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diff --git a/js/src/jit/x86-shared/AssemblerBuffer-x86-shared.h b/js/src/jit/x86-shared/AssemblerBuffer-x86-shared.h
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+++ b/js/src/jit/x86-shared/AssemblerBuffer-x86-shared.h
@@ -0,0 +1,256 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ *
+ * ***** BEGIN LICENSE BLOCK *****
+ * Copyright (C) 2008 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * ***** END LICENSE BLOCK ***** */
+
+#ifndef jit_x86_shared_AssemblerBuffer_x86_shared_h
+#define jit_x86_shared_AssemblerBuffer_x86_shared_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Likely.h"
+#include "mozilla/Vector.h"
+
+#include <stdarg.h>
+#include <stddef.h>
+#include <stdint.h>
+
+#include "jit/JitContext.h"
+#include "jit/JitSpewer.h"
+#include "jit/ProcessExecutableMemory.h"
+#include "js/AllocPolicy.h"
+#include "js/Vector.h"
+
+// Spew formatting helpers.
+#define PRETTYHEX(x)      \
+  (((x) < 0) ? "-" : ""), \
+      ((unsigned)((x) ^ ((x) >> 31)) + ((unsigned)(x) >> 31))
+
+#define MEM_o "%s0x%x"
+#define MEM_os MEM_o "(,%s,%d)"
+#define MEM_ob MEM_o "(%s)"
+#define MEM_obs MEM_o "(%s,%s,%d)"
+
+#define MEM_o32 "%s0x%04x"
+#define MEM_o32s MEM_o32 "(,%s,%d)"
+#define MEM_o32b MEM_o32 "(%s)"
+#define MEM_o32bs MEM_o32 "(%s,%s,%d)"
+#define MEM_o32r ".Lfrom%d(%%rip)"
+
+#define ADDR_o(offset) PRETTYHEX(offset)
+#define ADDR_os(offset, index, scale) \
+  ADDR_o(offset), GPRegName((index)), (1 << (scale))
+#define ADDR_ob(offset, base) ADDR_o(offset), GPRegName((base))
+#define ADDR_obs(offset, base, index, scale) \
+  ADDR_ob(offset, base), GPRegName((index)), (1 << (scale))
+
+#define ADDR_o32(offset) ADDR_o(offset)
+#define ADDR_o32s(offset, index, scale) ADDR_os(offset, index, scale)
+#define ADDR_o32b(offset, base) ADDR_ob(offset, base)
+#define ADDR_o32bs(offset, base, index, scale) \
+  ADDR_obs(offset, base, index, scale)
+#define ADDR_o32r(offset) (offset)
+
+namespace js {
+
+class JS_PUBLIC_API Sprinter;
+
+namespace jit {
+
+// AllocPolicy for AssemblerBuffer. OOMs when trying to allocate more than
+// MaxCodeBytesPerProcess bytes. Use private inheritance to make sure we
+// explicitly have to expose SystemAllocPolicy methods.
+class AssemblerBufferAllocPolicy : private SystemAllocPolicy {
+ public:
+  using SystemAllocPolicy::checkSimulatedOOM;
+  using SystemAllocPolicy::free_;
+  using SystemAllocPolicy::reportAllocOverflow;
+
+  template <typename T>
+  T* pod_realloc(T* p, size_t oldSize, size_t newSize) {
+    static_assert(
+        sizeof(T) == 1,
+        "AssemblerBufferAllocPolicy should only be used with byte vectors");
+    MOZ_ASSERT(oldSize <= MaxCodeBytesPerProcess);
+    if (MOZ_UNLIKELY(newSize > MaxCodeBytesPerProcess)) {
+      return nullptr;
+    }
+    return SystemAllocPolicy::pod_realloc<T>(p, oldSize, newSize);
+  }
+  template <typename T>
+  T* pod_malloc(size_t numElems) {
+    static_assert(
+        sizeof(T) == 1,
+        "AssemblerBufferAllocPolicy should only be used with byte vectors");
+    if (MOZ_UNLIKELY(numElems > MaxCodeBytesPerProcess)) {
+      return nullptr;
+    }
+    return SystemAllocPolicy::pod_malloc<T>(numElems);
+  }
+};
+
+class AssemblerBuffer {
+  template <size_t size, typename T>
+  MOZ_ALWAYS_INLINE void sizedAppendUnchecked(T value) {
+    m_buffer.infallibleAppend(reinterpret_cast<unsigned char*>(&value), size);
+  }
+
+  template <size_t size, typename T>
+  MOZ_ALWAYS_INLINE void sizedAppend(T value) {
+    if (MOZ_UNLIKELY(
+            !m_buffer.append(reinterpret_cast<unsigned char*>(&value), size))) {
+      oomDetected();
+    }
+  }
+
+ public:
+  AssemblerBuffer() : m_oom(false) {}
+
+  void ensureSpace(size_t space) {
+    // This should only be called with small |space| values to ensure
+    // we don't overflow below.
+    MOZ_ASSERT(space <= 16);
+    if (MOZ_UNLIKELY(!m_buffer.reserve(m_buffer.length() + space))) {
+      oomDetected();
+    }
+  }
+
+  bool isAligned(size_t alignment) const {
+    return !(m_buffer.length() & (alignment - 1));
+  }
+
+  MOZ_ALWAYS_INLINE void putByteUnchecked(int value) {
+    sizedAppendUnchecked<1>(value);
+  }
+  MOZ_ALWAYS_INLINE void putShortUnchecked(int value) {
+    sizedAppendUnchecked<2>(value);
+  }
+  MOZ_ALWAYS_INLINE void putIntUnchecked(int value) {
+    sizedAppendUnchecked<4>(value);
+  }
+  MOZ_ALWAYS_INLINE void putInt64Unchecked(int64_t value) {
+    sizedAppendUnchecked<8>(value);
+  }
+
+  MOZ_ALWAYS_INLINE void putByte(int value) { sizedAppend<1>(value); }
+  MOZ_ALWAYS_INLINE void putShort(int value) { sizedAppend<2>(value); }
+  MOZ_ALWAYS_INLINE void putInt(int value) { sizedAppend<4>(value); }
+  MOZ_ALWAYS_INLINE void putInt64(int64_t value) { sizedAppend<8>(value); }
+
+  [[nodiscard]] bool append(const unsigned char* values, size_t size) {
+    if (MOZ_UNLIKELY(!m_buffer.append(values, size))) {
+      oomDetected();
+      return false;
+    }
+    return true;
+  }
+
+  size_t size() const { return m_buffer.length(); }
+
+  bool oom() const { return m_oom; }
+
+  bool reserve(size_t size) { return !m_oom && m_buffer.reserve(size); }
+
+  bool swap(Vector<uint8_t, 0, SystemAllocPolicy>& bytes);
+
+  const unsigned char* buffer() const {
+    MOZ_RELEASE_ASSERT(!m_oom);
+    return m_buffer.begin();
+  }
+
+  unsigned char* data() { return m_buffer.begin(); }
+
+ protected:
+  /*
+   * OOM handling: This class can OOM in the ensureSpace() method trying
+   * to allocate a new buffer. In response to an OOM, we need to avoid
+   * crashing and report the error. We also want to make it so that
+   * users of this class need to check for OOM only at certain points
+   * and not after every operation.
+   *
+   * Our strategy for handling an OOM is to set m_oom, and then clear (but
+   * not free) m_buffer, preserving the current buffer. This way, the user
+   * can continue assembling into the buffer, deferring OOM checking
+   * until the user wants to read code out of the buffer.
+   *
+   * See also the |buffer| method.
+   */
+  void oomDetected() {
+    m_oom = true;
+    m_buffer.clear();
+#ifdef DEBUG
+    JitContext* context = MaybeGetJitContext();
+    if (context) {
+      context->setOOM();
+    }
+#endif
+  }
+
+  mozilla::Vector<unsigned char, 256, AssemblerBufferAllocPolicy> m_buffer;
+  bool m_oom;
+};
+
+class GenericAssembler {
+#ifdef JS_JITSPEW
+  Sprinter* printer;
+#endif
+ public:
+  GenericAssembler()
+#ifdef JS_JITSPEW
+      : printer(nullptr)
+#endif
+  {
+  }
+
+  void setPrinter(Sprinter* sp) {
+#ifdef JS_JITSPEW
+    printer = sp;
+#endif
+  }
+
+#ifdef JS_JITSPEW
+  inline void spew(const char* fmt, ...) MOZ_FORMAT_PRINTF(2, 3) {
+    if (MOZ_UNLIKELY(printer || JitSpewEnabled(JitSpew_Codegen))) {
+      va_list va;
+      va_start(va, fmt);
+      spew(fmt, va);
+      va_end(va);
+    }
+  }
+#else
+  MOZ_ALWAYS_INLINE void spew(const char* fmt, ...) MOZ_FORMAT_PRINTF(2, 3) {}
+#endif
+
+#ifdef JS_JITSPEW
+  MOZ_COLD void spew(const char* fmt, va_list va) MOZ_FORMAT_PRINTF(2, 0);
+#endif
+};
+
+}  // namespace jit
+}  // namespace js
+
+#endif /* jit_x86_shared_AssemblerBuffer_x86_shared_h */