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diff --git a/js/src/jit/shared/AtomicOperations-shared-jit.cpp b/js/src/jit/shared/AtomicOperations-shared-jit.cpp
new file mode 100644
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+++ b/js/src/jit/shared/AtomicOperations-shared-jit.cpp
@@ -0,0 +1,180 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "jit/AtomicOperations.h"
+
+#if defined(__arm__)
+#  include "jit/arm/Architecture-arm.h"
+#endif
+
+#ifdef JS_HAVE_GENERATED_ATOMIC_OPS
+
+#  include <atomic>
+
+#  include "js/GCAPI.h"
+
+using namespace js;
+using namespace js::jit;
+
+// A "block" is a sequence of bytes that is a reasonable quantum to copy to
+// amortize call overhead when implementing memcpy and memmove.  A block will
+// not fit in registers on all platforms and copying it without using
+// intermediate memory will therefore be sensitive to overlap.
+//
+// A "word" is an item that we can copy using only register intermediate storage
+// on all platforms; words can be individually copied without worrying about
+// overlap.
+//
+// Blocks and words can be aligned or unaligned; specific (generated) copying
+// functions handle this in platform-specific ways.
+
+static constexpr size_t WORDSIZE = sizeof(uintptr_t);
+static constexpr size_t BLOCKSIZE = 8 * WORDSIZE;  // Must be a power of 2
+
+static_assert(BLOCKSIZE % WORDSIZE == 0,
+              "A block is an integral number of words");
+
+// Constants must match the ones in GenerateAtomicOperations.py
+static_assert(JS_GENERATED_ATOMICS_BLOCKSIZE == BLOCKSIZE);
+static_assert(JS_GENERATED_ATOMICS_WORDSIZE == WORDSIZE);
+
+static constexpr size_t WORDMASK = WORDSIZE - 1;
+static constexpr size_t BLOCKMASK = BLOCKSIZE - 1;
+
+namespace js {
+namespace jit {
+
+static bool UnalignedAccessesAreOK() {
+#  ifdef DEBUG
+  const char* flag = getenv("JS_NO_UNALIGNED_MEMCPY");
+  if (flag && *flag == '1') return false;
+#  endif
+#  if defined(__x86_64__) || defined(__i386__)
+  return true;
+#  elif defined(__arm__)
+  return !HasAlignmentFault();
+#  elif defined(__aarch64__)
+  // This is not necessarily true but it's the best guess right now.
+  return true;
+#  else
+#    error "Unsupported platform"
+#  endif
+}
+
+#  ifndef JS_64BIT
+void AtomicCompilerFence() {
+  std::atomic_signal_fence(std::memory_order_acq_rel);
+}
+#  endif
+
+void AtomicMemcpyDownUnsynchronized(uint8_t* dest, const uint8_t* src,
+                                    size_t nbytes) {
+  JS::AutoSuppressGCAnalysis nogc;
+
+  const uint8_t* lim = src + nbytes;
+
+  // Set up bulk copying.  The cases are ordered the way they are on the
+  // assumption that if we can achieve aligned copies even with a little
+  // preprocessing then that is better than unaligned copying on a platform
+  // that supports it.
+
+  if (nbytes >= WORDSIZE) {
+    void (*copyBlock)(uint8_t* dest, const uint8_t* src);
+    void (*copyWord)(uint8_t* dest, const uint8_t* src);
+
+    if (((uintptr_t(dest) ^ uintptr_t(src)) & WORDMASK) == 0) {
+      const uint8_t* cutoff = (const uint8_t*)RoundUp(uintptr_t(src), WORDSIZE);
+      MOZ_ASSERT(cutoff <= lim);  // because nbytes >= WORDSIZE
+      while (src < cutoff) {
+        AtomicCopyByteUnsynchronized(dest++, src++);
+      }
+      copyBlock = AtomicCopyBlockDownUnsynchronized;
+      copyWord = AtomicCopyWordUnsynchronized;
+    } else if (UnalignedAccessesAreOK()) {
+      copyBlock = AtomicCopyBlockDownUnsynchronized;
+      copyWord = AtomicCopyWordUnsynchronized;
+    } else {
+      copyBlock = AtomicCopyUnalignedBlockDownUnsynchronized;
+      copyWord = AtomicCopyUnalignedWordDownUnsynchronized;
+    }
+
+    // Bulk copy, first larger blocks and then individual words.
+
+    const uint8_t* blocklim = src + ((lim - src) & ~BLOCKMASK);
+    while (src < blocklim) {
+      copyBlock(dest, src);
+      dest += BLOCKSIZE;
+      src += BLOCKSIZE;
+    }
+
+    const uint8_t* wordlim = src + ((lim - src) & ~WORDMASK);
+    while (src < wordlim) {
+      copyWord(dest, src);
+      dest += WORDSIZE;
+      src += WORDSIZE;
+    }
+  }
+
+  // Byte copy any remaining tail.
+
+  while (src < lim) {
+    AtomicCopyByteUnsynchronized(dest++, src++);
+  }
+}
+
+void AtomicMemcpyUpUnsynchronized(uint8_t* dest, const uint8_t* src,
+                                  size_t nbytes) {
+  JS::AutoSuppressGCAnalysis nogc;
+
+  const uint8_t* lim = src;
+
+  src += nbytes;
+  dest += nbytes;
+
+  if (nbytes >= WORDSIZE) {
+    void (*copyBlock)(uint8_t* dest, const uint8_t* src);
+    void (*copyWord)(uint8_t* dest, const uint8_t* src);
+
+    if (((uintptr_t(dest) ^ uintptr_t(src)) & WORDMASK) == 0) {
+      const uint8_t* cutoff = (const uint8_t*)(uintptr_t(src) & ~WORDMASK);
+      MOZ_ASSERT(cutoff >= lim);  // Because nbytes >= WORDSIZE
+      while (src > cutoff) {
+        AtomicCopyByteUnsynchronized(--dest, --src);
+      }
+      copyBlock = AtomicCopyBlockUpUnsynchronized;
+      copyWord = AtomicCopyWordUnsynchronized;
+    } else if (UnalignedAccessesAreOK()) {
+      copyBlock = AtomicCopyBlockUpUnsynchronized;
+      copyWord = AtomicCopyWordUnsynchronized;
+    } else {
+      copyBlock = AtomicCopyUnalignedBlockUpUnsynchronized;
+      copyWord = AtomicCopyUnalignedWordUpUnsynchronized;
+    }
+
+    const uint8_t* blocklim = src - ((src - lim) & ~BLOCKMASK);
+    while (src > blocklim) {
+      dest -= BLOCKSIZE;
+      src -= BLOCKSIZE;
+      copyBlock(dest, src);
+    }
+
+    const uint8_t* wordlim = src - ((src - lim) & ~WORDMASK);
+    while (src > wordlim) {
+      dest -= WORDSIZE;
+      src -= WORDSIZE;
+      copyWord(dest, src);
+    }
+  }
+
+  while (src > lim) {
+    AtomicCopyByteUnsynchronized(--dest, --src);
+  }
+}
+
+}  // namespace jit
+}  // namespace js
+
+#endif  // JS_HAVE_GENERATED_ATOMIC_OPS