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diff --git a/js/src/jit/shared/IonAssemblerBuffer.h b/js/src/jit/shared/IonAssemblerBuffer.h
new file mode 100644
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+++ b/js/src/jit/shared/IonAssemblerBuffer.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * 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/. */
+
+#ifndef jit_shared_IonAssemblerBuffer_h
+#define jit_shared_IonAssemblerBuffer_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/MathAlgorithms.h"
+
+#include <algorithm>
+
+#include "jit/shared/Assembler-shared.h"
+
+namespace js {
+namespace jit {
+
+// The offset into a buffer, in bytes.
+class BufferOffset {
+  int offset;
+
+ public:
+  friend BufferOffset nextOffset();
+
+  BufferOffset() : offset(INT_MIN) {}
+
+  explicit BufferOffset(int offset_) : offset(offset_) {
+    MOZ_ASSERT(offset >= 0);
+  }
+
+  explicit BufferOffset(Label* l) : offset(l->offset()) {
+    MOZ_ASSERT(offset >= 0);
+  }
+
+  int getOffset() const { return offset; }
+  bool assigned() const { return offset != INT_MIN; }
+
+  // A BOffImm is a Branch Offset Immediate. It is an architecture-specific
+  // structure that holds the immediate for a pc relative branch. diffB takes
+  // the label for the destination of the branch, and encodes the immediate
+  // for the branch. This will need to be fixed up later, since A pool may be
+  // inserted between the branch and its destination.
+  template <class BOffImm>
+  BOffImm diffB(BufferOffset other) const {
+    if (!BOffImm::IsInRange(offset - other.offset)) {
+      return BOffImm();
+    }
+    return BOffImm(offset - other.offset);
+  }
+
+  template <class BOffImm>
+  BOffImm diffB(Label* other) const {
+    MOZ_ASSERT(other->bound());
+    if (!BOffImm::IsInRange(offset - other->offset())) {
+      return BOffImm();
+    }
+    return BOffImm(offset - other->offset());
+  }
+};
+
+inline bool operator<(BufferOffset a, BufferOffset b) {
+  return a.getOffset() < b.getOffset();
+}
+
+inline bool operator>(BufferOffset a, BufferOffset b) {
+  return a.getOffset() > b.getOffset();
+}
+
+inline bool operator<=(BufferOffset a, BufferOffset b) {
+  return a.getOffset() <= b.getOffset();
+}
+
+inline bool operator>=(BufferOffset a, BufferOffset b) {
+  return a.getOffset() >= b.getOffset();
+}
+
+inline bool operator==(BufferOffset a, BufferOffset b) {
+  return a.getOffset() == b.getOffset();
+}
+
+inline bool operator!=(BufferOffset a, BufferOffset b) {
+  return a.getOffset() != b.getOffset();
+}
+
+template <int SliceSize>
+class BufferSlice {
+ protected:
+  BufferSlice<SliceSize>* prev_;
+  BufferSlice<SliceSize>* next_;
+
+  size_t bytelength_;
+
+ public:
+  mozilla::Array<uint8_t, SliceSize> instructions;
+
+ public:
+  explicit BufferSlice() : prev_(nullptr), next_(nullptr), bytelength_(0) {}
+
+  size_t length() const { return bytelength_; }
+  static inline size_t Capacity() { return SliceSize; }
+
+  BufferSlice* getNext() const { return next_; }
+  BufferSlice* getPrev() const { return prev_; }
+
+  void setNext(BufferSlice<SliceSize>* next) {
+    MOZ_ASSERT(next_ == nullptr);
+    MOZ_ASSERT(next->prev_ == nullptr);
+    next_ = next;
+    next->prev_ = this;
+  }
+
+  void putBytes(size_t numBytes, const void* source) {
+    MOZ_ASSERT(bytelength_ + numBytes <= SliceSize);
+    if (source) {
+      memcpy(&instructions[length()], source, numBytes);
+    }
+    bytelength_ += numBytes;
+  }
+
+  MOZ_ALWAYS_INLINE
+  void putU32Aligned(uint32_t value) {
+    MOZ_ASSERT(bytelength_ + 4 <= SliceSize);
+    MOZ_ASSERT((bytelength_ & 3) == 0);
+    MOZ_ASSERT((uintptr_t(&instructions[0]) & 3) == 0);
+    *reinterpret_cast<uint32_t*>(&instructions[bytelength_]) = value;
+    bytelength_ += 4;
+  }
+};
+
+template <int SliceSize, class Inst>
+class AssemblerBuffer {
+ protected:
+  typedef BufferSlice<SliceSize> Slice;
+
+  // Doubly-linked list of BufferSlices, with the most recent in tail position.
+  Slice* head;
+  Slice* tail;
+
+  bool m_oom;
+
+  // How many bytes has been committed to the buffer thus far.
+  // Does not include tail.
+  uint32_t bufferSize;
+
+  // How many bytes can be in the buffer.  Normally this is
+  // MaxCodeBytesPerBuffer, but for pasteup buffers where we handle far jumps
+  // explicitly it can be larger.
+  uint32_t maxSize;
+
+  // Finger for speeding up accesses.
+  Slice* finger;
+  int finger_offset;
+
+  LifoAlloc lifoAlloc_;
+
+ public:
+  explicit AssemblerBuffer()
+      : head(nullptr),
+        tail(nullptr),
+        m_oom(false),
+        bufferSize(0),
+        maxSize(MaxCodeBytesPerBuffer),
+        finger(nullptr),
+        finger_offset(0),
+        lifoAlloc_(8192) {}
+
+ public:
+  bool isAligned(size_t alignment) const {
+    MOZ_ASSERT(mozilla::IsPowerOfTwo(alignment));
+    return !(size() & (alignment - 1));
+  }
+
+  void setUnlimited() { maxSize = MaxCodeBytesPerProcess; }
+
+ private:
+  Slice* newSlice(LifoAlloc& a) {
+    if (size() > maxSize - sizeof(Slice)) {
+      fail_oom();
+      return nullptr;
+    }
+    Slice* tmp = static_cast<Slice*>(a.alloc(sizeof(Slice)));
+    if (!tmp) {
+      fail_oom();
+      return nullptr;
+    }
+    return new (tmp) Slice;
+  }
+
+ public:
+  bool ensureSpace(size_t size) {
+    // Space can exist in the most recent Slice.
+    if (tail && tail->length() + size <= tail->Capacity()) {
+      // Simulate allocation failure even when we don't need a new slice.
+      if (js::oom::ShouldFailWithOOM()) {
+        return fail_oom();
+      }
+
+      return true;
+    }
+
+    // Otherwise, a new Slice must be added.
+    Slice* slice = newSlice(lifoAlloc_);
+    if (slice == nullptr) {
+      return fail_oom();
+    }
+
+    // If this is the first Slice in the buffer, add to head position.
+    if (!head) {
+      head = slice;
+      finger = slice;
+      finger_offset = 0;
+    }
+
+    // Finish the last Slice and add the new Slice to the linked list.
+    if (tail) {
+      bufferSize += tail->length();
+      tail->setNext(slice);
+    }
+    tail = slice;
+
+    return true;
+  }
+
+  BufferOffset putByte(uint8_t value) {
+    return putBytes(sizeof(value), &value);
+  }
+
+  BufferOffset putShort(uint16_t value) {
+    return putBytes(sizeof(value), &value);
+  }
+
+  BufferOffset putInt(uint32_t value) {
+    return putBytes(sizeof(value), &value);
+  }
+
+  MOZ_ALWAYS_INLINE
+  BufferOffset putU32Aligned(uint32_t value) {
+    if (!ensureSpace(sizeof(value))) {
+      return BufferOffset();
+    }
+
+    BufferOffset ret = nextOffset();
+    tail->putU32Aligned(value);
+    return ret;
+  }
+
+  // Add numBytes bytes to this buffer.
+  // The data must fit in a single slice.
+  BufferOffset putBytes(size_t numBytes, const void* inst) {
+    if (!ensureSpace(numBytes)) {
+      return BufferOffset();
+    }
+
+    BufferOffset ret = nextOffset();
+    tail->putBytes(numBytes, inst);
+    return ret;
+  }
+
+  // Add a potentially large amount of data to this buffer.
+  // The data may be distrubuted across multiple slices.
+  // Return the buffer offset of the first added byte.
+  BufferOffset putBytesLarge(size_t numBytes, const void* data) {
+    BufferOffset ret = nextOffset();
+    while (numBytes > 0) {
+      if (!ensureSpace(1)) {
+        return BufferOffset();
+      }
+      size_t avail = tail->Capacity() - tail->length();
+      size_t xfer = numBytes < avail ? numBytes : avail;
+      MOZ_ASSERT(xfer > 0, "ensureSpace should have allocated a slice");
+      tail->putBytes(xfer, data);
+      data = (const uint8_t*)data + xfer;
+      numBytes -= xfer;
+    }
+    return ret;
+  }
+
+  unsigned int size() const {
+    if (tail) {
+      return bufferSize + tail->length();
+    }
+    return bufferSize;
+  }
+  BufferOffset nextOffset() const { return BufferOffset(size()); }
+
+  bool oom() const { return m_oom; }
+
+  bool fail_oom() {
+    m_oom = true;
+#ifdef DEBUG
+    JitContext* context = MaybeGetJitContext();
+    if (context) {
+      context->setOOM();
+    }
+#endif
+    return false;
+  }
+
+ private:
+  void update_finger(Slice* finger_, int fingerOffset_) {
+    finger = finger_;
+    finger_offset = fingerOffset_;
+  }
+
+  static const unsigned SliceDistanceRequiringFingerUpdate = 3;
+
+  Inst* getInstForwards(BufferOffset off, Slice* start, int startOffset,
+                        bool updateFinger = false) {
+    const int offset = off.getOffset();
+
+    int cursor = startOffset;
+    unsigned slicesSkipped = 0;
+
+    MOZ_ASSERT(offset >= cursor);
+
+    for (Slice* slice = start; slice != nullptr; slice = slice->getNext()) {
+      const int slicelen = slice->length();
+
+      // Is the offset within the bounds of this slice?
+      if (offset < cursor + slicelen) {
+        if (updateFinger ||
+            slicesSkipped >= SliceDistanceRequiringFingerUpdate) {
+          update_finger(slice, cursor);
+        }
+
+        MOZ_ASSERT(offset - cursor < (int)slice->length());
+        return (Inst*)&slice->instructions[offset - cursor];
+      }
+
+      cursor += slicelen;
+      slicesSkipped++;
+    }
+
+    MOZ_CRASH("Invalid instruction cursor.");
+  }
+
+  Inst* getInstBackwards(BufferOffset off, Slice* start, int startOffset,
+                         bool updateFinger = false) {
+    const int offset = off.getOffset();
+
+    int cursor = startOffset;  // First (lowest) offset in the start Slice.
+    unsigned slicesSkipped = 0;
+
+    MOZ_ASSERT(offset < int(cursor + start->length()));
+
+    for (Slice* slice = start; slice != nullptr;) {
+      // Is the offset within the bounds of this slice?
+      if (offset >= cursor) {
+        if (updateFinger ||
+            slicesSkipped >= SliceDistanceRequiringFingerUpdate) {
+          update_finger(slice, cursor);
+        }
+
+        MOZ_ASSERT(offset - cursor < (int)slice->length());
+        return (Inst*)&slice->instructions[offset - cursor];
+      }
+
+      // Move the cursor to the start of the previous slice.
+      Slice* prev = slice->getPrev();
+      cursor -= prev->length();
+
+      slice = prev;
+      slicesSkipped++;
+    }
+
+    MOZ_CRASH("Invalid instruction cursor.");
+  }
+
+ public:
+  Inst* getInstOrNull(BufferOffset off) {
+    if (!off.assigned()) {
+      return nullptr;
+    }
+    return getInst(off);
+  }
+
+  // Get a pointer to the instruction at offset |off| which must be within the
+  // bounds of the buffer. Use |getInstOrNull()| if |off| may be unassigned.
+  Inst* getInst(BufferOffset off) {
+    const int offset = off.getOffset();
+    // This function is hot, do not make the next line a RELEASE_ASSERT.
+    MOZ_ASSERT(off.assigned() && offset >= 0 && unsigned(offset) < size());
+
+    // Is the instruction in the last slice?
+    if (offset >= int(bufferSize)) {
+      return (Inst*)&tail->instructions[offset - bufferSize];
+    }
+
+    // How close is this offset to the previous one we looked up?
+    // If it is sufficiently far from the start and end of the buffer,
+    // use the finger to start midway through the list.
+    int finger_dist = abs(offset - finger_offset);
+    if (finger_dist < std::min(offset, int(bufferSize - offset))) {
+      if (finger_offset < offset) {
+        return getInstForwards(off, finger, finger_offset, true);
+      }
+      return getInstBackwards(off, finger, finger_offset, true);
+    }
+
+    // Is the instruction closer to the start or to the end?
+    if (offset < int(bufferSize - offset)) {
+      return getInstForwards(off, head, 0);
+    }
+
+    // The last slice was already checked above, so start at the
+    // second-to-last.
+    Slice* prev = tail->getPrev();
+    return getInstBackwards(off, prev, bufferSize - prev->length());
+  }
+
+  typedef AssemblerBuffer<SliceSize, Inst> ThisClass;
+
+  class AssemblerBufferInstIterator {
+    BufferOffset bo_;
+    ThisClass* buffer_;
+
+   public:
+    explicit AssemblerBufferInstIterator(BufferOffset bo, ThisClass* buffer)
+        : bo_(bo), buffer_(buffer) {}
+    void advance(int offset) { bo_ = BufferOffset(bo_.getOffset() + offset); }
+    Inst* next() {
+      advance(cur()->size());
+      return cur();
+    }
+    Inst* peek() {
+      return buffer_->getInst(BufferOffset(bo_.getOffset() + cur()->size()));
+    }
+    Inst* cur() const { return buffer_->getInst(bo_); }
+  };
+};
+
+}  // namespace jit
+}  // namespace js
+
+#endif  // jit_shared_IonAssemblerBuffer_h