Adding upstream version 127.0.upstream/127.0

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1743 insertions, 0 deletions

diff --git a/js/src/wasm/WasmPI.cpp b/js/src/wasm/WasmPI.cpp
new file mode 100644
index 0000000000..d11db9022d
--- /dev/null
+++ b/js/src/wasm/WasmPI.cpp
@@ -0,0 +1,1743 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ *
+ * Copyright 2016 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "wasm/WasmPI.h"
+
+#include "builtin/Promise.h"
+#include "jit/MIRGenerator.h"
+#include "js/CallAndConstruct.h"
+#include "vm/JSContext.h"
+#include "vm/JSObject.h"
+#include "vm/NativeObject.h"
+#include "vm/PromiseObject.h"
+#include "wasm/WasmConstants.h"
+#include "wasm/WasmContext.h"
+#include "wasm/WasmFeatures.h"
+#include "wasm/WasmGenerator.h"
+#include "wasm/WasmValidate.h"
+
+#include "vm/JSObject-inl.h"
+#include "wasm/WasmGcObject-inl.h"
+#include "wasm/WasmInstance-inl.h"
+
+#ifdef XP_WIN
+#  include "util/WindowsWrapper.h"
+#endif
+
+using namespace js;
+using namespace js::jit;
+
+#ifdef ENABLE_WASM_JSPI
+namespace js::wasm {
+
+SuspenderObjectData::SuspenderObjectData(void* stackMemory)
+    : stackMemory_(stackMemory),
+      suspendableFP_(nullptr),
+      suspendableSP_(static_cast<uint8_t*>(stackMemory) +
+                     SuspendableStackPlusRedZoneSize),
+      state_(SuspenderState::Initial) {}
+
+#  ifdef _WIN64
+// On WIN64, the Thread Information Block stack limits has to be modified to
+// avoid failures on SP checks.
+void SuspenderObjectData::updateTIBStackFields() {
+  _NT_TIB* tib = reinterpret_cast<_NT_TIB*>(::NtCurrentTeb());
+  savedStackBase_ = tib->StackBase;
+  savedStackLimit_ = tib->StackLimit;
+  uintptr_t stack_limit = (uintptr_t)stackMemory_ + SuspendableRedZoneSize;
+  uintptr_t stack_base = stack_limit + SuspendableStackSize;
+  tib->StackBase = (void*)stack_base;
+  tib->StackLimit = (void*)stack_limit;
+}
+
+void SuspenderObjectData::restoreTIBStackFields() {
+  _NT_TIB* tib = reinterpret_cast<_NT_TIB*>(::NtCurrentTeb());
+  tib->StackBase = savedStackBase_;
+  tib->StackLimit = savedStackLimit_;
+}
+#  endif
+
+// Slots that used in various JSFunctionExtended below.
+const size_t SUSPENDER_SLOT = 0;
+const size_t WRAPPED_FN_SLOT = 1;
+const size_t CONTINUE_ON_SUSPENDABLE_SLOT = 1;
+
+SuspenderContext::SuspenderContext()
+    : activeSuspender_(nullptr), suspendedStacks_() {}
+
+SuspenderContext::~SuspenderContext() {
+  MOZ_ASSERT(activeSuspender_ == nullptr);
+}
+
+SuspenderObject* SuspenderContext::activeSuspender() {
+  return activeSuspender_;
+}
+
+void SuspenderContext::setActiveSuspender(SuspenderObject* obj) {
+  activeSuspender_.set(obj);
+}
+
+void SuspenderContext::trace(JSTracer* trc) {
+  if (activeSuspender_) {
+    TraceEdge(trc, &activeSuspender_, "suspender");
+  }
+}
+
+void SuspenderContext::traceRoots(JSTracer* trc) {
+  for (const SuspenderObjectData& data : suspendedStacks_) {
+    void* startFP = data.suspendableFP();
+    void* returnAddress = data.suspendedReturnAddress();
+    void* exitFP = data.suspendableExitFP();
+    MOZ_ASSERT(startFP != exitFP);
+
+    WasmFrameIter iter(static_cast<FrameWithInstances*>(startFP),
+                       returnAddress);
+    MOZ_ASSERT(iter.stackSwitched());
+    uintptr_t highestByteVisitedInPrevWasmFrame = 0;
+    while (true) {
+      MOZ_ASSERT(!iter.done());
+      uint8_t* nextPC = iter.resumePCinCurrentFrame();
+      Instance* instance = iter.instance();
+      TraceInstanceEdge(trc, instance, "WasmFrameIter instance");
+      highestByteVisitedInPrevWasmFrame = instance->traceFrame(
+          trc, iter, nextPC, highestByteVisitedInPrevWasmFrame);
+      if (iter.frame() == exitFP) {
+        break;
+      }
+      ++iter;
+      if (iter.stackSwitched()) {
+        highestByteVisitedInPrevWasmFrame = 0;
+      }
+    }
+  }
+}
+
+static_assert(JS_STACK_GROWTH_DIRECTION < 0,
+              "JS-PI implemented only for native stacks that grows towards 0");
+
+static void DecrementSuspendableStacksCount(JSContext* cx) {
+  for (;;) {
+    uint32_t currentCount = cx->wasm().suspendableStacksCount;
+    MOZ_ASSERT(currentCount > 0);
+    if (cx->wasm().suspendableStacksCount.compareExchange(currentCount,
+                                                          currentCount - 1)) {
+      break;
+    }
+    // Failed to decrement suspendableStacksCount, repeat.
+  }
+}
+
+class SuspenderObject : public NativeObject {
+ public:
+  static const JSClass class_;
+
+  enum { DataSlot, PromisingPromiseSlot, SuspendingPromiseSlot, SlotCount };
+
+  static SuspenderObject* create(JSContext* cx) {
+    for (;;) {
+      uint32_t currentCount = cx->wasm().suspendableStacksCount;
+      if (currentCount >= SuspendableStacksMaxCount) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_JSPI_SUSPENDER_LIMIT);
+        return nullptr;
+      }
+      if (cx->wasm().suspendableStacksCount.compareExchange(currentCount,
+                                                            currentCount + 1)) {
+        break;
+      }
+      // Failed to increment suspendableStacksCount, repeat.
+    }
+
+    Rooted<SuspenderObject*> suspender(
+        cx, NewBuiltinClassInstance<SuspenderObject>(cx));
+    if (!suspender) {
+      DecrementSuspendableStacksCount(cx);
+      return nullptr;
+    }
+
+    void* stackMemory = js_malloc(SuspendableStackPlusRedZoneSize);
+    if (!stackMemory) {
+      DecrementSuspendableStacksCount(cx);
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+
+    SuspenderObjectData* data = js_new<SuspenderObjectData>(stackMemory);
+    if (!data) {
+      js_free(stackMemory);
+      DecrementSuspendableStacksCount(cx);
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    MOZ_RELEASE_ASSERT(data->state() != SuspenderState::Moribund);
+
+    suspender->initReservedSlot(DataSlot, PrivateValue(data));
+    suspender->initReservedSlot(PromisingPromiseSlot, NullValue());
+    suspender->initReservedSlot(SuspendingPromiseSlot, NullValue());
+    return suspender;
+  }
+
+  PromiseObject* promisingPromise() const {
+    return &getReservedSlot(PromisingPromiseSlot)
+                .toObject()
+                .as<PromiseObject>();
+  }
+
+  void setPromisingPromise(Handle<PromiseObject*> promise) {
+    setReservedSlot(PromisingPromiseSlot, ObjectOrNullValue(promise));
+  }
+
+  PromiseObject* suspendingPromise() const {
+    return &getReservedSlot(SuspendingPromiseSlot)
+                .toObject()
+                .as<PromiseObject>();
+  }
+
+  void setSuspendingPromise(Handle<PromiseObject*> promise) {
+    setReservedSlot(SuspendingPromiseSlot, ObjectOrNullValue(promise));
+  }
+
+  JS::NativeStackLimit getStackMemoryLimit() {
+    return JS::NativeStackLimit(data()->stackMemory()) + SuspendableRedZoneSize;
+  }
+
+  SuspenderState state() { return data()->state(); }
+
+  inline bool hasData() { return !getReservedSlot(DataSlot).isUndefined(); }
+
+  inline SuspenderObjectData* data() {
+    return static_cast<SuspenderObjectData*>(
+        getReservedSlot(DataSlot).toPrivate());
+  }
+
+  void setMoribund(JSContext* cx);
+  void setActive(JSContext* cx);
+  void setSuspended(JSContext* cx);
+
+  void enter(JSContext* cx);
+  void suspend(JSContext* cx);
+  void resume(JSContext* cx);
+  void leave(JSContext* cx);
+
+ private:
+  static const JSClassOps classOps_;
+
+  static void finalize(JS::GCContext* gcx, JSObject* obj);
+};
+
+static_assert(SuspenderObjectDataSlot == SuspenderObject::DataSlot);
+
+const JSClass SuspenderObject::class_ = {
+    "SuspenderObject",
+    JSCLASS_HAS_RESERVED_SLOTS(SlotCount) | JSCLASS_BACKGROUND_FINALIZE,
+    &SuspenderObject::classOps_};
+
+const JSClassOps SuspenderObject::classOps_ = {
+    nullptr,   // addProperty
+    nullptr,   // delProperty
+    nullptr,   // enumerate
+    nullptr,   // newEnumerate
+    nullptr,   // resolve
+    nullptr,   // mayResolve
+    finalize,  // finalize
+    nullptr,   // call
+    nullptr,   // construct
+    nullptr,   // trace
+};
+
+/* static */
+void SuspenderObject::finalize(JS::GCContext* gcx, JSObject* obj) {
+  SuspenderObject& suspender = obj->as<SuspenderObject>();
+  if (!suspender.hasData()) {
+    return;
+  }
+  SuspenderObjectData* data = suspender.data();
+  MOZ_RELEASE_ASSERT(data->state() == SuspenderState::Moribund);
+  js_free(data->stackMemory());
+  js_free(data);
+}
+
+void SuspenderObject::setMoribund(JSContext* cx) {
+  // TODO make sense to free stackMemory at this point to reduce memory usage?
+  MOZ_ASSERT(state() == SuspenderState::Active);
+  ResetInstanceStackLimits(cx);
+#  ifdef _WIN64
+  data()->restoreTIBStackFields();
+#  endif
+  SuspenderObjectData* data = this->data();
+  data->setState(SuspenderState::Moribund);
+  DecrementSuspendableStacksCount(cx);
+  MOZ_ASSERT(
+      !cx->wasm().promiseIntegration.suspendedStacks_.ElementProbablyInList(
+          data));
+}
+
+void SuspenderObject::setActive(JSContext* cx) {
+  data()->setState(SuspenderState::Active);
+  UpdateInstanceStackLimitsForSuspendableStack(cx, getStackMemoryLimit());
+#  ifdef _WIN64
+  data()->updateTIBStackFields();
+#  endif
+}
+
+void SuspenderObject::setSuspended(JSContext* cx) {
+  data()->setState(SuspenderState::Suspended);
+  ResetInstanceStackLimits(cx);
+#  ifdef _WIN64
+  data()->restoreTIBStackFields();
+#  endif
+}
+
+void SuspenderObject::enter(JSContext* cx) {
+  MOZ_ASSERT(state() == SuspenderState::Initial);
+  cx->wasm().promiseIntegration.setActiveSuspender(this);
+  setActive(cx);
+}
+
+void SuspenderObject::suspend(JSContext* cx) {
+  MOZ_ASSERT(state() == SuspenderState::Active);
+  setSuspended(cx);
+  cx->wasm().promiseIntegration.suspendedStacks_.pushFront(data());
+  cx->wasm().promiseIntegration.setActiveSuspender(nullptr);
+}
+
+void SuspenderObject::resume(JSContext* cx) {
+  MOZ_ASSERT(state() == SuspenderState::Suspended);
+  cx->wasm().promiseIntegration.setActiveSuspender(this);
+  setActive(cx);
+  cx->wasm().promiseIntegration.suspendedStacks_.remove(data());
+}
+
+void SuspenderObject::leave(JSContext* cx) {
+  cx->wasm().promiseIntegration.setActiveSuspender(nullptr);
+  // We are exiting alternative stack if state is active,
+  // otherwise the stack was just suspended.
+  switch (state()) {
+    case SuspenderState::Active:
+      setMoribund(cx);
+      break;
+    case SuspenderState::Suspended:
+      break;
+    case SuspenderState::Initial:
+    case SuspenderState::Moribund:
+      MOZ_CRASH();
+  }
+}
+
+bool ParseSuspendingPromisingString(JSContext* cx, HandleValue val,
+                                    SuspenderArgPosition& result) {
+  if (val.isNullOrUndefined()) {
+    result = SuspenderArgPosition::None;
+    return true;
+  }
+
+  RootedString str(cx, ToString(cx, val));
+  if (!str) {
+    return false;
+  }
+  Rooted<JSLinearString*> linear(cx, str->ensureLinear(cx));
+  if (!linear) {
+    return false;
+  }
+
+  if (StringEqualsLiteral(linear, "first")) {
+    result = SuspenderArgPosition::First;
+  } else if (StringEqualsLiteral(linear, "last")) {
+    result = SuspenderArgPosition::Last;
+  } else {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_JSPI_ARG_POSITION);
+    return false;
+  }
+  return true;
+}
+
+using CallImportData = Instance::WasmJSPICallImportData;
+
+/*static*/
+bool CallImportData::Call(CallImportData* data) {
+  Instance* instance = data->instance;
+  JSContext* cx = instance->cx();
+  return instance->callImport(cx, data->funcImportIndex, data->argc,
+                              data->argv);
+}
+
+bool CallImportOnMainThread(JSContext* cx, Instance* instance,
+                            int32_t funcImportIndex, int32_t argc,
+                            uint64_t* argv) {
+  CallImportData data = {instance, funcImportIndex, argc, argv};
+  Rooted<SuspenderObject*> suspender(
+      cx, cx->wasm().promiseIntegration.activeSuspender());
+  SuspenderObjectData* stacks = suspender->data();
+
+  cx->wasm().promiseIntegration.setActiveSuspender(nullptr);
+
+  MOZ_ASSERT(suspender->state() == SuspenderState::Active);
+  suspender->setSuspended(cx);
+
+  // The platform specific code below inserts offsets as strings into inline
+  // assembly. CHECK_OFFSETS verifies the specified literals in macros below.
+#  define CHECK_OFFSETS(MAIN_FP_OFFSET, MAIN_SP_OFFSET, SUSPENDABLE_FP_OFFSET, \
+                        SUSPENDABLE_SP_OFFSET)                                 \
+    static_assert((MAIN_FP_OFFSET) == SuspenderObjectData::offsetOfMainFP() && \
+                  (MAIN_SP_OFFSET) == SuspenderObjectData::offsetOfMainSP() && \
+                  (SUSPENDABLE_FP_OFFSET) ==                                   \
+                      SuspenderObjectData::offsetOfSuspendableFP() &&          \
+                  (SUSPENDABLE_SP_OFFSET) ==                                   \
+                      SuspenderObjectData::offsetOfSuspendableSP());
+
+  // The following assembly code temporarily switches FP/SP pointers to be on
+  // main stack, while maintaining frames linking.  After
+  // `CallImportData::Call` execution suspendable stack FP/SP will be restored.
+  //
+  // Because the assembly sequences contain a call, the trashed-register list
+  // must contain all the caller saved registers.  They must also contain "cc"
+  // and "memory" since both of those state elements could be modified by the
+  // call.  They also need a "volatile" qualifier to ensure that the they don't
+  // get optimised out or otherwise messed with by clang/gcc.
+  //
+  // `Registers::VolatileMask` (in the assembler complex) is useful in that it
+  // lists the caller-saved registers.
+
+  uintptr_t res;
+
+  // clang-format off
+#if defined(_M_ARM64) || defined(__aarch64__)
+#    define CALLER_SAVED_REGS \
+      "x0", "x1", "x2", "x3","x4", "x5", "x6", "x7", "x8", "x9", "x10",   \
+      "x11", "x12", "x13", "x14", "x15", "x16", "x17", /* "x18", */       \
+      "x19", "x20", /* it's unclear who saves these two, so be safe */    \
+      /* claim that all the vector regs are caller-saved, for safety */   \
+      "v0", "v1", "v2", "v3", "v4", "v5", "v6", "v7", "v8", "v9", "v10",  \
+      "v11", "v12", "v13", "v14", "v15", "v16", "v17", "v18", "v19",      \
+      "v20", "v21", "v22", "v23", "v24", "v25", "v26", "v27", "v28",      \
+      "v29", "v30", "v31"
+#    define INLINED_ASM(MAIN_FP, MAIN_SP, SUSPENDABLE_FP, SUSPENDABLE_SP) \
+      CHECK_OFFSETS(MAIN_FP, MAIN_SP, SUSPENDABLE_FP, SUSPENDABLE_SP);    \
+      asm volatile(                                                       \
+          "\n   mov     x0, %1"                                           \
+          "\n   mov     x27, sp "                                         \
+          "\n   str     x29, [x0, #" #SUSPENDABLE_FP "] "                 \
+          "\n   str     x27, [x0, #" #SUSPENDABLE_SP "] "                 \
+                                                                          \
+          "\n   ldr     x29, [x0, #" #MAIN_FP "] "                        \
+          "\n   ldr     x27, [x0, #" #MAIN_SP "] "                        \
+          "\n   mov     sp, x27 "                                         \
+                                                                          \
+          "\n   stp     x0, x27, [sp, #-16]! "                            \
+                                                                          \
+          "\n   mov     x0, %3"                                           \
+          "\n   blr     %2 "                                              \
+                                                                          \
+          "\n   ldp     x3, x27, [sp], #16 "                              \
+                                                                          \
+          "\n   mov     x27, sp "                                         \
+          "\n   str     x29, [x3, #" #MAIN_FP "] "                        \
+          "\n   str     x27, [x3, #" #MAIN_SP "] "                        \
+                                                                          \
+          "\n   ldr     x29, [x3, #" #SUSPENDABLE_FP "] "                 \
+          "\n   ldr     x27, [x3, #" #SUSPENDABLE_SP "] "                 \
+          "\n   mov     sp, x27 "                                         \
+          "\n   mov     %0, x0"                                           \
+          : "=r"(res)                                                     \
+          : "r"(stacks), "r"(CallImportData::Call), "r"(&data)            \
+          : "x0", "x3", "x27", CALLER_SAVED_REGS, "cc", "memory")
+  INLINED_ASM(24, 32, 40, 48);
+
+#  elif defined(_WIN64) && defined(_M_X64)
+#    define INLINED_ASM(MAIN_FP, MAIN_SP, SUSPENDABLE_FP, SUSPENDABLE_SP) \
+      CHECK_OFFSETS(MAIN_FP, MAIN_SP, SUSPENDABLE_FP, SUSPENDABLE_SP);    \
+      asm("\n   mov     %1, %%rcx"                                        \
+          "\n   mov     %%rbp, " #SUSPENDABLE_FP "(%%rcx)"                \
+          "\n   mov     %%rsp, " #SUSPENDABLE_SP "(%%rcx)"                \
+                                                                          \
+          "\n   mov     " #MAIN_FP "(%%rcx), %%rbp"                       \
+          "\n   mov     " #MAIN_SP "(%%rcx), %%rsp"                       \
+                                                                          \
+          "\n   push    %%rcx"                                            \
+          "\n   push    %%rdx"                                            \
+                                                                          \
+          "\n   mov     %3, %%rcx"                                        \
+          "\n   call    *%2"                                              \
+                                                                          \
+          "\n   pop    %%rdx"                                             \
+          "\n   pop    %%rcx"                                             \
+                                                                          \
+          "\n   mov     %%rbp, " #MAIN_FP "(%%rcx)"                       \
+          "\n   mov     %%rsp, " #MAIN_SP "(%%rcx)"                       \
+                                                                          \
+          "\n   mov     " #SUSPENDABLE_FP "(%%rcx), %%rbp"                \
+          "\n   mov     " #SUSPENDABLE_SP "(%%rcx), %%rsp"                \
+                                                                          \
+          "\n   movq     %%rax, %0"                                       \
+          : "=r"(res)                                                     \
+          : "r"(stacks), "r"(CallImportData::Call), "r"(&data)         \
+          : "rcx", "rax")
+  INLINED_ASM(24, 32, 40, 48);
+
+#  elif defined(__x86_64__)
+#    define INLINED_ASM(MAIN_FP, MAIN_SP, SUSPENDABLE_FP, SUSPENDABLE_SP) \
+      CHECK_OFFSETS(MAIN_FP, MAIN_SP, SUSPENDABLE_FP, SUSPENDABLE_SP);    \
+      asm("\n   mov     %1, %%rdi"                                        \
+          "\n   mov     %%rbp, " #SUSPENDABLE_FP "(%%rdi)"                \
+          "\n   mov     %%rsp, " #SUSPENDABLE_SP "(%%rdi)"                \
+                                                                          \
+          "\n   mov     " #MAIN_FP "(%%rdi), %%rbp"                       \
+          "\n   mov     " #MAIN_SP "(%%rdi), %%rsp"                       \
+                                                                          \
+          "\n   push    %%rdi"                                            \
+          "\n   push    %%rdx"                                            \
+                                                                          \
+          "\n   mov     %3, %%rdi"                                        \
+          "\n   call    *%2"                                              \
+                                                                          \
+          "\n   pop    %%rdx"                                             \
+          "\n   pop    %%rdi"                                             \
+                                                                          \
+          "\n   mov     %%rbp, " #MAIN_FP "(%%rdi)"                       \
+          "\n   mov     %%rsp, " #MAIN_SP "(%%rdi)"                       \
+                                                                          \
+          "\n   mov     " #SUSPENDABLE_FP "(%%rdi), %%rbp"                \
+          "\n   mov     " #SUSPENDABLE_SP "(%%rdi), %%rsp"                \
+                                                                          \
+          "\n   movq     %%rax, %0"                                       \
+          : "=r"(res)                                                     \
+          : "r"(stacks), "r"(CallImportData::Call), "r"(&data)         \
+          : "rdi", "rax")
+  INLINED_ASM(24, 32, 40, 48);
+
+#  else
+  MOZ_CRASH("Not supported for this platform");
+#  endif
+  // clang-format on
+
+  bool ok = res;
+  suspender->setActive(cx);
+  cx->wasm().promiseIntegration.setActiveSuspender(suspender);
+
+#  undef INLINED_ASM
+#  undef CHECK_OFFSETS
+#  undef CALLER_SAVED_REGS
+
+  return ok;
+}
+
+void UnwindStackSwitch(JSContext* cx) {
+  SuspenderObject* suspender = cx->wasm().promiseIntegration.activeSuspender();
+  MOZ_ASSERT(suspender);
+  cx->wasm().promiseIntegration.setActiveSuspender(nullptr);
+  suspender->setMoribund(cx);
+}
+
+// Suspending
+
+// Builds a wasm module with following structure:
+// (module
+//   (type $params (struct (field ..)*)))
+//   (type $results (struct (field ..)*)))
+//   (import "" "" (func $suspending.wrappedfn ..))
+//   (import "" "" (func $suspending.add-promise-reactions ..))
+//   (import "" "" (func $suspending.get-suspending-promise-result ..))
+//   (func $suspending.exported .. )
+//   (func $suspending.trampoline ..)
+//   (func $suspending.continue-on-suspendable ..)
+//   (export "" (func $suspending.exported))
+// )
+class SuspendingFunctionModuleFactory {
+ public:
+  enum TypeIdx {
+    ParamsTypeIndex,
+    ResultsTypeIndex,
+  };
+
+  enum FnIdx {
+    WrappedFnIndex,
+    GetSuspendingResultsFnIndex,
+    ExportedFnIndex,
+    TrampolineFnIndex,
+    ContinueOnSuspendableFnIndex
+  };
+
+ private:
+  // Builds function that will be imported to wasm module:
+  // (func $suspending.exported
+  //   (param $suspender externref)? (param ..)* (param $suspender externref)?
+  //   (result ..)*
+  //   (local $suspender externref)?
+  //   (local $results (ref $results))
+  //   ;; #if checkSuspender
+  //   local.get $suspender
+  //   call $builtin.check-suspender
+  //   ;; #else
+  //   call $builtin.current-suspender
+  //   local.tee $suspender
+  //   ;; #endif
+  //   ref.func $suspending.trampoline
+  //   local.get $i*
+  //   stuct.new $param-type
+  //   stack-switch SwitchToMain ;; <- (suspender,fn,data)
+  //   local.get $suspender
+  //   call $builtin.get-suspending-promise-result
+  //   ref.cast $results-type
+  //   local.set $results
+  //   (struct.get $results (local.get $results))*
+  // )
+  bool encodeExportedFunction(ModuleEnvironment& moduleEnv, uint32_t paramsSize,
+                              uint32_t resultSize, uint32_t paramsOffset,
+                              uint32_t suspenderIndex, bool checkSuspender,
+                              RefType resultType, Bytes& bytecode) {
+    Encoder encoder(bytecode, *moduleEnv.types);
+    ValTypeVector locals;
+    if (!checkSuspender && !locals.emplaceBack(RefType::extern_())) {
+      return false;
+    }
+    if (!locals.emplaceBack(resultType)) {
+      return false;
+    }
+    if (!EncodeLocalEntries(encoder, locals)) {
+      return false;
+    }
+
+    if (checkSuspender) {
+      if (!encoder.writeOp(Op::LocalGet) ||
+          !encoder.writeVarU32(suspenderIndex)) {
+        return false;
+      }
+      if (!encoder.writeOp(MozOp::CallBuiltinModuleFunc) ||
+          !encoder.writeVarU32((uint32_t)BuiltinModuleFuncId::CheckSuspender)) {
+        return false;
+      }
+    } else {
+      if (!encoder.writeOp(Op::I32Const) || !encoder.writeVarU32(0)) {
+        return false;
+      }
+      if (!encoder.writeOp(MozOp::CallBuiltinModuleFunc) ||
+          !encoder.writeVarU32(
+              (uint32_t)BuiltinModuleFuncId::CurrentSuspender)) {
+        return false;
+      }
+      if (!encoder.writeOp(Op::LocalTee) ||
+          !encoder.writeVarU32(suspenderIndex)) {
+        return false;
+      }
+    }
+
+    // Results local is located after all params and suspender.
+    // Adding 1 to paramSize for checkSuspender and ! cases.
+    int resultsIndex = paramsSize + 1;
+
+    if (!encoder.writeOp(Op::RefFunc) ||
+        !encoder.writeVarU32(TrampolineFnIndex)) {
+      return false;
+    }
+    for (uint32_t i = 0; i < paramsSize; i++) {
+      if (!encoder.writeOp(Op::LocalGet) ||
+          !encoder.writeVarU32(i + paramsOffset)) {
+        return false;
+      }
+    }
+    if (!encoder.writeOp(GcOp::StructNew) ||
+        !encoder.writeVarU32(ParamsTypeIndex)) {
+      return false;
+    }
+
+    if (!encoder.writeOp(MozOp::StackSwitch) ||
+        !encoder.writeVarU32(uint32_t(StackSwitchKind::SwitchToMain))) {
+      return false;
+    }
+
+    if (!encoder.writeOp(Op::LocalGet) ||
+        !encoder.writeVarU32(suspenderIndex)) {
+      return false;
+    }
+    if (!encoder.writeOp(MozOp::CallBuiltinModuleFunc) ||
+        !encoder.writeVarU32(
+            (uint32_t)BuiltinModuleFuncId::GetSuspendingPromiseResult)) {
+      return false;
+    }
+    if (!encoder.writeOp(GcOp::RefCast) ||
+        !encoder.writeVarU32(ResultsTypeIndex) ||
+        !encoder.writeOp(Op::LocalSet) || !encoder.writeVarU32(resultsIndex)) {
+      return false;
+    }
+    for (uint32_t i = 0; i < resultSize; i++) {
+      if (!encoder.writeOp(Op::LocalGet) ||
+          !encoder.writeVarU32(resultsIndex) ||
+          !encoder.writeOp(GcOp::StructGet) ||
+          !encoder.writeVarU32(ResultsTypeIndex) || !encoder.writeVarU32(i)) {
+        return false;
+      }
+    }
+    return encoder.writeOp(Op::End);
+  }
+
+  // Builds function that is called on main stack:
+  // (func $suspending.trampoline
+  //   (param $params (ref $suspender) (ref $param-type))
+  //   (result externref)
+  //   local.get $suspender ;; for call $process-promise
+  //   (struct.get $param-type $i (local.get $param))*
+  //   call $suspending.wrappedfn
+  //   ref.func $suspending.continue-on-suspendable
+  //   call $suspending.add-promise-reactions
+  // )
+  // The function calls suspending import and returns into the
+  // $promising.exported function because that was the top function
+  // on the main stack.
+  bool encodeTrampolineFunction(ModuleEnvironment& moduleEnv,
+                                uint32_t paramsSize, Bytes& bytecode) {
+    Encoder encoder(bytecode, *moduleEnv.types);
+    if (!EncodeLocalEntries(encoder, ValTypeVector())) {
+      return false;
+    }
+    const uint32_t SuspenderIndex = 0;
+    const uint32_t ParamsIndex = 1;
+
+    // For GetSuspendingResultsFnIndex call below.
+    if (!encoder.writeOp(Op::LocalGet) ||
+        !encoder.writeVarU32(SuspenderIndex)) {
+      return false;
+    }
+
+    for (uint32_t i = 0; i < paramsSize; i++) {
+      if (!encoder.writeOp(Op::LocalGet) || !encoder.writeVarU32(ParamsIndex)) {
+        return false;
+      }
+      if (!encoder.writeOp(GcOp::StructGet) ||
+          !encoder.writeVarU32(ParamsTypeIndex) || !encoder.writeVarU32(i)) {
+        return false;
+      }
+    }
+    if (!encoder.writeOp(Op::Call) || !encoder.writeVarU32(WrappedFnIndex)) {
+      return false;
+    }
+
+    if (!encoder.writeOp(Op::RefFunc) ||
+        !encoder.writeVarU32(ContinueOnSuspendableFnIndex)) {
+      return false;
+    }
+    if (!encoder.writeOp(Op::Call) ||
+        !encoder.writeVarU32(GetSuspendingResultsFnIndex)) {
+      return false;
+    }
+
+    return encoder.writeOp(Op::End);
+  }
+
+  // Builds function that is called on main stack:
+  // (func $suspending.continue-on-suspendable
+  //   (param $params (ref $suspender))
+  //   (result externref)
+  //   local.get $suspender
+  //   ref.null funcref
+  //   ref.null anyref
+  //   stack-switch ContinueOnSuspendable
+  // )
+  bool encodeContinueOnSuspendableFunction(ModuleEnvironment& moduleEnv,
+                                           uint32_t resultsSize,
+                                           Bytes& bytecode) {
+    Encoder encoder(bytecode, *moduleEnv.types);
+    if (!EncodeLocalEntries(encoder, ValTypeVector())) {
+      return false;
+    }
+
+    const uint32_t SuspenderIndex = 0;
+    if (!encoder.writeOp(Op::LocalGet) ||
+        !encoder.writeVarU32(SuspenderIndex)) {
+      return false;
+    }
+    if (!encoder.writeOp(Op::RefNull) ||
+        !encoder.writeValType(ValType(RefType::func()))) {
+      return false;
+    }
+    if (!encoder.writeOp(Op::RefNull) ||
+        !encoder.writeValType(ValType(RefType::any()))) {
+      return false;
+    }
+
+    if (!encoder.writeOp(MozOp::StackSwitch) ||
+        !encoder.writeVarU32(
+            uint32_t(StackSwitchKind::ContinueOnSuspendable))) {
+      return false;
+    }
+
+    return encoder.writeOp(Op::End);
+  }
+
+ public:
+  SharedModule build(JSContext* cx, HandleObject func, ValTypeVector&& params,
+                     ValTypeVector&& results,
+                     SuspenderArgPosition argPosition) {
+    FeatureOptions options;
+    options.isBuiltinModule = true;
+    options.requireGC = true;
+    options.requireTailCalls = true;
+
+    ScriptedCaller scriptedCaller;
+    SharedCompileArgs compileArgs =
+        CompileArgs::buildAndReport(cx, std::move(scriptedCaller), options);
+    if (!compileArgs) {
+      return nullptr;
+    }
+
+    ModuleEnvironment moduleEnv(compileArgs->features);
+    MOZ_ASSERT(IonAvailable(cx));
+    CompilerEnvironment compilerEnv(CompileMode::Once, Tier::Optimized,
+                                    DebugEnabled::False);
+    compilerEnv.computeParameters();
+
+    if (!moduleEnv.init()) {
+      return nullptr;
+    }
+
+    RefType suspenderType = RefType::extern_();
+    RefType promiseType = RefType::extern_();
+
+    size_t paramsSize, paramsOffset, suspenderIndex;
+    size_t resultsSize = results.length();
+    bool checkSuspender;
+    ValTypeVector paramsWithoutSuspender;
+    switch (argPosition) {
+      case SuspenderArgPosition::First:
+        paramsSize = params.length() - 1;
+        paramsOffset = 1;
+        suspenderIndex = 0;
+        if (!paramsWithoutSuspender.append(params.begin() + 1, params.end())) {
+          ReportOutOfMemory(cx);
+          return nullptr;
+        }
+        checkSuspender = true;
+        break;
+      case SuspenderArgPosition::Last:
+        paramsSize = params.length() - 1;
+        paramsOffset = 0;
+        suspenderIndex = paramsSize - 1;
+        if (!paramsWithoutSuspender.append(params.begin(), params.end() - 1)) {
+          ReportOutOfMemory(cx);
+          return nullptr;
+        }
+        checkSuspender = true;
+        break;
+      default:
+        paramsSize = params.length();
+        paramsOffset = 0;
+        suspenderIndex = paramsSize;
+        if (!paramsWithoutSuspender.append(params.begin(), params.end())) {
+          ReportOutOfMemory(cx);
+          return nullptr;
+        }
+        checkSuspender = false;
+        break;
+    }
+    ValTypeVector resultsRef;
+    if (!resultsRef.emplaceBack(promiseType)) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+
+    StructType boxedParamsStruct;
+    if (!StructType::createImmutable(paramsWithoutSuspender,
+                                     &boxedParamsStruct)) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    MOZ_ASSERT(moduleEnv.types->length() == ParamsTypeIndex);
+    if (!moduleEnv.types->addType(std::move(boxedParamsStruct))) {
+      return nullptr;
+    }
+
+    StructType boxedResultType;
+    if (!StructType::createImmutable(results, &boxedResultType)) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    MOZ_ASSERT(moduleEnv.types->length() == ResultsTypeIndex);
+    if (!moduleEnv.types->addType(std::move(boxedResultType))) {
+      return nullptr;
+    }
+
+    MOZ_ASSERT(moduleEnv.funcs.length() == WrappedFnIndex);
+    if (!moduleEnv.addDefinedFunc(std::move(paramsWithoutSuspender),
+                                  std::move(resultsRef))) {
+      return nullptr;
+    }
+
+    ValTypeVector paramsGetSuspendingResults, resultsGetSuspendingResults;
+    if (!paramsGetSuspendingResults.emplaceBack(suspenderType) ||
+        !paramsGetSuspendingResults.emplaceBack(promiseType) ||
+        !paramsGetSuspendingResults.emplaceBack(RefType::func())) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    MOZ_ASSERT(moduleEnv.funcs.length() == GetSuspendingResultsFnIndex);
+    if (!moduleEnv.addDefinedFunc(std::move(paramsGetSuspendingResults),
+                                  std::move(resultsGetSuspendingResults))) {
+      return nullptr;
+    }
+
+    // Imports names are not important, declare functions above as imports.
+    moduleEnv.numFuncImports = moduleEnv.funcs.length();
+
+    // We will be looking up and using the exports function by index so
+    // the name doesn't matter.
+    MOZ_ASSERT(moduleEnv.funcs.length() == ExportedFnIndex);
+    if (!moduleEnv.addDefinedFunc(std::move(params), std::move(results),
+                                  /*declareForRef = */ true,
+                                  mozilla::Some(CacheableName()))) {
+      return nullptr;
+    }
+
+    ValTypeVector paramsTrampoline, resultsTrampoline;
+    if (!paramsTrampoline.emplaceBack(suspenderType) ||
+        !paramsTrampoline.emplaceBack(RefType::fromTypeDef(
+            &(*moduleEnv.types)[ParamsTypeIndex], false))) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    MOZ_ASSERT(moduleEnv.funcs.length() == TrampolineFnIndex);
+    if (!moduleEnv.addDefinedFunc(std::move(paramsTrampoline),
+                                  std::move(resultsTrampoline),
+                                  /*declareForRef = */ true)) {
+      return nullptr;
+    }
+
+    ValTypeVector paramsContinueOnSuspendable, resultsContinueOnSuspendable;
+    if (!paramsContinueOnSuspendable.emplaceBack(suspenderType) ||
+        !paramsContinueOnSuspendable.emplaceBack(RefType::extern_())) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    MOZ_ASSERT(moduleEnv.funcs.length() == ContinueOnSuspendableFnIndex);
+    if (!moduleEnv.addDefinedFunc(std::move(paramsContinueOnSuspendable),
+                                  std::move(resultsContinueOnSuspendable),
+                                  /*declareForRef = */ true)) {
+      return nullptr;
+    }
+
+    ModuleGenerator mg(*compileArgs, &moduleEnv, &compilerEnv, nullptr, nullptr,
+                       nullptr);
+    if (!mg.init(nullptr)) {
+      return nullptr;
+    }
+    // Build functions and keep bytecodes around until the end.
+    Bytes bytecode;
+    if (!encodeExportedFunction(
+            moduleEnv, paramsSize, resultsSize, paramsOffset, suspenderIndex,
+            checkSuspender,
+            RefType::fromTypeDef(&(*moduleEnv.types)[ResultsTypeIndex], false),
+            bytecode)) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    if (!mg.compileFuncDef(ExportedFnIndex, 0, bytecode.begin(),
+                           bytecode.begin() + bytecode.length())) {
+      return nullptr;
+    }
+    Bytes bytecode2;
+    if (!encodeTrampolineFunction(moduleEnv, paramsSize, bytecode2)) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    if (!mg.compileFuncDef(TrampolineFnIndex, 0, bytecode2.begin(),
+                           bytecode2.begin() + bytecode2.length())) {
+      return nullptr;
+    }
+    Bytes bytecode3;
+    if (!encodeContinueOnSuspendableFunction(moduleEnv, paramsSize,
+                                             bytecode3)) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    if (!mg.compileFuncDef(ContinueOnSuspendableFnIndex, 0, bytecode3.begin(),
+                           bytecode3.begin() + bytecode3.length())) {
+      return nullptr;
+    }
+    if (!mg.finishFuncDefs()) {
+      return nullptr;
+    }
+
+    SharedBytes shareableBytes = js_new<ShareableBytes>();
+    if (!shareableBytes) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    return mg.finishModule(*shareableBytes);
+  }
+};
+
+// Reaction on resolved/rejected suspending promise.
+static bool WasmPISuspendTaskContinue(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  Rooted<JSFunction*> callee(cx, &args.callee().as<JSFunction>());
+  RootedValue suspender(cx, callee->getExtendedSlot(SUSPENDER_SLOT));
+
+  // Convert result of the promise into the parameters/arguments for the
+  // $suspending.continue-on-suspendable.
+  RootedFunction continueOnSuspendable(
+      cx, &callee->getExtendedSlot(CONTINUE_ON_SUSPENDABLE_SLOT)
+               .toObject()
+               .as<JSFunction>());
+  RootedValueVector argv(cx);
+  if (!argv.emplaceBack(suspender)) {
+    ReportOutOfMemory(cx);
+    return false;
+  }
+  MOZ_ASSERT(args.length() > 0);
+  if (!argv.emplaceBack(args[0])) {
+    ReportOutOfMemory(cx);
+    return false;
+  }
+
+  JS::Rooted<JS::Value> rval(cx);
+  if (Call(cx, UndefinedHandleValue, continueOnSuspendable, argv, &rval)) {
+    return true;
+  }
+
+  if (cx->isThrowingOutOfMemory()) {
+    return false;
+  }
+  Rooted<PromiseObject*> promise(
+      cx, suspender.toObject().as<SuspenderObject>().promisingPromise());
+  return RejectPromiseWithPendingError(cx, promise);
+}
+
+// Collects returned suspending promising, and registers callbacks to
+// react on it using WasmPISuspendTaskContinue.
+// Seen as $suspending.add-promise-reactions in wasm.
+static bool WasmPIAddPromiseReactions(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+
+  Rooted<SuspenderObject*> suspenderObject(
+      cx, &args[0].toObject().as<SuspenderObject>());
+  RootedValue rval(cx, args[1]);
+  RootedFunction fn(cx, &args[2].toObject().as<JSFunction>());
+
+  MOZ_ASSERT(rval.toObject().is<PromiseObject>(),
+             "WasmPIWrapSuspendingImport always returning a promise");
+  Rooted<PromiseObject*> promise(cx, &rval.toObject().as<PromiseObject>());
+  suspenderObject->setSuspendingPromise(promise);
+
+  // Pass fn here
+  RootedFunction then_(
+      cx, NewNativeFunction(cx, WasmPISuspendTaskContinue, 1, nullptr,
+                            gc::AllocKind::FUNCTION_EXTENDED, GenericObject));
+  then_->initExtendedSlot(SUSPENDER_SLOT, ObjectValue(*suspenderObject));
+  then_->initExtendedSlot(CONTINUE_ON_SUSPENDABLE_SLOT, ObjectValue(*fn));
+  return AddPromiseReactions(cx, promise, then_, then_);
+}
+
+// Wraps original import to catch all exceptions and convert result to a
+// promise.
+// Seen as $suspending.wrappedfn in wasm.
+static bool WasmPIWrapSuspendingImport(JSContext* cx, unsigned argc,
+                                       Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  Rooted<JSFunction*> callee(cx, &args.callee().as<JSFunction>());
+  RootedFunction originalImportFunc(
+      cx,
+      &callee->getExtendedSlot(WRAPPED_FN_SLOT).toObject().as<JSFunction>());
+
+  // Catching exceptions here.
+  RootedValue rval(cx);
+  if (Call(cx, UndefinedHandleValue, originalImportFunc, args, &rval)) {
+    // Convert the result to a resolved promise.
+    RootedObject promiseConstructor(cx, GetPromiseConstructor(cx));
+    RootedObject promiseObj(cx, PromiseResolve(cx, promiseConstructor, rval));
+    if (!promiseObj) {
+      return false;
+    }
+    args.rval().setObject(*promiseObj);
+    return true;
+  }
+
+  if (cx->isThrowingOutOfMemory()) {
+    return false;
+  }
+
+  // Convert failure to a rejected promise.
+  RootedObject promiseObject(cx, NewPromiseObject(cx, nullptr));
+  if (!promiseObject) {
+    return false;
+  }
+  args.rval().setObject(*promiseObject);
+
+  Rooted<PromiseObject*> promise(cx, &promiseObject->as<PromiseObject>());
+  return RejectPromiseWithPendingError(cx, promise);
+}
+
+JSFunction* WasmSuspendingFunctionCreate(JSContext* cx, HandleObject func,
+                                         ValTypeVector&& params,
+                                         ValTypeVector&& results,
+                                         SuspenderArgPosition argPosition) {
+  MOZ_ASSERT(IsCallable(ObjectValue(*func)) &&
+             !IsCrossCompartmentWrapper(func));
+
+  if (argPosition != SuspenderArgPosition::None) {
+    if (params.length() < 1 ||
+        params[argPosition == SuspenderArgPosition::Last ? params.length() - 1
+                                                         : 0] !=
+            RefType::extern_()) {
+      JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                JSMSG_JSPI_EXPECTED_SUSPENDER);
+      return nullptr;
+    }
+  }
+
+  SuspendingFunctionModuleFactory moduleFactory;
+  SharedModule module = moduleFactory.build(cx, func, std::move(params),
+                                            std::move(results), argPosition);
+  if (!module) {
+    return nullptr;
+  }
+
+  // Instantiate the module.
+  Rooted<ImportValues> imports(cx);
+
+  // Add $suspending.wrappedfn to imports.
+  RootedFunction funcWrapper(
+      cx, NewNativeFunction(cx, WasmPIWrapSuspendingImport, 0, nullptr,
+                            gc::AllocKind::FUNCTION_EXTENDED, GenericObject));
+  if (!funcWrapper) {
+    return nullptr;
+  }
+  funcWrapper->initExtendedSlot(WRAPPED_FN_SLOT, ObjectValue(*func));
+  if (!imports.get().funcs.append(funcWrapper)) {
+    ReportOutOfMemory(cx);
+    return nullptr;
+  }
+
+  // Add $suspending.add-promise-reactions to imports.
+  RootedFunction addPromiseReactions(
+      cx, NewNativeFunction(cx, WasmPIAddPromiseReactions, 3, nullptr,
+                            gc::AllocKind::FUNCTION_EXTENDED, GenericObject));
+  if (!addPromiseReactions) {
+    return nullptr;
+  }
+  if (!imports.get().funcs.append(addPromiseReactions)) {
+    ReportOutOfMemory(cx);
+    return nullptr;
+  }
+
+  Rooted<WasmInstanceObject*> instance(cx);
+  if (!module->instantiate(cx, imports.get(), nullptr, &instance)) {
+    // Can also trap on invalid input function.
+    return nullptr;
+  }
+
+  // Returns the $suspending.exported function.
+  RootedFunction wasmFunc(cx);
+  if (!WasmInstanceObject::getExportedFunction(
+          cx, instance, SuspendingFunctionModuleFactory::ExportedFnIndex,
+          &wasmFunc)) {
+    return nullptr;
+  }
+  return wasmFunc;
+}
+
+JSFunction* WasmSuspendingFunctionCreate(JSContext* cx, HandleObject func,
+                                         const FuncType& type) {
+  ValTypeVector params, results;
+  if (!params.append(type.args().begin(), type.args().end()) ||
+      !results.append(type.results().begin(), type.results().end())) {
+    ReportOutOfMemory(cx);
+    return nullptr;
+  }
+  return WasmSuspendingFunctionCreate(cx, func, std::move(params),
+                                      std::move(results),
+                                      SuspenderArgPosition::None);
+}
+
+// Promising
+
+// Builds a wasm module with following structure:
+// (module
+//   (type $params (struct (field ..)*)))
+//   (type $results (struct (field ..)*)))
+//   (import "" "" (func $promising.wrappedfn ..))
+//   (import "" "" (func $promising.create-suspender ..))
+//   (func $promising.exported .. )
+//   (func $promising.trampoline ..)
+//   (export "" (func $promising.exported))
+// )
+class PromisingFunctionModuleFactory {
+ public:
+  enum TypeIdx {
+    ParamsTypeIndex,
+    ResultsTypeIndex,
+  };
+
+  enum FnIdx {
+    WrappedFnIndex,
+    CreateSuspenderFnIndex,
+    ExportedFnIndex,
+    TrampolineFnIndex,
+  };
+
+ private:
+  // Builds function that will be exported for JS:
+  // (func $promising.exported
+  //   (param ..)* (result externref)
+  //   (local $promise externref)
+  //   call $promising.create-suspender ;; -> (suspender,promise)
+  //   local.set $promise
+  //   ref.func $promising.trampoline
+  //   local.get $i*
+  //   stuct.new $param-type
+  //   stack-switch SwitchToSuspendable ;; <- (suspender,fn,data)
+  //   local.get $promise
+  // )
+  bool encodeExportedFunction(ModuleEnvironment& moduleEnv, uint32_t paramsSize,
+                              Bytes& bytecode) {
+    Encoder encoder(bytecode, *moduleEnv.types);
+    ValTypeVector locals;
+    if (!locals.emplaceBack(RefType::extern_())) {
+      return false;
+    }
+    if (!EncodeLocalEntries(encoder, locals)) {
+      return false;
+    }
+
+    const uint32_t PromiseIndex = paramsSize;
+    if (!encoder.writeOp(Op::Call) ||
+        !encoder.writeVarU32(CreateSuspenderFnIndex)) {
+      return false;
+    }
+    if (!encoder.writeOp(Op::LocalSet) || !encoder.writeVarU32(PromiseIndex)) {
+      return false;
+    }
+
+    if (!encoder.writeOp(Op::RefFunc) ||
+        !encoder.writeVarU32(TrampolineFnIndex)) {
+      return false;
+    }
+    for (uint32_t i = 0; i < paramsSize; i++) {
+      if (!encoder.writeOp(Op::LocalGet) || !encoder.writeVarU32(i)) {
+        return false;
+      }
+    }
+    if (!encoder.writeOp(GcOp::StructNew) ||
+        !encoder.writeVarU32(ParamsTypeIndex)) {
+      return false;
+    }
+    if (!encoder.writeOp(MozOp::StackSwitch) ||
+        !encoder.writeVarU32(uint32_t(StackSwitchKind::SwitchToSuspendable))) {
+      return false;
+    }
+
+    if (!encoder.writeOp(Op::LocalGet) || !encoder.writeVarU32(PromiseIndex)) {
+      return false;
+    }
+    return encoder.writeOp(Op::End);
+  }
+
+  // Builds function that is called on alternative stack:
+  // (func $promising.trampoline
+  //   (param $suspender externref) (param $params (ref $param-type))
+  //   (result externref)
+  //   local.get $suspender ;; for call $set-results
+  //   (local.get $suspender)?
+  //   (struct.get $param-type $i (local.get $param))*
+  //   (local.get $suspender)?
+  //   call $promising.wrappedfn
+  //   struct.new $result-type
+  //   call $builtin.set-promising-promise-results
+  // )
+  bool encodeTrampolineFunction(ModuleEnvironment& moduleEnv,
+                                uint32_t paramsSize,
+                                SuspenderArgPosition argPosition,
+                                Bytes& bytecode) {
+    Encoder encoder(bytecode, *moduleEnv.types);
+    if (!EncodeLocalEntries(encoder, ValTypeVector())) {
+      return false;
+    }
+    const uint32_t SuspenderIndex = 0;
+    const uint32_t ParamsIndex = 1;
+
+    // Reserved for SetResultsFnIndex call at the end
+    if (!encoder.writeOp(Op::LocalGet) ||
+        !encoder.writeVarU32(SuspenderIndex)) {
+      return false;
+    }
+
+    if (argPosition == SuspenderArgPosition::First) {
+      if (!encoder.writeOp(Op::LocalGet) ||
+          !encoder.writeVarU32(SuspenderIndex)) {
+        return false;
+      }
+    }
+    for (uint32_t i = 0; i < paramsSize; i++) {
+      if (!encoder.writeOp(Op::LocalGet) || !encoder.writeVarU32(ParamsIndex)) {
+        return false;
+      }
+      if (!encoder.writeOp(GcOp::StructGet) ||
+          !encoder.writeVarU32(ParamsTypeIndex) || !encoder.writeVarU32(i)) {
+        return false;
+      }
+    }
+    if (argPosition == SuspenderArgPosition::Last) {
+      if (!encoder.writeOp(Op::LocalGet) ||
+          !encoder.writeVarU32(SuspenderIndex)) {
+        return false;
+      }
+    }
+    if (!encoder.writeOp(Op::Call) || !encoder.writeVarU32(WrappedFnIndex)) {
+      return false;
+    }
+
+    if (!encoder.writeOp(GcOp::StructNew) ||
+        !encoder.writeVarU32(ResultsTypeIndex)) {
+      return false;
+    }
+    if (!encoder.writeOp(MozOp::CallBuiltinModuleFunc) ||
+        !encoder.writeVarU32(
+            (uint32_t)BuiltinModuleFuncId::SetPromisingPromiseResults)) {
+      return false;
+    }
+
+    return encoder.writeOp(Op::End);
+  }
+
+ public:
+  SharedModule build(JSContext* cx, HandleFunction fn, ValTypeVector&& params,
+                     ValTypeVector&& results,
+                     SuspenderArgPosition argPosition) {
+    const FuncType& fnType = fn->wasmTypeDef()->funcType();
+    size_t paramsSize = params.length();
+
+    RefType suspenderType = RefType::extern_();
+    RefType promiseType = RefType::extern_();
+
+    FeatureOptions options;
+    options.isBuiltinModule = true;
+    options.requireGC = true;
+    options.requireTailCalls = true;
+
+    ScriptedCaller scriptedCaller;
+    SharedCompileArgs compileArgs =
+        CompileArgs::buildAndReport(cx, std::move(scriptedCaller), options);
+    if (!compileArgs) {
+      return nullptr;
+    }
+
+    ModuleEnvironment moduleEnv(compileArgs->features);
+    MOZ_ASSERT(IonAvailable(cx));
+    CompilerEnvironment compilerEnv(CompileMode::Once, Tier::Optimized,
+                                    DebugEnabled::False);
+    compilerEnv.computeParameters();
+
+    if (!moduleEnv.init()) {
+      return nullptr;
+    }
+
+    StructType boxedParamsStruct;
+    if (!StructType::createImmutable(params, &boxedParamsStruct)) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    MOZ_ASSERT(moduleEnv.types->length() == ParamsTypeIndex);
+    if (!moduleEnv.types->addType(std::move(boxedParamsStruct))) {
+      return nullptr;
+    }
+
+    StructType boxedResultType;
+    if (!StructType::createImmutable(fnType.results(), &boxedResultType)) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    MOZ_ASSERT(moduleEnv.types->length() == ResultsTypeIndex);
+    if (!moduleEnv.types->addType(std::move(boxedResultType))) {
+      return nullptr;
+    }
+
+    ValTypeVector paramsForWrapper, resultsForWrapper;
+    if (!paramsForWrapper.append(fnType.args().begin(), fnType.args().end()) ||
+        !resultsForWrapper.append(fnType.results().begin(),
+                                  fnType.results().end())) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    MOZ_ASSERT(moduleEnv.funcs.length() == WrappedFnIndex);
+    if (!moduleEnv.addDefinedFunc(std::move(paramsForWrapper),
+                                  std::move(resultsForWrapper))) {
+      return nullptr;
+    }
+
+    ValTypeVector paramsCreateSuspender, resultsCreateSuspender;
+    if (!resultsCreateSuspender.emplaceBack(suspenderType) ||
+        !resultsCreateSuspender.emplaceBack(promiseType)) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    MOZ_ASSERT(moduleEnv.funcs.length() == CreateSuspenderFnIndex);
+    if (!moduleEnv.addDefinedFunc(std::move(paramsCreateSuspender),
+                                  std::move(resultsCreateSuspender))) {
+      return nullptr;
+    }
+
+    // Imports names are not important, declare functions above as imports.
+    moduleEnv.numFuncImports = moduleEnv.funcs.length();
+
+    // We will be looking up and using the exports function by index so
+    // the name doesn't matter.
+    MOZ_ASSERT(moduleEnv.funcs.length() == ExportedFnIndex);
+    if (!moduleEnv.addDefinedFunc(std::move(params), std::move(results),
+                                  /* declareFoRef = */ true,
+                                  mozilla::Some(CacheableName()))) {
+      return nullptr;
+    }
+
+    ValTypeVector paramsTrampoline, resultsTrampoline;
+    if (!paramsTrampoline.emplaceBack(suspenderType) ||
+        !paramsTrampoline.emplaceBack(RefType::fromTypeDef(
+            &(*moduleEnv.types)[ParamsTypeIndex], false))) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    MOZ_ASSERT(moduleEnv.funcs.length() == TrampolineFnIndex);
+    if (!moduleEnv.addDefinedFunc(std::move(paramsTrampoline),
+                                  std::move(resultsTrampoline),
+                                  /* declareFoRef = */ true)) {
+      return nullptr;
+    }
+
+    ModuleGenerator mg(*compileArgs, &moduleEnv, &compilerEnv, nullptr, nullptr,
+                       nullptr);
+    if (!mg.init(nullptr)) {
+      return nullptr;
+    }
+    // Build functions and keep bytecodes around until the end.
+    Bytes bytecode;
+    if (!encodeExportedFunction(moduleEnv, paramsSize, bytecode)) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    if (!mg.compileFuncDef(ExportedFnIndex, 0, bytecode.begin(),
+                           bytecode.begin() + bytecode.length())) {
+      return nullptr;
+    }
+    Bytes bytecode2;
+    if (!encodeTrampolineFunction(moduleEnv, paramsSize, argPosition,
+                                  bytecode2)) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    if (!mg.compileFuncDef(TrampolineFnIndex, 0, bytecode2.begin(),
+                           bytecode2.begin() + bytecode2.length())) {
+      return nullptr;
+    }
+    if (!mg.finishFuncDefs()) {
+      return nullptr;
+    }
+
+    SharedBytes shareableBytes = js_new<ShareableBytes>();
+    if (!shareableBytes) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    return mg.finishModule(*shareableBytes);
+  }
+};
+
+// Creates a suspender and promise (that will be returned to JS code).
+// Seen as $promising.create-suspender to wasm.
+static bool WasmPICreateSuspender(JSContext* cx, unsigned argc, Value* vp) {
+  Rooted<SuspenderObject*> suspenderObject(cx, SuspenderObject::create(cx));
+  RootedObject promiseObject(cx, NewPromiseObject(cx, nullptr));
+  if (!promiseObject) {
+    return false;
+  }
+
+  Rooted<PromiseObject*> promise(cx, &promiseObject->as<PromiseObject>());
+  suspenderObject->setPromisingPromise(promise);
+
+  CallArgs args = CallArgsFromVp(argc, vp);
+  Rooted<ArrayObject*> results(cx, NewDenseEmptyArray(cx));
+  if (!NewbornArrayPush(cx, results, ObjectValue(*suspenderObject))) {
+    return false;
+  }
+  if (!NewbornArrayPush(cx, results, ObjectValue(*promise))) {
+    return false;
+  }
+  args.rval().setObject(*results);
+  return true;
+}
+
+// Wraps call to wasm $promising.exported function to catch an exception and
+// return a promise instead.
+static bool WasmPIPromisingFunction(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  Rooted<JSFunction*> callee(cx, &args.callee().as<JSFunction>());
+  RootedFunction fn(
+      cx,
+      &callee->getExtendedSlot(WRAPPED_FN_SLOT).toObject().as<JSFunction>());
+
+  // Catching exceptions here.
+  if (Call(cx, UndefinedHandleValue, fn, args, args.rval())) {
+    return true;
+  }
+
+  if (cx->isThrowingOutOfMemory()) {
+    return false;
+  }
+
+  RootedObject promiseObject(cx, NewPromiseObject(cx, nullptr));
+  if (!promiseObject) {
+    return false;
+  }
+  args.rval().setObject(*promiseObject);
+
+  Rooted<PromiseObject*> promise(cx, &promiseObject->as<PromiseObject>());
+  return RejectPromiseWithPendingError(cx, promise);
+}
+
+JSFunction* WasmPromisingFunctionCreate(JSContext* cx, HandleObject func,
+                                        ValTypeVector&& params,
+                                        ValTypeVector&& results,
+                                        SuspenderArgPosition argPosition) {
+  RootedFunction wrappedWasmFunc(cx, &func->as<JSFunction>());
+  MOZ_ASSERT(wrappedWasmFunc->isWasm());
+  const FuncType& wrappedWasmFuncType =
+      wrappedWasmFunc->wasmTypeDef()->funcType();
+
+  if (argPosition != SuspenderArgPosition::None) {
+    if (results.length() != 1 || results[0] != RefType::extern_()) {
+      JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                JSMSG_JSPI_EXPECTED_PROMISE);
+      return nullptr;
+    }
+
+    size_t paramsOffset, suspenderIndex;
+    switch (argPosition) {
+      case SuspenderArgPosition::First:
+        paramsOffset = 1;
+        suspenderIndex = 0;
+        break;
+      case SuspenderArgPosition::Last:
+        paramsOffset = 0;
+        suspenderIndex = params.length();
+        break;
+      default:
+        MOZ_CRASH();
+    }
+
+    if (wrappedWasmFuncType.args().length() != params.length() + 1 ||
+        wrappedWasmFuncType.args()[suspenderIndex] != RefType::extern_()) {
+      JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                JSMSG_JSPI_EXPECTED_SUSPENDER);
+      return nullptr;
+    }
+    for (size_t i = 0; i < params.length(); i++) {
+      if (params[i] != wrappedWasmFuncType.args()[i + paramsOffset]) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_JSPI_SIGNATURE_MISMATCH);
+        return nullptr;
+      }
+    }
+  } else {
+    MOZ_ASSERT(results.length() == 0 && params.length() == 0);
+    if (!results.append(RefType::extern_())) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+    if (!params.append(wrappedWasmFuncType.args().begin(),
+                       wrappedWasmFuncType.args().end())) {
+      ReportOutOfMemory(cx);
+      return nullptr;
+    }
+  }
+
+  PromisingFunctionModuleFactory moduleFactory;
+  SharedModule module = moduleFactory.build(
+      cx, wrappedWasmFunc, std::move(params), std::move(results), argPosition);
+  // Instantiate the module.
+  Rooted<ImportValues> imports(cx);
+
+  // Add wrapped function ($promising.wrappedfn) to imports.
+  if (!imports.get().funcs.append(func)) {
+    ReportOutOfMemory(cx);
+    return nullptr;
+  }
+
+  // Add $promising.create-suspender to imports.
+  RootedFunction createSuspenderFunc(
+      cx, NewNativeFunction(cx, WasmPICreateSuspender, 0, nullptr,
+                            gc::AllocKind::FUNCTION_EXTENDED, GenericObject));
+  if (!createSuspenderFunc) {
+    return nullptr;
+  }
+  if (!imports.get().funcs.append(createSuspenderFunc)) {
+    ReportOutOfMemory(cx);
+    return nullptr;
+  }
+
+  Rooted<WasmInstanceObject*> instance(cx);
+  if (!module->instantiate(cx, imports.get(), nullptr, &instance)) {
+    MOZ_ASSERT(cx->isThrowingOutOfMemory());
+    return nullptr;
+  }
+
+  // Wrap $promising.exported function for exceptions/traps handling.
+  RootedFunction wasmFunc(cx);
+  if (!WasmInstanceObject::getExportedFunction(
+          cx, instance, PromisingFunctionModuleFactory::ExportedFnIndex,
+          &wasmFunc)) {
+    return nullptr;
+  }
+
+  RootedFunction wasmFuncWrapper(
+      cx, NewNativeFunction(cx, WasmPIPromisingFunction, 0, nullptr,
+                            gc::AllocKind::FUNCTION_EXTENDED, GenericObject));
+  if (!wasmFuncWrapper) {
+    return nullptr;
+  }
+  wasmFuncWrapper->initExtendedSlot(WRAPPED_FN_SLOT, ObjectValue(*wasmFunc));
+  return wasmFuncWrapper;
+}
+
+// Gets active suspender.
+// The reserved parameter is a workaround for limitation in the
+// WasmBuiltinModule.yaml generator to always have params.
+SuspenderObject* CurrentSuspender(Instance* instance, int32_t reserved) {
+  MOZ_ASSERT(SASigCurrentSuspender.failureMode == FailureMode::FailOnNullPtr);
+  JSContext* cx = instance->cx();
+  SuspenderObject* suspender = cx->wasm().promiseIntegration.activeSuspender();
+  if (!suspender) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_JSPI_INVALID_STATE);
+    return nullptr;
+  }
+  return suspender;
+}
+
+// Checks suspender value.
+SuspenderObject* CheckSuspender(Instance* instance, JSObject* maybeSuspender) {
+  MOZ_ASSERT(SASigCheckSuspender.failureMode == FailureMode::FailOnNullPtr);
+  JSContext* cx = instance->cx();
+  if (!maybeSuspender || !maybeSuspender->is<SuspenderObject>() ||
+      &maybeSuspender->as<SuspenderObject>() !=
+          cx->wasm().promiseIntegration.activeSuspender()) {
+    // Wrong suspender
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_JSPI_INVALID_SUSPENDER);
+    return nullptr;
+  }
+  SuspenderObject* suspenderObject = &maybeSuspender->as<SuspenderObject>();
+  if (suspenderObject->state() != SuspenderState::Active) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_JSPI_INVALID_STATE);
+    return nullptr;
+  }
+  return suspenderObject;
+}
+
+// Converts promise results into actual function result, or exception/trap
+// if rejected.
+JSObject* GetSuspendingPromiseResult(Instance* instance,
+                                     SuspenderObject* suspender) {
+  MOZ_ASSERT(SASigGetSuspendingPromiseResult.failureMode ==
+             FailureMode::FailOnNullPtr);
+  JSContext* cx = instance->cx();
+  Rooted<SuspenderObject*> suspenderObject(cx, suspender);
+
+  Rooted<PromiseObject*> promise(cx, suspenderObject->suspendingPromise());
+
+  if (promise->state() == JS::PromiseState::Rejected) {
+    RootedValue reason(cx, promise->reason());
+    // Result is also the reason of promise rejection.
+    cx->setPendingException(reason, ShouldCaptureStack::Maybe);
+    return nullptr;
+  }
+
+  // Construct the results object.
+  Rooted<WasmStructObject*> results(
+      cx, instance->constantStructNewDefault(
+              cx, SuspendingFunctionModuleFactory::ResultsTypeIndex));
+  const StructFieldVector& fields = results->typeDef().structType().fields_;
+
+  MOZ_ASSERT(fields.length() <= 1);
+  if (fields.length() == 1) {
+    RootedValue jsValue(cx, promise->value());
+
+    // The struct object is constructed based on returns of exported function.
+    // It is the only way we can get ValType for Val::fromJSValue call.
+    auto bestTier = instance->code().bestTier();
+    const wasm::FuncExport& funcExport =
+        instance->metadata(bestTier).lookupFuncExport(
+            SuspendingFunctionModuleFactory::ExportedFnIndex);
+    const wasm::FuncType& sig =
+        instance->metadata().getFuncExportType(funcExport);
+
+    RootedVal val(cx);
+    MOZ_ASSERT(sig.result(0).storageType() == fields[0].type);
+    if (!Val::fromJSValue(cx, sig.result(0), jsValue, &val)) {
+      return nullptr;
+    }
+    results->storeVal(val, 0);
+  }
+  return results;
+}
+
+// Resolves the promise using results packed by wasm.
+int32_t SetPromisingPromiseResults(Instance* instance,
+                                   SuspenderObject* suspender,
+                                   WasmStructObject* results) {
+  MOZ_ASSERT(SASigSetPromisingPromiseResults.failureMode ==
+             FailureMode::FailOnNegI32);
+  JSContext* cx = instance->cx();
+  Rooted<WasmStructObject*> res(cx, results);
+  Rooted<SuspenderObject*> suspenderObject(cx, suspender);
+  RootedObject promise(cx, suspenderObject->promisingPromise());
+
+  const StructType& resultType = res->typeDef().structType();
+  RootedValue val(cx);
+  // Unbox the result value from the struct, if any.
+  if (resultType.fields_.length() > 0) {
+    MOZ_RELEASE_ASSERT(resultType.fields_.length() == 1);
+    if (!res->getField(cx, /*index=*/0, &val)) {
+      return -1;
+    }
+  }
+  ResolvePromise(cx, promise, val);
+  return 0;
+}
+
+void UpdateSuspenderState(Instance* instance, SuspenderObject* suspender,
+                          UpdateSuspenderStateAction action) {
+  MOZ_ASSERT(SASigUpdateSuspenderState.failureMode == FailureMode::Infallible);
+
+  JSContext* cx = instance->cx();
+  switch (action) {
+    case UpdateSuspenderStateAction::Enter:
+      suspender->enter(cx);
+      break;
+    case UpdateSuspenderStateAction::Suspend:
+      suspender->suspend(cx);
+      break;
+    case UpdateSuspenderStateAction::Resume:
+      suspender->resume(cx);
+      break;
+    case UpdateSuspenderStateAction::Leave:
+      suspender->leave(cx);
+      break;
+    default:
+      MOZ_CRASH();
+  }
+}
+
+}  // namespace js::wasm
+#endif  // ENABLE_WASM_JSPI