diff --git a/benchmarks/runtime/README.md b/benchmarks/runtime/README.md
index f3a444ea98..149c348d04 100644
--- a/benchmarks/runtime/README.md
+++ b/benchmarks/runtime/README.md
@@ -21,6 +21,7 @@ Application-shaped workloads for measuring the speed of compiled executables. Ea
 | `json-records` | cpu | `JSON.stringify`, typed `JSON.parse`, Map aggregation |
 | `json-unions` | cpu | `JSON.parse` into discriminated unions: an event log with eight kinds and nested expression trees with nine, evaluated after the cast |
 | `keyed-option-writes` | cpu | class methods whose names are also keys of option records written through element accesses: `enabled[name] = true` over a union of literal names and `totals["count"] = n` |
+| `layered-errors` | cpu | validation errors thrown below a stack of small functions that hold strings and arrays, and caught per entry in a loop: about half of 640,000 entries fail |
 | `line-reader` | cpu | `readline` over a file stream: 'line' events for a 67 MB access log (every 50th line not ASCII), with a status count, a path tally and byte totals |
 | `log-lines` | cpu | 200k immediate `console.log`/`process.stdout.write` lines into the runner's pipe |
 | `log-summary`, `inventory-report` | cpu | the multi-module applications from `benchmarks/builds`, on 300k-line inputs |
diff --git a/benchmarks/runtime/workloads/layered-errors.json b/benchmarks/runtime/workloads/layered-errors.json
new file mode 100644
index 0000000000..70971552c4
--- /dev/null
+++ b/benchmarks/runtime/workloads/layered-errors.json
@@ -0,0 +1,6 @@
+{
+  "entry": "workloads/layered-errors.ts",
+  "args": [
+    "1"
+  ]
+}
diff --git a/benchmarks/runtime/workloads/layered-errors.ts b/benchmarks/runtime/workloads/layered-errors.ts
new file mode 100644
index 0000000000..bf2ff581cb
--- /dev/null
+++ b/benchmarks/runtime/workloads/layered-errors.ts
@@ -0,0 +1,95 @@
+// A configuration loader whose validation errors are thrown at the bottom
+// of a stack of small layers and caught per entry at the top: each layer
+// holds strings and arrays while the error passes through it.
+class ConfigError extends Error {
+  readonly key: string;
+  constructor(key: string, message: string) {
+    super(message);
+    this.key = key;
+  }
+}
+
+let seed = 4242;
+function random(): number {
+  seed = (Math.imul(seed, 1103515245) + 12345) >>> 0;
+  return seed / 4294967296;
+}
+
+function fail(key: string, text: string): never {
+  throw new ConfigError(key, `invalid value "${text}" for ${key}`);
+}
+function digits(key: string, text: string): number {
+  let value = 0;
+  for (let i = 0; i < text.length; i++) {
+    const c = text.charCodeAt(i);
+    if (c < 48 || c > 57) fail(key, text);
+    value = value * 10 + (c - 48);
+  }
+  return value;
+}
+function parseNumber(key: string, text: string): number {
+  return text.length === 0 ? fail(key, text) : digits(key, text);
+}
+function checkRange(key: string, text: string, max: number): number {
+  const value = parseNumber(key, text);
+  return value > max ? fail(key, text) : value;
+}
+function coerce(key: string, raw: string, max: number): number {
+  const trimmed = raw.trim();
+  const parts = [key, trimmed];
+  const value = checkRange(key, trimmed, max);
+  return value + parts.length;
+}
+function resolveValue(key: string, raw: string, defaults: Map<string, string>): number {
+  const text = raw === "default" ? (defaults.get(key) ?? "") : raw;
+  const label = "value:" + key;
+  return coerce(key, text, 100000) + label.length;
+}
+function loadEntry(section: string, entry: string, defaults: Map<string, string>): number {
+  const eq = entry.indexOf("=");
+  const key = section + "." + entry.slice(0, eq);
+  const path = [section, key];
+  return resolveValue(key, entry.slice(eq + 1), defaults) + path.length;
+}
+function loadEntryWithContext(section: string, entry: string, defaults: Map<string, string>): number {
+  const context = `${section}/${entry.length}`;
+  return loadEntry(section, entry, defaults) + context.length;
+}
+
+function makeEntry(i: number): string {
+  const r = random();
+  const name = "k" + (i % 97);
+  if (r < 0.3) return `${name}=x${i}`;
+  if (r < 0.4) return `${name}=`;
+  if (r < 0.5) return `${name}=${200000 + i}`;
+  if (r < 0.55) return `${name}=default`;
+  return `${name}= ${Math.floor(random() * 90000)} `;
+}
+
+const rounds = Number(process.argv[2] ?? "1");
+const defaults = new Map<string, string>();
+for (let i = 0; i < 97; i += 2) defaults.set(`net.k${i}`, String(i * 10));
+const sections = ["net", "db", "cache", "auth"];
+const entries: string[] = [];
+for (let i = 0; i < 4000; i++) entries.push(makeEntry(i));
+
+let loaded = 0;
+let sum = 0;
+let failed = 0;
+const failedKeys = new Map<string, number>();
+for (let round = 0; round < rounds * 40; round++) {
+  for (const section of sections) {
+    for (const entry of entries) {
+      try {
+        sum += loadEntryWithContext(section, entry, defaults);
+        loaded++;
+      } catch (error) {
+        if (!(error instanceof ConfigError)) throw error;
+        failed++;
+        failedKeys.set(error.key, (failedKeys.get(error.key) ?? 0) + 1);
+      }
+    }
+  }
+}
+console.log(`loaded ${loaded} failed ${failed} sum ${sum} distinct failing keys ${failedKeys.size}`);
+console.log(`net.k0 failures ${failedKeys.get("net.k0") ?? 0}`);
diff --git a/packages/compiler/src/backend/llvm/blocks.ts b/packages/compiler/src/backend/llvm/blocks.ts
index 3ae65cb0f0..2aef040a19 100644
--- a/packages/compiler/src/backend/llvm/blocks.ts
+++ b/packages/compiler/src/backend/llvm/blocks.ts
@@ -97,12 +97,22 @@ export class BlockBuilder {
       if (s === UNWIND_RESUME) {
         const exception = this.tmp();
         this.line(`${exception} = load { ptr, i32 }, ptr ${UNWIND_SLOT}`);
-        s = `resume { ptr, i32 } ${exception}`;
+        this.resumeUnwind(exception);
+        return;
       }
       this.cur.term = `  ${this.withDebugLocation(s)}`;
     }
   }
 
+  /** Ends the block by continuing an unwind with `exception`, the value
+   * of a landing pad (UNWIND_RESUME_DECLARATION). */
+  resumeUnwind(exception: string): void {
+    const token = this.tmp();
+    this.line(`${token} = extractvalue { ptr, i32 } ${exception}, 0`);
+    this.line(`call void @scr_unwind_resume(ptr ${token})`);
+    this.terminate("unreachable");
+  }
+
   br(label: string, attachment = ""): void {
     this.terminate(`br label %${label}${attachment}`);
   }
diff --git a/packages/compiler/src/backend/llvm/emit-function.ts b/packages/compiler/src/backend/llvm/emit-function.ts
index 02c12305c5..aab4459e0c 100644
--- a/packages/compiler/src/backend/llvm/emit-function.ts
+++ b/packages/compiler/src/backend/llvm/emit-function.ts
@@ -62,7 +62,7 @@ import { emitIntegerClone, emitIntegerDispatcher } from "./integer-clones.js";
 import { beginFunctionLoops, LOOPS } from "./loop-state.js";
 import { runningExtremumUpdates } from "./running-extrema.js";
 import { findIntegerOrNaNLocals } from "./wrapping-sums.js";
-import { UNWIND_PERSONALITY, UNWIND_SLOT } from "./unwind-abi.js";
+import { UNWIND_PERSONALITY, UNWIND_RESUME_DECLARATION, UNWIND_SLOT } from "./unwind-abi.js";
 
 export function emitFunction(emitter: LlEmitter, fn: IrFunction): string {
   const B = new BlockBuilder();
@@ -681,7 +681,10 @@ export function emitFunction(emitter: LlEmitter, fn: IrFunction): string {
   emitter.privateGlobalSlots.clear();
   // Landing pads (emitDirectCall) need the personality.
   const personality = emitter.usesLandingPads ? ` ${UNWIND_PERSONALITY}` : "";
-  if (emitter.usesLandingPads) emitter.declare(`declare i32 @__gcc_personality_v0(...)`);
+  if (emitter.usesLandingPads) {
+    emitter.declare(`declare i32 @__gcc_personality_v0(...)`);
+    emitter.declare(UNWIND_RESUME_DECLARATION);
+  }
   let body = `define internal ${ret} @${symbol}${int32Abi ? INT32_ABI_SUFFIX : ""}(${params.join(", ")}) ${attrs}${personality}${debug} { ; ${fn.name}\n${B.render()}\n}`;
   if (int32Abi) body += "\n" + emitter.emitInt32AbiWrapper(fn, symbol, int32Abi);
   if (emitter.integerClones.members.has(fn.name)) {
diff --git a/packages/compiler/src/backend/llvm/emitter.ts b/packages/compiler/src/backend/llvm/emitter.ts
index e25d78e53d..9c2d3bfe64 100644
--- a/packages/compiler/src/backend/llvm/emitter.ts
+++ b/packages/compiler/src/backend/llvm/emitter.ts
@@ -160,6 +160,7 @@ import {
   UNWIND_PERSONALITY,
   UNWIND_RAISE,
   UNWIND_RAISE_DECLARATION,
+  UNWIND_RESUME_DECLARATION,
   UNWIND_RESUME,
   UNWIND_SLOT,
   UnwindAbiViolation,
@@ -6110,7 +6111,8 @@ export class LlEmitter {
       const exception = B.tmp();
       B.line(`${exception} = landingpad { ptr, i32 } cleanup`);
       for (const index of borrowed) this.releaseFrame(`%p${index}`, fn.params[index]!.type);
-      B.terminate(`resume { ptr, i32 } ${exception}`);
+      this.declare(UNWIND_RESUME_DECLARATION);
+      B.resumeUnwind(exception);
       B.startBlock(ok);
     } else B.line(ret === "void" ? call : `%result = ${call}`);
     // A borrowed result is reachable from the parameters: own it first. A
diff --git a/packages/compiler/src/backend/llvm/unwind-abi.test.ts b/packages/compiler/src/backend/llvm/unwind-abi.test.ts
index bcef1c3796..da490324b7 100644
--- a/packages/compiler/src/backend/llvm/unwind-abi.test.ts
+++ b/packages/compiler/src/backend/llvm/unwind-abi.test.ts
@@ -93,7 +93,9 @@ test("a method of a class outside any hierarchy is on the unwinding ABI; closure
   // closure over mid is a pointer target and returns with the flag.
   const add = body(ir, "Counter_add");
   expect(add).toMatch(/invoke double @sc_b?f_[^(]*mid/);
-  expect(add).toMatch(/resume \{ ptr, i32 \} %[\w.]+/);
+  expect(add).toMatch(/call void @scr_unwind_resume\(ptr %[\w.]+\)\n\s+unreachable/);
+  expect(add).not.toMatch(/\bresume \{/);
+  expect(ir).toContain("declare void @scr_unwind_resume(ptr) cold");
   expect(add).not.toContain("@scr_unwind_raise");
   const closures = ir
     .split(/\n(?=define )/)
@@ -212,7 +214,10 @@ test("an invoked wrapper of a raising function resumes the unwind; its address m
   expect(out).toContain(
     "%r = invoke double @sc_bf_body.i32(i32 %i) to label %ub.ok.0 unwind label %ub.lp.0",
   );
-  expect(out).toContain("resume { ptr, i32 } %ub.v.0");
+  expect(out).toContain(
+    "%ub.e.0 = extractvalue { ptr, i32 } %ub.v.0, 0\n  call void @scr_unwind_resume(ptr %ub.e.0)\n  unreachable",
+  );
+  expect(out).toContain("declare void @scr_unwind_resume(ptr) cold");
   expect(() =>
     bridgePlainUnwindCalls([
       `${raising}\n@table = internal constant [1 x ptr] [ptr @sc_bf_body.i32]`,
@@ -277,11 +282,27 @@ test("calls that cannot unwind are marked nounwind", () => {
     "  unreachable",
     "}",
   ].join("\n");
-  const [out] = bridgePlainUnwindCalls([raising]);
+  const resuming = [
+    "define internal void @sc_bf_resumes(ptr %b) #0 personality ptr @__gcc_personality_v0 {",
+    "entry:",
+    "  invoke void @sc_bf_raises(ptr %b) to label %ok unwind label %lp",
+    "ok:",
+    "  ret void",
+    "lp:",
+    "  %e = landingpad { ptr, i32 } cleanup",
+    "  %t = extractvalue { ptr, i32 } %e, 0",
+    "  call void @scr_unwind_resume(ptr %t)",
+    "  unreachable",
+    "}",
+  ].join("\n");
+  const [out] = bridgePlainUnwindCalls([[raising, resuming].join("\n")]);
   expect(out).toContain("%n = call i64 @scr_bytes_length(ptr %b) nounwind ; length");
   expect(out).toContain("call void @scr_bytes_release(ptr %b) nounwind, !dbg !4");
   expect(out).toContain("%r = call ptr %b(ptr %b) nounwind");
   expect(out).toContain("  call void @scr_unwind_raise()\n");
+  // The resume unwinds: where LLVM inlines the function at an invoke, the
+  // call becomes an invoke of the caller's landing pad.
+  expect(out).toContain("  call void @scr_unwind_resume(ptr %t)\n");
 });
 
 test("functions without a plain call of a raising function change only their call lines", () => {
diff --git a/packages/compiler/src/backend/llvm/unwind-abi.ts b/packages/compiler/src/backend/llvm/unwind-abi.ts
index fdd0a2f18b..2c0721a133 100644
--- a/packages/compiler/src/backend/llvm/unwind-abi.ts
+++ b/packages/compiler/src/backend/llvm/unwind-abi.ts
@@ -76,9 +76,11 @@ function recursiveFunctions(edges: ReadonlyMap<string, readonly string[]>): Set<
  * than a `call`'s callee, and not called from an async or generator body
  * (coroutine bodies keep plain calls).
  *
- * A native unwind costs about 2 µs a raise and 0.1 µs a frame, against a
- * few nanoseconds for a flag test, so the functions an exception is likely to
- * cross many of keep the flag: those with a `throw` or rethrow of their own
+ * A native unwind costs about 0.05 µs a raise and 10 to 25 ns a frame on
+ * x86-64 (scr_unwind.c's cached plans) and about 2 µs a raise and 0.1 µs a
+ * frame through libgcc's unwinder, against a few nanoseconds for a flag
+ * test, so the functions an exception is likely to cross many of keep the
+ * flag: those with a `throw` or rethrow of their own
  * and those up to THROW_LEVELS calls above one (a handler near the throw
  * takes it without a native unwind), and the recursive ones (deep stacks
  * are recursions). */
@@ -254,8 +256,8 @@ export function unwindAbiFunctions(
  * its own takes the exception. */
 export const UNWIND_RAISE = "call void @scr_unwind_raise()\n  unreachable";
 /** What the unwind of a landing pad ends with when no handler of the
- * function takes the exception: the unwind goes on (`resume`) with the
- * exception the landing pad stored in UNWIND_SLOT. The block builder
+ * function takes the exception: the unwind goes on (scr_unwind_resume) with
+ * the exception the landing pad stored in UNWIND_SLOT. The block builder
  * expands it (BlockBuilder.terminate). */
 export const UNWIND_RESUME = "resume { ptr, i32 } <unwind slot>";
 /** The function's slot for the exception its landing pads take. */
@@ -264,13 +266,17 @@ export const UNWIND_SLOT = "%unw.exc";
  * would call the sanitizer's no-return hook before it, which fails on fiber
  * stacks; scr_unwind_raise clears the unwound frames' shadow itself. */
 export const UNWIND_RAISE_DECLARATION = "declare void @scr_unwind_raise() cold";
+/** scr_unwind_resume's declaration. Landing pads call it with the
+ * exception they took to continue the unwind (BlockBuilder.resumeUnwind);
+ * like scr_unwind_raise, it is not `noreturn`. */
+export const UNWIND_RESUME_DECLARATION = "declare void @scr_unwind_resume(ptr) cold";
 
 /** The personality of functions with landing pads: the landing pads are
- * cleanups that scr_unwind_raise's forced unwind runs. */
+ * cleanups that scr_unwind_raise's unwind runs. */
 export const UNWIND_PERSONALITY = "personality ptr @__gcc_personality_v0";
 
-/** An unwind leaving the function: a raise, or a landing pad's `resume`. */
-const UNWINDS = /call void @scr_unwind_raise\(\)|\n\s+resume /;
+/** An unwind leaving the function: a raise, or a landing pad's resume. */
+const UNWINDS = /call void @scr_unwind_raise\(\)|call void @scr_unwind_resume\(/;
 /** A pending-flag test: the thread's exception word, the active cell, or the runtime query. */
 const PENDING_TEST = /@scr_exc_flag\b|%exc\.cell\b|@scr_exc_active\b|@scr_exc_pending\(/;
 
@@ -307,15 +313,16 @@ function argumentsEnd(line: string, open: number): number {
   return close;
 }
 
-/** A call that cannot unwind, marked so: only scr_unwind_raise and the
- * raising functions propagate an unwind (runtime code and every function
- * reached through a pointer return normally), so every other call is
- * `nounwind`, and the inliner keeps it a call where it inlines a raising
- * function at an invoke. */
+/** A call that cannot unwind, marked so: only scr_unwind_raise,
+ * scr_unwind_resume and the raising functions propagate an unwind (runtime
+ * code and every function reached through a pointer return normally), so
+ * every other call is `nounwind`, and the inliner keeps it a call where it
+ * inlines a raising function at an invoke. */
 function markNounwind(line: string): string {
   const call = line.indexOf("call ");
   const callee = CALLEE.exec(line.slice(call));
-  if (callee === null || callee[0] === "@scr_unwind_raise(") return line;
+  if (callee === null || callee[0] === "@scr_unwind_raise(" || callee[0] === "@scr_unwind_resume(")
+    return line;
   const close = argumentsEnd(line, call + callee.index + callee[0].length - 1);
   return close >= line.length
     ? line
@@ -446,6 +453,7 @@ export function bridgePlainUnwindCalls(parts: string[]): string[] {
       }
   if (violations.size > 0) throw new UnwindAbiViolation(violations);
   let personality = false;
+  let resumes = false;
   let counter = 0;
   const out = split.map((chunks) =>
     chunks
@@ -525,8 +533,11 @@ export function bridgePlainUnwindCalls(parts: string[]): string[] {
             result.push(
               `${pad}:`,
               `${indent}%ub.v.${n} = landingpad { ptr, i32 } cleanup`,
-              `${indent}resume { ptr, i32 } %ub.v.${n}`,
+              `${indent}%ub.e.${n} = extractvalue { ptr, i32 } %ub.v.${n}, 0`,
+              `${indent}call void @scr_unwind_resume(ptr %ub.e.${n})`,
+              `${indent}unreachable`,
             );
+            resumes = true;
             result.push(`${ok}:`);
           } else if (m[2] !== undefined) {
             const type = (m[3] ?? "").replace(RETURN_ATTRIBUTES, "").trim();
@@ -585,5 +596,7 @@ export function bridgePlainUnwindCalls(parts: string[]): string[] {
   );
   if (personality && !out.some((part) => part.includes("@__gcc_personality_v0(...)")))
     out[0] = `declare i32 @__gcc_personality_v0(...)\n${out[0]}`;
+  if (resumes && !out.some((part) => part.includes(UNWIND_RESUME_DECLARATION)))
+    out[0] = `${UNWIND_RESUME_DECLARATION}\n${out[0]}`;
   return out;
 }
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/unwind-landing-pad-chains.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/unwind-landing-pad-chains.ts.json
new file mode 100644
index 0000000000..84c925f1ed
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/unwind-landing-pad-chains.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/unwind-landing-pad-chains.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/runtime/src/scr_exception.h b/packages/runtime/src/scr_exception.h
index 168b2fe2d7..6c5f83358c 100644
--- a/packages/runtime/src/scr_exception.h
+++ b/packages/runtime/src/scr_exception.h
@@ -129,6 +129,8 @@ _Static_assert(offsetof(ScrExcCell, kind) == 0 && sizeof(ScrExcKind) == 4 && SCR
  * functions, unwind-abi.ts). Linux (glibc and musl) only; it never
  * returns. */
 SCR_COLD _Noreturn void scr_unwind_raise(void);
+/* Continues the unwind from a landing pad with the exception it took. */
+SCR_COLD void scr_unwind_resume(void *exc);
 SCR_COLD void scr_throw_f64(double v);
 SCR_COLD void scr_throw_bool(bool v);
 SCR_COLD void scr_throw_str(ScrStr *v); /* takes ownership */
diff --git a/packages/runtime/src/scr_unwind.c b/packages/runtime/src/scr_unwind.c
index 50474e82e4..339f85088b 100644
--- a/packages/runtime/src/scr_unwind.c
+++ b/packages/runtime/src/scr_unwind.c
@@ -1,6 +1,24 @@
 /* scr_unwind_raise: the native unwind of the unwinding exception ABI of
- * emitted functions (the compiler's unwind-abi.ts). Linux (glibc and musl)
- * only; elsewhere this unit is empty. */
+ * emitted functions (the compiler's unwind-abi.ts), and scr_unwind_resume,
+ * which emitted landing pads call to continue it. Linux (glibc and musl)
+ * only; elsewhere this unit is empty.
+ *
+ * Emitted landing pads are cleanups: they release the frame's references
+ * and then either take the exception (a handler of theirs, or a frame on
+ * the pending-flag ABI that returns with it pending) or resume the unwind.
+ * So the unwind is a single pass through every landing pad; the exception
+ * itself stays in the active cell.
+ *
+ * On x86-64 the unwind walks with plans cached per return address. The
+ * first raise through a return address finds the function's FDE through
+ * the .eh_frame_hdr of the object that contains it, runs the CFI up to the
+ * call, and finds the call site's landing pad in the LSDA. The plan records
+ * how to compute the frame's CFA, where each callee-saved register is
+ * saved, and the landing pad; later raises only look plans up. A landing
+ * pad runs with rax holding the raise's token and rdx 0 (a cleanup). When a
+ * frame's FDE uses CFI or encodings the plans do not express, or a return
+ * address has no FDE, the unwind goes on from the frame the walk started at
+ * with libgcc's forced unwind, as on other Linux targets. */
 #include "scr_runtime.h"
 
 #if defined(__linux__) && !defined(__ANDROID__) && !defined(__wasi__)
@@ -16,6 +34,7 @@
  * which link it in. */
 #pragma weak _Unwind_ForcedUnwind
 #pragma weak _Unwind_GetCFA
+#pragma weak _Unwind_Resume
 
 #if defined(__SANITIZE_ADDRESS__)
 #define SCR_UNWIND_ASAN 1
@@ -34,11 +53,6 @@
 static SCR_TL uintptr_t scr_unwind_low;
 #endif
 
-/* Emitted landing pads are cleanups: they release the frame's references
- * and then either take the exception (a handler of theirs, or a frame on
- * the pending-flag ABI that returns with it pending) or resume the unwind.
- * So the unwind is forced, a single pass through every landing pad; the
- * exception itself stays in the active cell. */
 static _Unwind_Reason_Code scr_unwind_stop(int version, _Unwind_Action actions,
                                            _Unwind_Exception_Class cls,
                                            struct _Unwind_Exception *exc,
@@ -69,7 +83,10 @@ static _Unwind_Reason_Code scr_unwind_stop(int version, _Unwind_Action actions,
 
 static SCR_TL struct _Unwind_Exception scr_unwind_exception;
 
-void scr_unwind_raise(void) {
+/* libgcc's forced unwind, from the caller of the function that calls this.
+ * It does not return; it is not declared noreturn, so sanitized builds call
+ * no sanitizer hook before it (see scr_unwind_stop). */
+static __attribute__((noinline)) void scr_unwind_forced(void) {
   struct _Unwind_Exception *exc = &scr_unwind_exception;
   memset(exc, 0, sizeof *exc);
   exc->exception_class = 0x5343524950540000ull; /* "SCRIPT\0\0" */
@@ -80,4 +97,686 @@ void scr_unwind_raise(void) {
   fputs("scriptc: the native unwinder could not raise an exception\n", stderr);
   abort();
 }
+
+#if defined(__x86_64__)
+#include <link.h>
+
+/* DWARF register numbers. */
+enum {
+  SCR_UNW_RBX = 3,
+  SCR_UNW_RBP = 6,
+  SCR_UNW_RSP = 7,
+  SCR_UNW_R12 = 12,
+  SCR_UNW_R13 = 13,
+  SCR_UNW_R14 = 14,
+  SCR_UNW_R15 = 15,
+  SCR_UNW_RA = 16,
+  SCR_UNW_REGS = 17
+};
+
+/* The callee-saved registers a plan restores, in the order of its bits. */
+static const uint8_t scr_unwind_callee[6] = {SCR_UNW_RBX, SCR_UNW_RBP, SCR_UNW_R12,
+                                             SCR_UNW_R13, SCR_UNW_R14, SCR_UNW_R15};
+
+typedef struct {
+  uint64_t pc;      /* the return address the plan is for (0: an empty slot) */
+  uint64_t pad;     /* the call site's landing pad, or 0 */
+  int32_t cfa_off;  /* the CFA is cfa_reg's value plus cfa_off */
+  uint8_t cfa_reg;  /* SCR_UNW_RSP or SCR_UNW_RBP */
+  uint8_t saved;    /* bit i: scr_unwind_callee[i] is saved at CFA + off[i] * 8 */
+  int8_t off[6];
+  uint8_t fallback; /* the frame is left to libgcc's unwinder */
+} ScrUnwindPlan;
+
+_Static_assert(sizeof(ScrUnwindPlan) == 32, "a plan is half a cache line");
+
+#define SCR_UNWIND_PLAN_BITS 14
+#define SCR_UNWIND_PLAN_SLOTS (1u << SCR_UNWIND_PLAN_BITS)
+static ScrUnwindPlan scr_unwind_plans[SCR_UNWIND_PLAN_SLOTS];
+static const ScrUnwindPlan scr_unwind_fallback_plan = {.fallback = 1};
+static size_t scr_unwind_plan_count;
+/* Inserts take this lock; lookups take none. A plan is published by the
+ * release store of its pc. */
+static int scr_unwind_lock;
+
+/* Reads a value in DWARF pointer encoding enc. datarel is the base of
+ * datarel values (0 where they do not occur). Sets *bad for an encoding the
+ * plans do not read, which includes indirect values. Like libgcc, a zero
+ * value stays zero. */
+static uint64_t scr_unwind_read(const uint8_t **p, uint8_t enc, uint64_t datarel, int *bad) {
+  const uint8_t *start = *p;
+  uint64_t v = 0;
+  switch (enc & 0x0f) {
+  case 0x00: /* absptr */
+  case 0x04: /* udata8 */
+  case 0x0c: /* sdata8 */
+    memcpy(&v, *p, 8);
+    *p += 8;
+    break;
+  case 0x01: { /* uleb128 */
+    unsigned shift = 0;
+    uint8_t b;
+    do {
+      b = *(*p)++;
+      if (shift < 64) v |= (uint64_t)(b & 0x7f) << shift;
+      shift += 7;
+    } while (b & 0x80);
+    break;
+  }
+  case 0x02: {
+    uint16_t x;
+    memcpy(&x, *p, 2);
+    *p += 2;
+    v = x;
+    break;
+  }
+  case 0x03: {
+    uint32_t x;
+    memcpy(&x, *p, 4);
+    *p += 4;
+    v = x;
+    break;
+  }
+  case 0x0a: {
+    int16_t x;
+    memcpy(&x, *p, 2);
+    *p += 2;
+    v = (uint64_t)(int64_t)x;
+    break;
+  }
+  case 0x0b: {
+    int32_t x;
+    memcpy(&x, *p, 4);
+    *p += 4;
+    v = (uint64_t)(int64_t)x;
+    break;
+  }
+  default: /* sleb128 and unknown formats */
+    *bad = 1;
+    return 0;
+  }
+  if (v == 0) return 0;
+  switch (enc & 0x70) {
+  case 0x00:
+    break;
+  case 0x10: /* pcrel */
+    v += (uint64_t)(uintptr_t)start;
+    break;
+  case 0x30: /* datarel */
+    if (datarel == 0) *bad = 1;
+    v += datarel;
+    break;
+  default:
+    *bad = 1;
+    return 0;
+  }
+  if (enc & 0x80) *bad = 1; /* indirect: read only through scr_unwind_skip */
+  return v;
+}
+
+static uint64_t scr_unwind_uleb(const uint8_t **p) {
+  int bad = 0;
+  return scr_unwind_read(p, 0x01, 0, &bad);
+}
+
+static int64_t scr_unwind_sleb(const uint8_t **p) {
+  int64_t v = 0;
+  unsigned shift = 0;
+  uint8_t b;
+  do {
+    b = *(*p)++;
+    if (shift < 64) v |= (int64_t)((uint64_t)(b & 0x7f) << shift);
+    shift += 7;
+  } while (b & 0x80);
+  if (shift < 64 && (b & 0x40)) v |= (int64_t)(~(uint64_t)0 << shift);
+  return v;
+}
+
+/* A search for the loaded object that contains pc, and its .eh_frame_hdr
+ * search table. */
+typedef struct {
+  uint64_t pc;
+  const uint8_t *hdr;
+  const uint8_t *table;
+  size_t count;
+} ScrUnwindObject;
+
+static int scr_unwind_find_object(struct dl_phdr_info *info, size_t size, void *data) {
+  (void)size;
+  ScrUnwindObject *object = data;
+  int contains = 0;
+  const uint8_t *h = NULL;
+  for (int i = 0; i < info->dlpi_phnum; i++) {
+    const ElfW(Phdr) *phdr = &info->dlpi_phdr[i];
+    uint64_t start = (uint64_t)info->dlpi_addr + phdr->p_vaddr;
+    if (phdr->p_type == PT_LOAD && object->pc >= start && object->pc - start < phdr->p_memsz)
+      contains = 1;
+    else if (phdr->p_type == PT_GNU_EH_FRAME)
+      h = (const uint8_t *)(uintptr_t)start;
+  }
+  if (!contains) return 0;
+  /* Version 1 with the table LLVM's and GNU's linkers write: datarel
+   * sdata4 pairs, sorted by initial location. */
+  if (h != NULL && h[0] == 1 && h[1] != 0xff && h[2] != 0xff && h[3] == 0x3b) {
+    int bad = 0;
+    const uint8_t *p = h + 4;
+    (void)scr_unwind_read(&p, h[1], (uint64_t)(uintptr_t)h, &bad);
+    uint64_t count = scr_unwind_read(&p, h[2], (uint64_t)(uintptr_t)h, &bad);
+    if (!bad) {
+      object->hdr = h;
+      object->table = p;
+      object->count = (size_t)count;
+    }
+  }
+  return 1;
+}
+
+/* The FDE whose initial location is the greatest one at or below pc in
+ * the table of the object that contains pc. */
+static const uint8_t *scr_unwind_find_fde(uint64_t pc) {
+  ScrUnwindObject object = {.pc = pc};
+  dl_iterate_phdr(scr_unwind_find_object, &object);
+  if (object.table == NULL) return NULL;
+  uint64_t base = (uint64_t)(uintptr_t)object.hdr;
+  size_t lo = 0, hi = object.count;
+  while (lo < hi) {
+    size_t mid = lo + (hi - lo) / 2;
+    int32_t start;
+    memcpy(&start, object.table + mid * 8, 4);
+    if (base + (uint64_t)(int64_t)start <= pc) lo = mid + 1;
+    else hi = mid;
+  }
+  if (lo == 0) return NULL;
+  int32_t fde;
+  memcpy(&fde, object.table + (lo - 1) * 8 + 4, 4);
+  return (const uint8_t *)(uintptr_t)(base + (uint64_t)(int64_t)fde);
+}
+
+#define SCR_UNWIND_UNSAVED INT32_MIN
+
+typedef struct {
+  uint64_t cfa_reg;
+  int64_t cfa_off;
+  int32_t saved[SCR_UNW_REGS]; /* CFA-relative offsets, or SCR_UNWIND_UNSAVED */
+  uint64_t args_size;
+} ScrUnwindRules;
+
+/* Whether the plans track register reg: the callee-saved registers, the
+ * stack pointer and the return address. */
+static int scr_unwind_tracked(uint64_t reg) {
+  return reg == SCR_UNW_RBX || reg == SCR_UNW_RBP || reg == SCR_UNW_RSP ||
+         (reg >= SCR_UNW_R12 && reg <= SCR_UNW_RA);
+}
+
+static void scr_unwind_save(ScrUnwindRules *r, uint64_t reg, int64_t off, int *bad) {
+  if (reg >= SCR_UNW_REGS) return; /* vector and other registers stay as they are */
+  if (off < INT32_MIN + 1 || off > INT32_MAX) *bad = 1;
+  else r->saved[reg] = (int32_t)off;
+}
+
+/* Runs CFI instructions up to the location pc. Sets *bad for an
+ * instruction the plans do not express. */
+static void scr_unwind_run_cfi(const uint8_t *p, const uint8_t *end, uint64_t code_align,
+                               int64_t data_align, uint64_t loc, uint64_t pc, ScrUnwindRules *r,
+                               const ScrUnwindRules *initial, int *bad) {
+  ScrUnwindRules stack[8];
+  int depth = 0;
+  while (p < end && loc <= pc && !*bad) {
+    uint8_t op = *p++;
+    uint8_t low = op & 0x3f;
+    switch (op & 0xc0) {
+    case 0x40: /* advance_loc */
+      loc += low * code_align;
+      continue;
+    case 0x80: /* offset */
+      scr_unwind_save(r, low, (int64_t)scr_unwind_uleb(&p) * data_align, bad);
+      continue;
+    case 0xc0: /* restore */
+      if (low < SCR_UNW_REGS) r->saved[low] = initial->saved[low];
+      continue;
+    default:
+      break;
+    }
+    switch (op) {
+    case 0x00: /* nop */
+      break;
+    case 0x02: /* advance_loc1 */
+      loc += *p++ * code_align;
+      break;
+    case 0x03: { /* advance_loc2 */
+      uint16_t d;
+      memcpy(&d, p, 2);
+      p += 2;
+      loc += d * code_align;
+      break;
+    }
+    case 0x04: { /* advance_loc4 */
+      uint32_t d;
+      memcpy(&d, p, 4);
+      p += 4;
+      loc += d * code_align;
+      break;
+    }
+    case 0x05: { /* offset_extended */
+      uint64_t reg = scr_unwind_uleb(&p);
+      scr_unwind_save(r, reg, (int64_t)scr_unwind_uleb(&p) * data_align, bad);
+      break;
+    }
+    case 0x06: { /* restore_extended */
+      uint64_t reg = scr_unwind_uleb(&p);
+      if (reg < SCR_UNW_REGS) r->saved[reg] = initial->saved[reg];
+      break;
+    }
+    case 0x07: /* undefined */
+    case 0x08: { /* same_value */
+      uint64_t reg = scr_unwind_uleb(&p);
+      if (op == 0x07 && scr_unwind_tracked(reg)) *bad = 1;
+      else if (reg < SCR_UNW_REGS) r->saved[reg] = SCR_UNWIND_UNSAVED;
+      break;
+    }
+    case 0x09: { /* register */
+      uint64_t reg = scr_unwind_uleb(&p);
+      (void)scr_unwind_uleb(&p);
+      if (scr_unwind_tracked(reg)) *bad = 1;
+      break;
+    }
+    case 0x0a: /* remember_state */
+      if (depth == 8) *bad = 1;
+      else stack[depth++] = *r;
+      break;
+    case 0x0b: { /* restore_state: the CFA rule and args size stay */
+      if (depth == 0) {
+        *bad = 1;
+        break;
+      }
+      uint64_t args_size = r->args_size;
+      *r = stack[--depth];
+      r->args_size = args_size;
+      break;
+    }
+    case 0x0c: /* def_cfa */
+      r->cfa_reg = scr_unwind_uleb(&p);
+      r->cfa_off = (int64_t)scr_unwind_uleb(&p);
+      break;
+    case 0x0d: /* def_cfa_register */
+      r->cfa_reg = scr_unwind_uleb(&p);
+      break;
+    case 0x0e: /* def_cfa_offset */
+      r->cfa_off = (int64_t)scr_unwind_uleb(&p);
+      break;
+    case 0x11: { /* offset_extended_sf */
+      uint64_t reg = scr_unwind_uleb(&p);
+      scr_unwind_save(r, reg, scr_unwind_sleb(&p) * data_align, bad);
+      break;
+    }
+    case 0x12: /* def_cfa_sf */
+      r->cfa_reg = scr_unwind_uleb(&p);
+      r->cfa_off = scr_unwind_sleb(&p) * data_align;
+      break;
+    case 0x13: /* def_cfa_offset_sf */
+      r->cfa_off = scr_unwind_sleb(&p) * data_align;
+      break;
+    case 0x2e: /* GNU_args_size */
+      r->args_size = scr_unwind_uleb(&p);
+      break;
+    default: /* expressions, val_offset, set_loc and vendor instructions */
+      *bad = 1;
+      break;
+    }
+  }
+}
+
+/* The landing pad of the call site that contains pc in an LSDA, or 0. */
+static uint64_t scr_unwind_lsda_pad(const uint8_t *p, uint64_t func_start, uint64_t pc, int *bad) {
+  uint8_t lpstart_enc = *p++;
+  uint64_t lpstart = func_start;
+  if (lpstart_enc != 0xff) lpstart = scr_unwind_read(&p, lpstart_enc, 0, bad);
+  uint8_t ttype_enc = *p++;
+  if (ttype_enc != 0xff) (void)scr_unwind_uleb(&p);
+  uint8_t cs_enc = *p++;
+  uint64_t cs_len = scr_unwind_uleb(&p);
+  const uint8_t *cs_end = p + cs_len;
+  while (p < cs_end && !*bad) {
+    uint64_t start = scr_unwind_read(&p, cs_enc, 0, bad);
+    uint64_t len = scr_unwind_read(&p, cs_enc, 0, bad);
+    uint64_t pad = scr_unwind_read(&p, cs_enc, 0, bad);
+    (void)scr_unwind_uleb(&p); /* action */
+    if (pc < func_start + start) return 0;
+    if (pc < func_start + start + len) return pad ? lpstart + pad : 0;
+  }
+  return 0;
+}
+
+/* The personality routine of emitted functions; the walk reads their
+ * LSDAs itself. Frames with any other personality are left to libgcc. */
+extern _Unwind_Reason_Code __gcc_personality_v0(int, _Unwind_Action, _Unwind_Exception_Class,
+                                                struct _Unwind_Exception *,
+                                                struct _Unwind_Context *);
+#pragma weak __gcc_personality_v0
+
+/* Builds the plan of return address ra. */
+static void scr_unwind_build(ScrUnwindPlan *plan, uint64_t ra) {
+  memset(plan, 0, sizeof *plan);
+  plan->fallback = 1;
+  uint64_t pc = ra - 1; /* within the call instruction */
+  const uint8_t *fde = scr_unwind_find_fde(pc);
+  if (fde == NULL) return;
+  uint32_t fde_len;
+  memcpy(&fde_len, fde, 4);
+  if (fde_len == 0 || fde_len == 0xffffffffu) return;
+  const uint8_t *fde_end = fde + 4 + fde_len;
+  int32_t cie_off;
+  memcpy(&cie_off, fde + 4, 4);
+  if (cie_off == 0) return;
+  const uint8_t *cie = fde + 4 - cie_off;
+  uint32_t cie_len;
+  memcpy(&cie_len, cie, 4);
+  if (cie_len == 0 || cie_len == 0xffffffffu) return;
+  const uint8_t *cie_end = cie + 4 + cie_len;
+  const uint8_t *q = cie + 8; /* past the length and the CIE id */
+  uint8_t version = *q++;
+  if (version != 1 && version != 3) return;
+  const char *aug = (const char *)q;
+  q += strlen(aug) + 1;
+  uint64_t code_align = scr_unwind_uleb(&q);
+  int64_t data_align = scr_unwind_sleb(&q);
+  uint64_t ra_reg = version == 1 ? *q++ : scr_unwind_uleb(&q);
+  if (ra_reg != SCR_UNW_RA) return;
+  int bad = 0;
+  uint8_t fde_enc = 0x00, lsda_enc = 0xff;
+  if (aug[0] == 'z') {
+    uint64_t aug_len = scr_unwind_uleb(&q);
+    const uint8_t *aug_end = q + aug_len;
+    for (const char *a = aug + 1; *a != '\0'; a++) {
+      if (*a == 'R') {
+        fde_enc = *q++;
+      } else if (*a == 'L') {
+        lsda_enc = *q++;
+      } else if (*a == 'P') {
+        uint8_t enc = *q++;
+        uint64_t slot = scr_unwind_read(&q, enc & 0x7f, 0, &bad);
+        uint64_t personality = slot;
+        if ((enc & 0x80) && slot != 0) memcpy(&personality, (const void *)(uintptr_t)slot, 8);
+        if (personality != (uint64_t)(uintptr_t)&__gcc_personality_v0) bad = 1;
+      } else {
+        return; /* signal frames and unknown augmentations */
+      }
+    }
+    q = aug_end;
+  } else if (aug[0] != '\0') {
+    return;
+  }
+  if (bad) return;
+  ScrUnwindRules initial = {.cfa_reg = SCR_UNW_RSP, .cfa_off = 8};
+  for (int i = 0; i < SCR_UNW_REGS; i++) initial.saved[i] = SCR_UNWIND_UNSAVED;
+  scr_unwind_run_cfi(q, cie_end, code_align, data_align, 0, UINT64_MAX, &initial, &initial, &bad);
+  const uint8_t *f = fde + 8;
+  uint64_t func_start = scr_unwind_read(&f, fde_enc, 0, &bad);
+  uint64_t func_len = scr_unwind_read(&f, fde_enc & 0x0f, 0, &bad);
+  if (bad || pc < func_start || pc - func_start >= func_len) return;
+  const uint8_t *lsda = NULL;
+  if (aug[0] == 'z') {
+    uint64_t aug_len = scr_unwind_uleb(&f);
+    const uint8_t *aug_end = f + aug_len;
+    if (lsda_enc != 0xff && aug_len != 0)
+      lsda = (const uint8_t *)(uintptr_t)scr_unwind_read(&f, lsda_enc, 0, &bad);
+    f = aug_end;
+  }
+  ScrUnwindRules r = initial;
+  scr_unwind_run_cfi(f, fde_end, code_align, data_align, func_start, pc, &r, &initial, &bad);
+  if (bad || r.args_size != 0) return;
+  if (r.cfa_reg != SCR_UNW_RSP && r.cfa_reg != SCR_UNW_RBP) return;
+  if (r.cfa_off < INT32_MIN || r.cfa_off > INT32_MAX) return;
+  if (r.saved[SCR_UNW_RA] != -8) return;
+  uint8_t saved = 0;
+  for (int i = 0; i < 6; i++) {
+    int32_t o = r.saved[scr_unwind_callee[i]];
+    if (o == SCR_UNWIND_UNSAVED) continue;
+    if (o % 8 != 0 || o / 8 < INT8_MIN || o / 8 > INT8_MAX) return;
+    saved |= (uint8_t)(1u << i);
+    plan->off[i] = (int8_t)(o / 8);
+  }
+  uint64_t pad = lsda ? scr_unwind_lsda_pad(lsda, func_start, pc, &bad) : 0;
+  if (bad) return;
+  plan->cfa_reg = (uint8_t)r.cfa_reg;
+  plan->cfa_off = (int32_t)r.cfa_off;
+  plan->saved = saved;
+  plan->pad = pad;
+  plan->fallback = 0;
+}
+
+static uint64_t scr_unwind_hash(uint64_t ra) {
+  return (ra * 0x9E3779B97F4A7C15ull) >> (64 - SCR_UNWIND_PLAN_BITS);
+}
+
+static __attribute__((noinline)) const ScrUnwindPlan *scr_unwind_insert(uint64_t ra) {
+  while (__atomic_exchange_n(&scr_unwind_lock, 1, __ATOMIC_ACQUIRE))
+    while (__atomic_load_n(&scr_unwind_lock, __ATOMIC_RELAXED)) __builtin_ia32_pause();
+  const ScrUnwindPlan *result = &scr_unwind_fallback_plan;
+  uint64_t h = scr_unwind_hash(ra);
+  ScrUnwindPlan *slot = &scr_unwind_plans[h];
+  while (slot->pc != 0 && slot->pc != ra) {
+    h = (h + 1) & (SCR_UNWIND_PLAN_SLOTS - 1);
+    slot = &scr_unwind_plans[h];
+  }
+  if (slot->pc == ra) {
+    result = slot;
+  } else if (scr_unwind_plan_count < SCR_UNWIND_PLAN_SLOTS / 4 * 3) {
+    /* A full table leaves further return addresses to libgcc. */
+    scr_unwind_plan_count++;
+    ScrUnwindPlan built;
+    scr_unwind_build(&built, ra);
+    /* Lookups read pc atomically, so the plan's other fields are written
+     * first and pc only by the release store that publishes them. */
+    memcpy((char *)slot + sizeof slot->pc, (const char *)&built + sizeof built.pc,
+           sizeof *slot - sizeof slot->pc);
+    __atomic_store_n(&slot->pc, ra, __ATOMIC_RELEASE);
+    result = slot;
+  }
+  __atomic_store_n(&scr_unwind_lock, 0, __ATOMIC_RELEASE);
+  return result;
+}
+
+static const ScrUnwindPlan *scr_unwind_plan(uint64_t ra) {
+  uint64_t h = scr_unwind_hash(ra);
+  for (;;) {
+    const ScrUnwindPlan *plan = &scr_unwind_plans[h];
+    uint64_t pc = __atomic_load_n(&plan->pc, __ATOMIC_ACQUIRE);
+    if (pc == ra) return plan;
+    if (pc == 0) return scr_unwind_insert(ra);
+    h = (h + 1) & (SCR_UNWIND_PLAN_SLOTS - 1);
+  }
+}
+
+/* Restores the callee-saved registers and the stack pointer of regs
+ * (indexed by DWARF number) and jumps to the landing pad with rax holding
+ * the token and rdx 0. */
+static _Noreturn __attribute__((naked)) void scr_unwind_install(const uint64_t *regs, uint64_t pad,
+                                                                const void *token) {
+  __asm__ volatile("movq %rdx, %rax\n\t"
+                   "movq %rsi, %r11\n\t"
+                   "movq 24(%rdi), %rbx\n\t"
+                   "movq 48(%rdi), %rbp\n\t"
+                   "movq 96(%rdi), %r12\n\t"
+                   "movq 104(%rdi), %r13\n\t"
+                   "movq 112(%rdi), %r14\n\t"
+                   "movq 120(%rdi), %r15\n\t"
+                   "movq 56(%rdi), %rsp\n\t"
+                   "xorl %edx, %edx\n\t"
+                   "jmpq *%r11\n\t");
+}
+
+/* Identifies a walk's raise to the landing pads it runs: they pass it to
+ * scr_unwind_resume. The thrown value is in the active cell. */
+static const char scr_unwind_token;
+
+#ifdef SCR_UNWIND_TESTING
+/* The walk's raises and resumes that went on with libgcc's unwinder, and
+ * the frames whose stack pointer or callee-saved registers differ from
+ * libgcc's for the same return address. */
+size_t scr_unwind_test_fallbacks;
+size_t scr_unwind_test_mismatches;
+
+typedef struct {
+  uint64_t ip, cfa, regs[6];
+} ScrUnwindTestFrame;
+
+typedef struct {
+  ScrUnwindTestFrame frames[512];
+  int count, next;
+} ScrUnwindTestTrace;
+
+static _Thread_local ScrUnwindTestTrace scr_unwind_test_trace;
+
+static _Unwind_Reason_Code scr_unwind_test_record(struct _Unwind_Context *context, void *arg) {
+  ScrUnwindTestTrace *trace = arg;
+  if (trace->count == 512) return _URC_END_OF_STACK;
+  ScrUnwindTestFrame *frame = &trace->frames[trace->count++];
+  frame->ip = _Unwind_GetIP(context);
+  frame->cfa = _Unwind_GetCFA(context);
+  for (int i = 0; i < 6; i++) frame->regs[i] = _Unwind_GetGR(context, scr_unwind_callee[i]);
+  return _URC_NO_REASON;
+}
+
+/* Records libgcc's frames from here, before the walk changes anything. */
+static void scr_unwind_test_start(void) {
+  scr_unwind_test_trace.count = 0;
+  scr_unwind_test_trace.next = 0;
+  _Unwind_Backtrace(scr_unwind_test_record, &scr_unwind_test_trace);
+}
+
+/* Compares the walk's frame, regs at its call, with libgcc's: libgcc's
+ * CFA there is the callee's, the frame's stack pointer at the call. */
+static void scr_unwind_test_check(const uint64_t *regs) {
+  ScrUnwindTestTrace *trace = &scr_unwind_test_trace;
+  while (trace->next < trace->count && trace->frames[trace->next].ip != regs[SCR_UNW_RA])
+    trace->next++;
+  if (trace->next == trace->count) {
+    if (trace->count < 512) __atomic_add_fetch(&scr_unwind_test_mismatches, 1, __ATOMIC_RELAXED);
+    return;
+  }
+  const ScrUnwindTestFrame *frame = &trace->frames[trace->next++];
+  int same = frame->cfa == regs[SCR_UNW_RSP];
+  for (int i = 0; i < 6; i++) same &= frame->regs[i] == regs[scr_unwind_callee[i]];
+  if (!same) __atomic_add_fetch(&scr_unwind_test_mismatches, 1, __ATOMIC_RELAXED);
+}
+
+#define SCR_UNWIND_FALLBACK() __atomic_add_fetch(&scr_unwind_test_fallbacks, 1, __ATOMIC_RELAXED)
+#define SCR_UNWIND_TEST_START() scr_unwind_test_start()
+#define SCR_UNWIND_TEST_CHECK(regs) scr_unwind_test_check(regs)
+#else
+#define SCR_UNWIND_FALLBACK() ((void)0)
+#define SCR_UNWIND_TEST_START() ((void)0)
+#define SCR_UNWIND_TEST_CHECK(regs) ((void)0)
+#endif
+
+/* Walks from the frame regs describes, a caller's registers at its call,
+ * and installs the first frame with a landing pad. Returns when a frame is
+ * left to libgcc, before anything has been installed. */
+static __attribute__((no_sanitize("address"))) void scr_unwind_walk(uint64_t *regs) {
+  SCR_UNWIND_TEST_START();
+  for (;;) {
+    const ScrUnwindPlan *plan = scr_unwind_plan(regs[SCR_UNW_RA]);
+    if (plan->fallback) return;
+    SCR_UNWIND_TEST_CHECK(regs);
+    if (plan->pad != 0) {
+#ifdef SCR_UNWIND_ASAN
+      /* The frames below the landing pad's never return: clear their
+       * redzones' shadow, up to the pad frame's stack pointer. */
+      uintptr_t sp;
+      __asm__ volatile("movq %%rsp, %0" : "=r"(sp));
+      if (regs[SCR_UNW_RSP] > sp)
+        __asan_unpoison_memory_region((void *)sp, regs[SCR_UNW_RSP] - sp);
+#endif
+      scr_unwind_install(regs, plan->pad, &scr_unwind_token);
+    }
+    uint64_t cfa = regs[plan->cfa_reg] + (uint64_t)(int64_t)plan->cfa_off;
+    const uint64_t *frame = (const uint64_t *)(uintptr_t)cfa;
+    for (unsigned m = plan->saved; m != 0; m &= m - 1) {
+      int i = __builtin_ctz(m);
+      regs[scr_unwind_callee[i]] = frame[plan->off[i]];
+    }
+    regs[SCR_UNW_RA] = frame[-1];
+    regs[SCR_UNW_RSP] = cfa;
+  }
+}
+
+/* Called by the entry stubs below with their caller's registers. */
+__attribute__((visibility("hidden"), used)) void scr_unwind_walk_raise(uint64_t *regs) {
+  scr_unwind_walk(regs);
+}
+
+__attribute__((visibility("hidden"), used)) void scr_unwind_walk_resume(uint64_t *regs,
+                                                                        const void *exc) {
+  if (exc == &scr_unwind_token) scr_unwind_walk(regs);
+}
+
+/* Where the entry stubs go when the walk returns, with the stack as it was
+ * at their entry: the caller is the emitted frame. */
+__attribute__((visibility("hidden"), used, no_sanitize("address"))) void
+scr_unwind_raise_slow(void) {
+  SCR_UNWIND_FALLBACK();
+  scr_unwind_forced();
+}
+
+__attribute__((visibility("hidden"), used, no_sanitize("address"))) void
+scr_unwind_resume_slow(void *exc) {
+  /* A landing pad that libgcc's unwind ran passes libgcc's exception. */
+  if (exc != &scr_unwind_token && _Unwind_Resume != NULL) _Unwind_Resume(exc);
+  SCR_UNWIND_FALLBACK();
+  scr_unwind_forced();
+}
+
+/* The entry stubs store the caller's callee-saved registers, stack pointer
+ * after the return and return address into a register array on the stack,
+ * indexed by DWARF number (17 slots: 136 bytes, which keeps the stack
+ * 16-byte aligned at the call). */
+#define SCR_UNWIND_CAPTURE                                                                         \
+  "subq $136, %rsp\n\t"                                                                            \
+  ".cfi_adjust_cfa_offset 136\n\t"                                                                \
+  "movq %rbx, 24(%rsp)\n\t"                                                                        \
+  "movq %rbp, 48(%rsp)\n\t"                                                                        \
+  "movq %r12, 96(%rsp)\n\t"                                                                        \
+  "movq %r13, 104(%rsp)\n\t"                                                                       \
+  "movq %r14, 112(%rsp)\n\t"                                                                       \
+  "movq %r15, 120(%rsp)\n\t"                                                                       \
+  "movq 136(%rsp), %rax\n\t"                                                                       \
+  "movq %rax, 128(%rsp)\n\t"                                                                       \
+  "leaq 144(%rsp), %rax\n\t"                                                                       \
+  "movq %rax, 56(%rsp)\n\t"
+
+__attribute__((naked)) void scr_unwind_raise(void) {
+  __asm__ volatile(SCR_UNWIND_CAPTURE "movq %rsp, %rdi\n\t"
+                                      "call scr_unwind_walk_raise\n\t"
+                                      "addq $136, %rsp\n\t"
+                                      ".cfi_adjust_cfa_offset -136\n\t"
+                                      "jmp scr_unwind_raise_slow\n\t");
+}
+
+/* Landing pads continue the unwind here with the exception they took. */
+__attribute__((naked)) void scr_unwind_resume(void *exc) {
+  __asm__ volatile(SCR_UNWIND_CAPTURE "movq %rdi, 40(%rsp)\n\t"
+                                      "movq %rdi, %rsi\n\t"
+                                      "movq %rsp, %rdi\n\t"
+                                      "call scr_unwind_walk_resume\n\t"
+                                      "movq 40(%rsp), %rdi\n\t"
+                                      "addq $136, %rsp\n\t"
+                                      ".cfi_adjust_cfa_offset -136\n\t"
+                                      "jmp scr_unwind_resume_slow\n\t");
+}
+
+#else
+
+void scr_unwind_raise(void) {
+  scr_unwind_forced();
+  __builtin_unreachable();
+}
+
+/* Landing pads continue the unwind here with the exception they took. */
+void scr_unwind_resume(void *exc) {
+  if (_Unwind_Resume != NULL) _Unwind_Resume(exc);
+  scr_unwind_forced();
+}
+
+#endif
 #endif
diff --git a/packages/runtime/test/test_unwind.c b/packages/runtime/test/test_unwind.c
new file mode 100644
index 0000000000..5edabcde4f
--- /dev/null
+++ b/packages/runtime/test/test_unwind.c
@@ -0,0 +1,165 @@
+/* Raises through the synthetic frames of test_unwind_frames.ll and
+ * test_unwind_frames.S with scr_unwind.c's walk: landing pads run inner
+ * first, the catching frame sees its callee-saved registers restored, every
+ * frame the walk steps through has the stack pointer and callee-saved
+ * registers libgcc's unwinder computes for it, and only frames with CFI the
+ * plans do not express go to libgcc's unwinder.
+ * Each case runs three times (the first builds plans, the others reuse
+ * them), and again from several threads at once. */
+#include <pthread.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+typedef void Frame(void **chain);
+extern Frame ir_raise, ir_plain, ir_plain_fp, ir_alloca, ir_pad, ir_pad_fp;
+extern Frame asm_remember, asm_rbp, asm_expr;
+extern int64_t ir_catch(void **chain, int64_t a, int64_t b, int64_t c, int64_t d, int64_t e,
+                        int64_t f);
+extern size_t scr_unwind_test_fallbacks;
+extern size_t scr_unwind_test_mismatches;
+
+static _Thread_local void **cleanup_base;
+static _Thread_local int cleanups[512];
+static _Thread_local int cleanup_count;
+
+/* A landing pad of the frame at chain[-1] ran. */
+void test_cleanup(void **chain) {
+  if (cleanup_count < 512) cleanups[cleanup_count] = (int)(chain - cleanup_base) - 1;
+  cleanup_count++;
+}
+
+void test_touch(void *p) { memset(p, 0x5a, 16); }
+
+void test_return(void **chain) { (void)chain; }
+
+static int64_t checksum(int64_t a, int64_t b, int64_t c, int64_t d, int64_t e, int64_t f) {
+  uint64_t s = (uint64_t)a * 3 ^ (uint64_t)b * 5;
+  s += (uint64_t)c * 7;
+  s ^= (uint64_t)d * 11;
+  s += (uint64_t)e * 13;
+  s ^= (uint64_t)f * 17;
+  return (int64_t)(s & INT64_MAX);
+}
+
+/* Writes over the stack the unwound frames used: in sanitized builds, a
+ * redzone the unwind left poisoned would be reported here. */
+static __attribute__((noinline)) void scrub(void) {
+  volatile char big[32768];
+  memset((char *)big, 0, sizeof big);
+}
+
+static void fail(const char *name, const char *what) {
+  fprintf(stderr, "%s: %s\n", name, what);
+  exit(1);
+}
+
+/* Runs frames[0..n) below ir_catch. A case that ends in test_return
+ * raises nothing. Returns the case's fallbacks to libgcc. */
+static size_t run(const char *name, Frame *const *frames, int n, int64_t seed) {
+  void *chain[512];
+  int expected[512];
+  int pads = 0;
+  if (n >= 511) fail(name, "too many frames");
+  for (int i = 0; i < n; i++) chain[i] = (void *)frames[i];
+  chain[n] = NULL;
+  int raises = frames[n - 1] == ir_raise;
+  if (raises)
+    for (int i = n - 1; i >= 0; i--)
+      if (frames[i] == ir_pad || frames[i] == ir_pad_fp) expected[pads++] = i;
+  size_t before = __atomic_load_n(&scr_unwind_test_fallbacks, __ATOMIC_RELAXED);
+  cleanup_base = chain;
+  cleanup_count = 0;
+  int64_t a = seed, b = seed ^ 0x1234, c = seed + 77, d = seed * 31, e = ~seed, f = seed << 3;
+  int64_t got = ir_catch(chain, a, b, c, d, e, f);
+  int64_t want = checksum(a, b, c, d, e, f) | (raises ? INT64_MIN : 0);
+  if (got != want) fail(name, "the catching frame's values or raise flag differ");
+  if (cleanup_count != pads) fail(name, "a different number of landing pads ran");
+  for (int i = 0; i < pads; i++)
+    if (cleanups[i] != expected[i]) fail(name, "landing pads ran out of order");
+  scrub();
+  if (__atomic_load_n(&scr_unwind_test_mismatches, __ATOMIC_RELAXED) != 0)
+    fail(name, "a frame's stack pointer or callee-saved registers differ from libgcc's");
+  return __atomic_load_n(&scr_unwind_test_fallbacks, __ATOMIC_RELAXED) - before;
+}
+
+typedef struct {
+  const char *name;
+  Frame *frames[16];
+  int count;
+  size_t fallbacks; /* raises or resumes that go to libgcc */
+} Case;
+
+/* Sanitized builds realign a frame with a dynamic alloca through r10 and
+ * describe it with DWARF expressions, which go to libgcc. */
+#if defined(__SANITIZE_ADDRESS__)
+#define ALLOCA_FALLBACKS 1
+#elif defined(__has_feature)
+#if __has_feature(address_sanitizer)
+#define ALLOCA_FALLBACKS 1
+#endif
+#endif
+#ifndef ALLOCA_FALLBACKS
+#define ALLOCA_FALLBACKS 0
+#endif
+
+static const Case cases[] = {
+    {"returns", {ir_pad, ir_plain, test_return}, 3, 0},
+    {"raise", {ir_raise}, 1, 0},
+    {"plain frames", {ir_plain, ir_plain_fp, ir_plain, ir_raise}, 4, 0},
+    {"dynamic alloca", {ir_pad, ir_alloca, ir_pad, ir_raise}, 4, ALLOCA_FALLBACKS},
+    {"landing pads", {ir_pad, ir_plain, ir_pad_fp, ir_pad, ir_raise}, 5, 0},
+    {"remember and rbp", {asm_remember, ir_pad, asm_rbp, asm_remember, ir_raise}, 5, 0},
+    {"libgcc above a pad", {asm_expr, ir_pad, ir_raise}, 3, 1},
+    {"libgcc between pads", {ir_pad, asm_expr, ir_pad_fp, ir_raise}, 4, 1},
+    {"libgcc at the raise", {ir_pad, asm_expr, ir_raise}, 3, 1},
+};
+#define CASES (sizeof cases / sizeof cases[0])
+
+static void run_cases(int64_t seed) {
+  for (size_t i = 0; i < CASES; i++) {
+    size_t fallbacks = run(cases[i].name, cases[i].frames, cases[i].count, seed + (int64_t)i);
+    if (fallbacks != cases[i].fallbacks) fail(cases[i].name, "a different number of fallbacks");
+  }
+}
+
+/* A deep recursion of every frame kind. */
+static void run_deep(int64_t seed) {
+  static Frame *const kinds[] = {ir_pad, ir_plain, asm_remember, asm_rbp, ir_pad_fp, ir_plain_fp};
+  Frame *frames[301];
+  for (int i = 0; i < 300; i++) frames[i] = kinds[i % 6];
+  frames[300] = ir_raise;
+  if (run("deep", frames, 301, seed) != 0) fail("deep", "a fallback to libgcc");
+}
+
+static void *thread_main(void *arg) {
+  int64_t seed = (int64_t)(intptr_t)arg;
+  for (int round = 0; round < 200; round++) {
+    for (size_t i = 0; i < CASES; i++) {
+      if (cases[i].fallbacks != 0) continue; /* the shared counter is checked alone */
+      if (run(cases[i].name, cases[i].frames, cases[i].count, seed + round) != 0)
+        fail(cases[i].name, "a fallback to libgcc in a thread");
+    }
+    if (round % 20 == 0) run_deep(seed);
+  }
+  return NULL;
+}
+
+/* With an argument, the threads run first, so they build the plans
+ * concurrently. */
+int main(int argc, char **argv) {
+  (void)argv;
+  if (argc == 1)
+    for (int round = 0; round < 3; round++) {
+      run_cases(round * 1000);
+      run_deep(round);
+    }
+  pthread_t threads[8];
+  for (int i = 0; i < 8; i++)
+    if (pthread_create(&threads[i], NULL, thread_main, (void *)(intptr_t)(i * 7919)) != 0)
+      fail("threads", "pthread_create failed");
+  for (int i = 0; i < 8; i++) pthread_join(threads[i], NULL);
+  puts("unwind checks passed");
+  return 0;
+}
diff --git a/packages/runtime/test/test_unwind_frames.S b/packages/runtime/test/test_unwind_frames.S
new file mode 100644
index 0000000000..af46685927
--- /dev/null
+++ b/packages/runtime/test/test_unwind_frames.S
@@ -0,0 +1,101 @@
+/* Frames for test_unwind.c with CFI that compilers emit rarely. Like the
+ * frames in test_unwind_frames.ll, each takes a chain and calls its first
+ * function with the rest. */
+  .text
+
+/* Saves rbx and r12, and leaves through an early epilogue under
+ * remember_state before its call, so the call's rules come from
+ * restore_state. */
+  .globl asm_remember
+  .type asm_remember,@function
+asm_remember:
+  .cfi_startproc
+  pushq %rbx
+  .cfi_def_cfa_offset 16
+  .cfi_offset %rbx, -16
+  pushq %r12
+  .cfi_def_cfa_offset 24
+  .cfi_offset %r12, -24
+  subq $8, %rsp
+  .cfi_def_cfa_offset 32
+  movq (%rdi), %rax
+  testq %rax, %rax
+  jne 1f
+  .cfi_remember_state
+  addq $8, %rsp
+  .cfi_def_cfa_offset 24
+  popq %r12
+  .cfi_def_cfa_offset 16
+  .cfi_restore %r12
+  popq %rbx
+  .cfi_def_cfa_offset 8
+  .cfi_restore %rbx
+  retq
+  .cfi_restore_state
+1:
+  movabsq $0x5151515151515151, %rbx
+  movabsq $0x5252525252525252, %r12
+  addq $8, %rdi
+  callq *%rax
+  addq $8, %rsp
+  .cfi_def_cfa_offset 24
+  popq %r12
+  .cfi_def_cfa_offset 16
+  popq %rbx
+  .cfi_def_cfa_offset 8
+  retq
+  .cfi_endproc
+  .size asm_remember, .-asm_remember
+
+/* An rbp frame that saves r13 and r14 below rbp and moves the stack
+ * pointer by an amount that depends on the chain's address. */
+  .globl asm_rbp
+  .type asm_rbp,@function
+asm_rbp:
+  .cfi_startproc
+  pushq %rbp
+  .cfi_def_cfa_offset 16
+  .cfi_offset %rbp, -16
+  movq %rsp, %rbp
+  .cfi_def_cfa_register %rbp
+  pushq %r13
+  pushq %r14
+  .cfi_offset %r13, -24
+  .cfi_offset %r14, -32
+  movq %rdi, %rax
+  andq $0x70, %rax
+  subq %rax, %rsp
+  movabsq $0x5353535353535353, %r13
+  movabsq $0x5454545454545454, %r14
+  movq (%rdi), %rax
+  addq $8, %rdi
+  callq *%rax
+  leaq -16(%rbp), %rsp
+  popq %r14
+  popq %r13
+  popq %rbp
+  .cfi_def_cfa %rsp, 8
+  retq
+  .cfi_endproc
+  .size asm_rbp, .-asm_rbp
+
+/* Defines its CFA with a DWARF expression (rsp + 16), which the plans do
+ * not express: raises through it go to libgcc's unwinder. */
+  .globl asm_expr
+  .type asm_expr,@function
+asm_expr:
+  .cfi_startproc
+  pushq %rbx
+  .cfi_escape 0x0f, 0x02, 0x77, 0x10
+  .cfi_offset %rbx, -16
+  movabsq $0x5555555555555555, %rbx
+  movq (%rdi), %rax
+  addq $8, %rdi
+  callq *%rax
+  popq %rbx
+  .cfi_def_cfa %rsp, 8
+  retq
+  .cfi_endproc
+  .size asm_expr, .-asm_expr
+
+  .section .note.GNU-stack,"",@progbits
diff --git a/packages/runtime/test/test_unwind_frames.ll b/packages/runtime/test/test_unwind_frames.ll
new file mode 100644
index 0000000000..08415e7253
--- /dev/null
+++ b/packages/runtime/test/test_unwind_frames.ll
@@ -0,0 +1,131 @@
+; Frames for test_unwind.c, shaped like emitted code on the unwinding
+; exception ABI. Every frame takes a chain: a null-terminated array of frame
+; functions, of which it calls the first with the rest. Each clobbers the
+; callee-saved registers, so its prologue saves them and its CFI says where.
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @scr_unwind_raise() cold
+declare void @scr_unwind_resume(ptr) cold
+declare i32 @__gcc_personality_v0(...)
+declare void @test_cleanup(ptr)
+declare void @test_touch(ptr)
+
+; Clobbers every callee-saved register the frame pointer leaves free.
+define private void @clobber() alwaysinline {
+  call void asm sideeffect "movq $$0x6161616161616161, %rbx\0Amovq $$0x6262626262626262, %r12\0Amovq $$0x6363636363636363, %r13\0Amovq $$0x6464646464646464, %r14\0Amovq $$0x6565656565656565, %r15", "~{rbx},~{r12},~{r13},~{r14},~{r15}"()
+  ret void
+}
+
+; A local that escapes, so sanitized builds give the frame redzones.
+define private void @local() alwaysinline sanitize_address {
+  %buf = alloca [48 x i8], align 16
+  call void @test_touch(ptr %buf)
+  ret void
+}
+
+define void @ir_raise(ptr %chain) sanitize_address {
+  call void @local()
+  call void @clobber()
+  call void @scr_unwind_raise()
+  unreachable
+}
+
+define void @ir_plain(ptr %chain) sanitize_address {
+  call void @local()
+  %fn = load ptr, ptr %chain
+  %rest = getelementptr ptr, ptr %chain, i64 1
+  call void @clobber()
+  call void %fn(ptr %rest)
+  ret void
+}
+
+define void @ir_plain_fp(ptr %chain) sanitize_address "frame-pointer"="all" {
+  call void @local()
+  %fn = load ptr, ptr %chain
+  %rest = getelementptr ptr, ptr %chain, i64 1
+  call void @clobber()
+  call void %fn(ptr %rest)
+  ret void
+}
+
+; A frame whose stack pointer moves by a runtime amount: its CFA is rbp's.
+define void @ir_alloca(ptr %chain) sanitize_address {
+  %bits = ptrtoint ptr %chain to i64
+  %n = and i64 %bits, 112
+  %size = add i64 %n, 16
+  %dyn = alloca i8, i64 %size, align 16
+  call void @test_touch(ptr %dyn)
+  %fn = load ptr, ptr %chain
+  %rest = getelementptr ptr, ptr %chain, i64 1
+  call void @clobber()
+  call void %fn(ptr %rest)
+  ret void
+}
+
+; A frame with a landing pad: it records its chain position and continues
+; the unwind. The chain position is live across the call.
+define void @ir_pad(ptr %chain) sanitize_address personality ptr @__gcc_personality_v0 {
+entry:
+  call void @local()
+  %fn = load ptr, ptr %chain
+  %rest = getelementptr ptr, ptr %chain, i64 1
+  call void @clobber()
+  invoke void %fn(ptr %rest) to label %ok unwind label %lp
+ok:
+  ret void
+lp:
+  %e = landingpad { ptr, i32 } cleanup
+  call void @test_cleanup(ptr %chain)
+  %token = extractvalue { ptr, i32 } %e, 0
+  call void @scr_unwind_resume(ptr %token)
+  unreachable
+}
+
+define void @ir_pad_fp(ptr %chain) sanitize_address "frame-pointer"="all" personality ptr @__gcc_personality_v0 {
+entry:
+  call void @local()
+  %fn = load ptr, ptr %chain
+  %rest = getelementptr ptr, ptr %chain, i64 1
+  call void @clobber()
+  invoke void %fn(ptr %rest) to label %ok unwind label %lp
+ok:
+  ret void
+lp:
+  %e = landingpad { ptr, i32 } cleanup
+  call void @test_cleanup(ptr %chain)
+  %token = extractvalue { ptr, i32 } %e, 0
+  call void @scr_unwind_resume(ptr %token)
+  unreachable
+}
+
+; Takes the exception, as a pending-flag frame does: returns a checksum of
+; six values live across the call, which hold the callee-saved registers,
+; and whether the call raised in bit 63.
+define i64 @ir_catch(ptr %chain, i64 %a, i64 %b, i64 %c, i64 %d, i64 %e, i64 %f) sanitize_address personality ptr @__gcc_personality_v0 {
+entry:
+  call void @local()
+  %x1 = mul i64 %a, 3
+  %x2 = mul i64 %b, 5
+  %x3 = mul i64 %c, 7
+  %x4 = mul i64 %d, 11
+  %x5 = mul i64 %e, 13
+  %x6 = mul i64 %f, 17
+  %fn = load ptr, ptr %chain
+  %rest = getelementptr ptr, ptr %chain, i64 1
+  invoke void %fn(ptr %rest) to label %ok unwind label %lp
+ok:
+  br label %done
+lp:
+  %l = landingpad { ptr, i32 } cleanup
+  br label %done
+done:
+  %raised = phi i64 [ 0, %ok ], [ -9223372036854775808, %lp ]
+  %s1 = xor i64 %x1, %x2
+  %s2 = add i64 %s1, %x3
+  %s3 = xor i64 %s2, %x4
+  %s4 = add i64 %s3, %x5
+  %s5 = xor i64 %s4, %x6
+  %s6 = and i64 %s5, 9223372036854775807
+  %r = or i64 %s6, %raised
+  ret i64 %r
+}
diff --git a/packages/runtime/test/unwind.test.ts b/packages/runtime/test/unwind.test.ts
new file mode 100644
index 0000000000..95fc5a0fd5
--- /dev/null
+++ b/packages/runtime/test/unwind.test.ts
@@ -0,0 +1,55 @@
+import { execFile } from "node:child_process";
+import { mkdir } from "node:fs/promises";
+import { join } from "node:path";
+import { promisify } from "node:util";
+import { expect, test } from "vitest";
+
+const execFileAsync = promisify(execFile);
+const testDir = import.meta.dirname;
+
+// scr_unwind.c walks with cached plans on x86-64 Linux only.
+test.runIf(process.platform === "linux" && process.arch === "x64")(
+  "the unwinder runs landing pads in order, restores callee-saved registers and falls back to libgcc",
+  async () => {
+    const buildDir = join(testDir, "build");
+    await mkdir(buildDir, { recursive: true });
+    const frames = [join(testDir, "test_unwind_frames.ll"), join(testDir, "test_unwind_frames.S")];
+    // The frames also run from a shared library, whose FDEs are in its own
+    // .eh_frame_hdr.
+    const library = join(buildDir, "libtest_unwind_frames.so");
+    await execFileAsync("clang", ["-O1", "-shared", "-fPIC", "-o", library, ...frames]);
+    for (const build of ["plain", "asan", "tsan", "shared"] as const) {
+      const bin = join(buildDir, `test_unwind_${build}`);
+      await execFileAsync("clang", [
+        "-std=c11",
+        "-O1",
+        "-Wall",
+        "-Wextra",
+        "-D_GNU_SOURCE",
+        "-DSCR_UNWIND_TESTING",
+        ...(build === "asan" ? ["-fsanitize=address"] : []),
+        ...(build === "tsan" ? ["-fsanitize=thread"] : []),
+        "-pthread",
+        "-o",
+        bin,
+        join(testDir, "test_unwind.c"),
+        ...(build === "shared" ? [library, `-Wl,-rpath,${buildDir}`] : frames),
+        join(testDir, "../src/scr_unwind.c"),
+        // The library's landing pads and the program share libgcc_s's unwinder.
+        ...(build === "shared" ? ["-shared-libgcc"] : ["-lgcc_eh"]),
+      ]);
+      const env = {
+        ...process.env,
+        ASAN_OPTIONS: "detect_leaks=0:halt_on_error=1",
+        TSAN_OPTIONS: "halt_on_error=1",
+      };
+      // ThreadSanitizer checks the threads' concurrent plan inserts and
+      // lookups; the libgcc fallback cases run in the other builds.
+      for (const args of build === "tsan" ? [["threads"]] : [[], ["threads"]]) {
+        const run = await execFileAsync(bin, args, { env });
+        expect(run.stdout).toBe("unwind checks passed\n");
+      }
+    }
+  },
+  120_000,
+);
diff --git a/scripts/zero-cost-compare.mjs b/scripts/zero-cost-compare.mjs
index 8d612db243..40f7287fbb 100644
--- a/scripts/zero-cost-compare.mjs
+++ b/scripts/zero-cost-compare.mjs
@@ -415,7 +415,9 @@ export function nodeIslandSymbols(names) {
  * whose parameters the lib types `any`: captures a match can leave unset
  * and results the checker types `any`. bench/record-untyped-writes, also
  * added later, writes typed records through `unknown` and `Object.assign`
- * views. Every other bench workload is fully typed and must link none. The gate and
+ * views. bench/layered-errors, also added later, constructs and throws
+ * Error subclass instances, as bench/validate-errors does. Every other bench
+ * workload is fully typed and must link none. The gate and
  * tests/harness/bench-dynamic-tier.test.ts enforce that, and the test also
  * fails when a listed workload stops linking them, so remove a workload here
  * when a leak fix makes its binary clean. */
@@ -440,6 +442,7 @@ export const DYNAMIC_WORKLOADS = new Set([
   "bench/throw-chains",
   "bench/regex-replacers",
   "bench/record-untyped-writes",
+  "bench/layered-errors",
 ]);
 
 /** Names of the functions (text and weak symbols) in `nm` output. Mach-O
diff --git a/tests/corpus/unwind-landing-pad-chains.ts b/tests/corpus/unwind-landing-pad-chains.ts
new file mode 100644
index 0000000000..ba605586dc
--- /dev/null
+++ b/tests/corpus/unwind-landing-pad-chains.ts
@@ -0,0 +1,104 @@
+// Exceptions thrown below functions that hold references and propagate by
+// unwinding: landing pads release a frame's strings and arrays and continue
+// the unwind, also where the frame is inlined into a caller whose own handler
+// takes the exception, and through methods, closures and deep chains.
+function leaf(n: number): number {
+  if (n % 3 === 0) throw new RangeError("leaf " + n);
+  return n * 2;
+}
+function first(n: number): number {
+  return leaf(n) + 1;
+}
+function second(n: number): number {
+  return first(n) * 3;
+}
+function third(n: number): number {
+  return second(n) - 2;
+}
+function holder(n: number): number {
+  const parts: string[] = ["a" + n];
+  const r = third(n);
+  parts.push("b");
+  return r + parts.length;
+}
+function labelled(n: number): number {
+  const label = "u" + n;
+  return holder(n) + label.length;
+}
+function deeper(n: number, depth: number): number {
+  const name = "d" + depth;
+  if (depth === 0) return labelled(n) + name.length;
+  return deeper(n, depth - 1) + name.length;
+}
+class Counter {
+  total = 0;
+  add(n: number): void {
+    this.total += labelled(n);
+  }
+}
+
+function caught(n: number): string {
+  try {
+    return "ok " + labelled(n);
+  } catch (e) {
+    return "caught " + (e as Error).message;
+  }
+}
+
+const out: string[] = [];
+for (let i = 0; i < 7; i++) out.push(caught(i));
+console.log(out.join(", "));
+
+const counter = new Counter();
+let failures = 0;
+for (let round = 0; round < 2000; round++) {
+  for (let i = 0; i < 6; i++) {
+    try {
+      counter.add(i);
+    } catch {
+      failures++;
+    }
+  }
+}
+console.log("counter", counter.total, "failures", failures);
+
+let deep = 0;
+let deepFailures = 0;
+for (let i = 0; i < 300; i++) {
+  try {
+    deep += deeper(i, i % 40);
+  } catch (e) {
+    deepFailures++;
+    if (i < 10) console.log((e as Error).name, (e as Error).message);
+  }
+}
+console.log("deep", deep, "failures", deepFailures);
+
+const viaClosure = (n: number): number => labelled(n) + 1;
+let closureFailures = 0;
+for (let i = 0; i < 30; i++) {
+  try {
+    viaClosure(i);
+  } catch {
+    closureFailures++;
+  }
+}
+console.log("closure failures", closureFailures);
+
+function finallyOrder(n: number): string {
+  const log: string[] = [];
+  try {
+    try {
+      log.push("try");
+      labelled(n);
+      log.push("after");
+    } finally {
+      log.push("finally");
+    }
+  } catch (e) {
+    log.push("catch " + (e as Error).message);
+  }
+  return log.join(" > ");
+}
+console.log(finallyOrder(3));
+console.log(finallyOrder(4));
diff --git a/tests/harness/corpus-costs/unwind-landing-pad-chains.ts.json b/tests/harness/corpus-costs/unwind-landing-pad-chains.ts.json
new file mode 100644
index 0000000000..97b189b08f
--- /dev/null
+++ b/tests/harness/corpus-costs/unwind-landing-pad-chains.ts.json
@@ -0,0 +1 @@
+{ "plain": 4100, "san": 69700 }