(`define internal ptr @${mangleClassNew(name)}(`);
+    const body = llvm.slice(start, llvm.indexOf("\n}\n", start));
+    // The constructor stores the 4-byte header itself; a 4-byte instance
+    // (Marker) clears nothing, and Toggle's field at offset 4 is cleared.
+    expect(body).toContain("%sa.z = getelementptr inbounds i8, ptr %sa.b, i64 4");
+    expect(body).toContain("%sa.zn = sub i64 %sa.n, 4");
+    expect(body).toContain("call void @llvm.memset.p0.i64(ptr align 4 %sa.z,");
+  }
+});
+
 test("published ProgramLifetime instances use the published mark every other object uses", () => {
   const node = { kind: "object" as const, className: "Node" };
   const leaf = { kind: "object" as const, className: "Leaf" };
diff --git a/packages/compiler/src/backend/llvm/classes.ts b/packages/compiler/src/backend/llvm/classes.ts
index 061bd06a2..1634f1f9e 100644
--- a/packages/compiler/src/backend/llvm/classes.ts
+++ b/packages/compiler/src/backend/llvm/classes.ts
@@ -1098,7 +1098,9 @@ export function emitClassShapes(
           host,
           sizeOf,
           traced && !lifetime ? { type: cycType } : null,
-          meta.hierarchy && !meta.idInRc ? 16 : 8,
+          // The words stored next: a compact instance's 4-byte header, the
+          // rc word, or the rc word and the vtable word.
+          meta.compact ? 4 : meta.hierarchy && !meta.idInRc ? 16 : 8,
         ),
       );
     if (meta.compact && !meta.hierarchy)
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/4635-contract-program-lifetime-layouts.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/4635-contract-program-lifetime-layouts.ts.json
new file mode 100644
index 000000000..31a42c38b
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/4635-contract-program-lifetime-layouts.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/4635-contract-program-lifetime-layouts.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/4635-contract-program-lifetime-layouts.ts b/tests/corpus/4635-contract-program-lifetime-layouts.ts
index b62b7fa1a..7b300091e 100644
--- a/tests/corpus/4635-contract-program-lifetime-layouts.ts
+++ b/tests/corpus/4635-contract-program-lifetime-layouts.ts
@@ -222,3 +222,64 @@ const fresh = new Binary(new Ident(16, "y"), "-", new Literal(20, 3));
 fresh.precedence = 11;
 fresh.right.flagged = true;
 console.log("fresh", describe(fresh), describe(fresh.right), classify(fresh.left), classify(published.root));
+
+// Instances in recycled blocks: freed 8- and 16-byte objects leave their
+// blocks on the allocator's free lists, and the next instances of the same
+// sizes reuse them. Marker has no fields (a 4-byte instance); Toggle's
+// field and Pair's first field sit right after the header.
+class Scrap {
+  readonly n: number;
+  constructor(n: number) {
+    this.n = n;
+  }
+}
+
+class Blank {}
+
+class Marker implements ProgramLifetime {}
+
+class Toggle implements ProgramLifetime {
+  on: boolean;
+  constructor(on: boolean) {
+    this.on = on;
+  }
+}
+
+class Pair implements ProgramLifetime {
+  readonly low: number;
+  readonly high: Node | undefined;
+  constructor(low: number, high: Node | undefined) {
+    this.low = low | 0;
+    this.high = high;
+  }
+}
+
+const blanks: Blank[] = [];
+const scraps: Scrap[] = [];
+const markers: Marker[] = [];
+const toggles: Toggle[] = [];
+const pairs: Pair[] = [];
+for (let round = 0; round < 3; round++) {
+  for (let i = 0; i < 200; i++) {
+    blanks.push(new Blank());
+    scraps.push(new Scrap(i * 1.5));
+  }
+  let sum = 0;
+  for (const s of scraps) sum += s.n;
+  blanks.length = 0;
+  scraps.length = 0;
+  for (let i = 0; i < 200; i++) {
+    markers.push(new Marker());
+    toggles.push(new Toggle(i % 3 === 0));
+    pairs.push(new Pair(i - 100, i % 2 === 0 ? undefined : published.root));
+  }
+  let on = 0;
+  let low = 0;
+  let high = 0;
+  for (const t of toggles) if (t.on) on++;
+  for (const p of pairs) {
+    low += p.low;
+    if (p.high !== undefined) high++;
+  }
+  console.log("recycled", round, sum, markers.length, on, low, high);
+}
diff --git a/tests/harness/corpus-costs/4635-contract-program-lifetime-layouts.ts.json b/tests/harness/corpus-costs/4635-contract-program-lifetime-layouts.ts.json
index 2b790bed2..4eba5d094 100644
--- a/tests/harness/corpus-costs/4635-contract-program-lifetime-layouts.ts.json
+++ b/tests/harness/corpus-costs/4635-contract-program-lifetime-layouts.ts.json
@@ -1 +1 @@
-{ "plain": 4944, "san": 7176 }
+{ "plain": 4944, "san": 7757 }

From 9d898d0affc1a32146c41962ec219a0aec4dea85 Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Wed, 21 Oct 2026 09:25:11 -0500
Subject: [PATCH 3/4] Name the stored header size for compact instances

---
 packages/compiler/src/backend/llvm/classes.ts | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/packages/compiler/src/backend/llvm/classes.ts b/packages/compiler/src/backend/llvm/classes.ts
index 1634f1f9e..9963fad5b 100644
--- a/packages/compiler/src/backend/llvm/classes.ts
+++ b/packages/compiler/src/backend/llvm/classes.ts
@@ -569,6 +569,16 @@ export const COMPACT_HEADER_TAG = 0x70000000;
 export const COMPACT_PUBLISHED = 0x80000000;
 export const COMPACT_ID_MASK = 0x0fffffff;
 
+/** The leading bytes of a new instance that its allocation function stores
+ * right after allocating it (emitObjectAlloc's `stored`), which a recycled
+ * block therefore need not clear: a compact instance's 4-byte header word
+ * (its fields start at offset 4), the 8-byte rc word, or the rc word and
+ * the vtable word of a hierarchy member that keeps its class in a vtable. */
+export function storedHeaderBytes(meta: LlClassMeta): number {
+  if (meta.compact) return 4;
+  return meta.hierarchy && !meta.idInRc ? 16 : 8;
+}
+
 /** The header word a compact instance is created with. */
 export function compactHeaderWord(meta: LlClassMeta): number {
   return COMPACT_HEADER_TAG | (meta.hierarchy ? meta.pre + 1 : 0);
@@ -1098,9 +1108,7 @@ export function emitClassShapes(
           host,
           sizeOf,
           traced && !lifetime ? { type: cycType } : null,
-          // The words stored next: a compact instance's 4-byte header, the
-          // rc word, or the rc word and the vtable word.
-          meta.compact ? 4 : meta.hierarchy && !meta.idInRc ? 16 : 8,
+          storedHeaderBytes(meta),
         ),
       );
     if (meta.compact && !meta.hierarchy)

From 6fd37b33c5470901f7b2558cf527cbfdc48202af Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Wed, 21 Oct 2026 09:55:28 -0500
Subject: [PATCH 4/4] Mark compact header loads invariant only where the header
 cannot change

---
 .../compiler/src/backend/llvm/classes.test.ts | 23 ++++++++++++++++++-
 packages/compiler/src/backend/llvm/classes.ts | 23 ++++++++++++-------
 2 files changed, 37 insertions(+), 9 deletions(-)

diff --git a/packages/compiler/src/backend/llvm/classes.test.ts b/packages/compiler/src/backend/llvm/classes.test.ts
index 2f554b458..5daaf4300 100644
--- a/packages/compiler/src/backend/llvm/classes.test.ts
+++ b/packages/compiler/src/backend/llvm/classes.test.ts
@@ -1,6 +1,12 @@
 import { expect, test } from "vitest";
 import { DYN, F64, STRING, VOID, type IrModule } from "../../ir/ir.js";
-import { buildClassGraph, classMembershipIntervals, emitClassMembershipHelper } from "./classes.js";
+import {
+  buildClassGraph,
+  classIdLoadLines,
+  classMembershipIntervals,
+  emitClassMembershipHelper,
+  type LlClassMeta,
+} from "./classes.js";
 import { emitLlvmModule } from "./emitter.js";
 import {
   mangleClassNew,
@@ -214,6 +220,21 @@ test("a recycled block is cleared from the end of a ProgramLifetime instance's 3
   }
 });
 
+test("invariant class id reads of compact instances leave out the published bit", () => {
+  const group = ", !invariant.group !{}";
+  const compact = (post: number) => ({ compact: true, root: { post } }) as unknown as LlClassMeta;
+  // Publication sets the header word's top bit; ids below 2^16 are read
+  // from the low 16 bits, which never change after construction.
+  expect(classIdLoadLines(compact(40), "%o", "%id", group)).toEqual([
+    "  %id.h = load i16, ptr %o, !invariant.group !{}",
+    "  %id = zext i16 %id.h to i32",
+  ]);
+  // Wider ids, and reads without the metadata, mask the whole word.
+  const masked = ["  %id.w = load i32, ptr %o", "  %id = and i32 %id.w, 268435455"];
+  expect(classIdLoadLines(compact(0xffff), "%o", "%id", group)).toEqual(masked);
+  expect(classIdLoadLines(compact(40), "%o", "%id", "")).toEqual(masked);
+});
+
 test("published ProgramLifetime instances use the published mark every other object uses", () => {
   const node = { kind: "object" as const, className: "Node" };
   const leaf = { kind: "object" as const, className: "Leaf" };
diff --git a/packages/compiler/src/backend/llvm/classes.ts b/packages/compiler/src/backend/llvm/classes.ts
index 9963fad5b..03ee8caa1 100644
--- a/packages/compiler/src/backend/llvm/classes.ts
+++ b/packages/compiler/src/backend/llvm/classes.ts
@@ -526,18 +526,24 @@ export function classIdVtLines(
 
 /** Lines that load the class id of the LlClassMeta.idInRc instance `obj`
  * into the i32 `id`: the high half of the rc word, or the low 28 bits of a
- * compact instance's header word (compactHeaderWord). */
+ * compact instance's header word (compactHeaderWord). `group` is the
+ * metadata of the load (`, !invariant.group !{}` for a load LLVM may treat
+ * as invariant), and it only goes on bytes that never change: publication
+ * sets the top bit of a compact header word, so a compact instance's id is
+ * then read as the word's low 16 bits, which hold the whole id when the
+ * hierarchy's ids are below 2^16, and otherwise as the masked word without
+ * the metadata. */
 export function classIdLoadLines(
   meta: LlClassMeta,
   obj: string,
   id: string,
   group: string,
 ): string[] {
-  if (meta.compact)
-    return [
-      `  ${id}.w = load i32, ptr ${obj}${group}`,
-      `  ${id} = and i32 ${id}.w, ${COMPACT_ID_MASK}`,
-    ];
+  if (meta.compact) {
+    if (group !== "" && meta.root.post + 1 <= 0xffff)
+      return [`  ${id}.h = load i16, ptr ${obj}${group}`, `  ${id} = zext i16 ${id}.h to i32`];
+    return [`  ${id}.w = load i32, ptr ${obj}`, `  ${id} = and i32 ${id}.w, ${COMPACT_ID_MASK}`];
+  }
   return [
     `  ${id}p = getelementptr inbounds i8, ptr ${obj}, i64 ${CLASS_ID_OFFSET}`,
     `  ${id} = load i32, ptr ${id}p${group}`,
@@ -1115,8 +1121,9 @@ export function emitClassShapes(
       nw.push(`  store i32 ${compactHeaderWord(meta)}, ptr %o ; no count (compactHeaderWord)`);
     else if (meta.compact) {
       // The header word: the class id and no count (compactHeaderWord).
-      // Only publication writes it, setting COMPACT_PUBLISHED, which the
-      // class id reads mask off, so they keep !invariant.group.
+      // Only publication writes it again, setting COMPACT_PUBLISHED in the
+      // top byte, so the class id reads that carry !invariant.group read
+      // the low 16 bits only (classIdLoadLines).
       host.declare(`declare ptr @llvm.launder.invariant.group.p0(ptr)`);
       nw.push(
         `  %o.vt = call ptr @llvm.launder.invariant.group.p0(ptr %o)`,