+/*
+ * Copyright 2017 Facebook, Inc.
+ *
+ * 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.
+ */
+
+#pragma once
+
+#include <assert.h>
+#include <stdint.h>
+
+namespace folly {
+
+/**
+ * StampedPtr packs both a pointer to T and a uint16_t into a 64-bit value,
+ * exploiting the fact that current addresses are limited to 48 bits on
+ * all current x86-64 and ARM64 processors.
+ *
+ * For both x86-64 and ARM64, 64-bit pointers have a canonical
+ * form in which the upper 16 bits are equal to bit 47. Intel has
+ * announced a 57-bit addressing mode (see https://software.intel.com/
+ * sites/default/files/managed/2b/80/5-level_paging_white_paper.pdf),
+ * but it is not yet available. The first problematic platform will
+ * probably be ARMv8.2, which supports 52-bit virtual addresses.
+ *
+ * This code works on all of the platforms I have available for test,
+ * and probably on all currently-shipping platforms that have a hope of
+ * compiling folly. Rather than enumerating the supported platforms via
+ * ifdef, this code dynamically validates its packing assumption in debug
+ * builds on each call to a mutating function. Presumably by the time we
+ * are running this process in an operating system image that can address
+ * more than 256TB of memory, RAM cost and the latency of 128-bit CAS
+ * will have improved enough that this optimization is no longer impactful.
+ *
+ * A common approach to this kind of packing seems to be to just assume
+ * the top 16 bits are zero, but https://github.com/LuaJIT/LuaJIT/issues/49
+ * indicates that ARM64 platforms in the wild are actually setting bit 47
+ * in their stack addresses. That means that we need to extend bit 47 to
+ * do the right thing (it's not expensive, it compiles to one instruction
+ * on x86-64 and arm64).
+ *
+ * Compare to PackedSyncPtr and DiscriminatedPtr, which perform similar
+ * packing but add additional functionality. The name is taken from
+ * Java's AtomicStampedReference. Unlike PackedSyncPtr, which tries to
+ * act pointer-like, this class acts more like a pair whose elements are
+ * named ptr and stamp. It also allows direct access to the internal
+ * raw field: since we're already at the metal you might want to play
+ * additional games. It is guaranteed that a zero raw value gets decoded
+ * as a (ptr,stamp) of (nullptr,0).
+ */
+template <typename T>
+struct StampedPtr {
+ /**
+ * The packing is not guaranteed, except that it is guaranteed that
+ * raw == 0 iff ptr() == nullptr && stamp() == 0.
+ */
+ uint64_t raw;
+
+ /* IMPORTANT: default initialization doesn't result in a sane state */
+
+ T* ptr() const {
+ return unpackPtr(raw);
+ }
+
+ uint16_t stamp() const {
+ return unpackStamp(raw);
+ }
+
+ void set(T* ptr, uint16_t stamp) {
+ raw = pack(ptr, stamp);
+ }
+
+ void setPtr(T* ptr) {
+ raw = pack(ptr, unpackStamp(raw));
+ }
+
+ void setStamp(uint16_t stamp) {
+ raw = pack(unpackPtr(raw), stamp);
+ }
+
+ static T* unpackPtr(uint64_t raw) {
+ // Canonical form means we need to extend bit 47 of the pointer to
+ // bits 48..63 (unless the operating system never hands those pointers
+ // to us, which is difficult to prove). Signed right-shift of a
+ // negative number is implementation-defined in C++ (not undefined!),
+ // but actually does the right thing on all the platforms I can find.
+ auto extended = static_cast<int64_t>(raw) >> kInternalStampBits;
+ return reinterpret_cast<T*>(static_cast<intptr_t>(extended));
+ }
+
+ static uint16_t unpackStamp(uint64_t raw) {
+ return static_cast<uint16_t>(raw);
+ }
+
+ static uint64_t pack(T* ptr, uint16_t stamp) {
+ auto shifted = static_cast<uint64_t>(reinterpret_cast<uintptr_t>(ptr))
+ << kInternalStampBits;
+ uint64_t raw = shifted | stamp;
+ assert(unpackPtr(raw) == ptr);
+ assert(unpackStamp(raw) == stamp);
+ return raw;
+ }
+
+ private:
+ // On 32-bit platforms it works okay to store a ptr in the top 48
+ // bits of a 64-bit value, but it will result in unnecessary work.
+ // If we align the pointer part at word granularity when we have the
+ // space then no shifting will ever be needed.
+ static constexpr unsigned kInternalStampBits = sizeof(void*) == 4 ? 32 : 16;
+};
+
+template <typename T>
+StampedPtr<T> makeStampedPtr(T* ptr, uint16_t stamp) {
+ return StampedPtr<T>{StampedPtr<T>::pack(ptr, stamp)};
+}
+
+} // namespace folly
projects / folly.git / commitdiff