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/*
* Copyright (c) Facebook, Inc. and its affiliates.
*
* 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 <folly/lang/SafeAssert.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;
FOLLY_SAFE_DCHECK(unpackPtr(raw) == ptr, "ptr mismatch.");
FOLLY_SAFE_DCHECK(unpackStamp(raw) == stamp, "stamp mismatch.");
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
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