如何创建一个不会破坏严格别名的uint8_t数组？ [英] How to create an uint8_t array that does not undermine strict aliasing?
问题描述
我最近问过这个问题：
问题是我正在写一个类型的数组 uint8_t ，并且编译器将其视为可以使用方法的 this 指针$ c> struct T * ），因为 void * 和 code> uint8_t * ）可以总是别名C ++中的任何其他指针。此行为导致错过优化机会。我想避免这一点，当然。所以问题是：我可以声明一个强制执行严格别名的 uint8_t 数组，即编译器将其视为从未使用任何其他类型的指针别名吗？也就是说，我正在寻找类似于 strict_uint8_t 类型的类，它是一个具有特殊别名行为的 uint8_t 。是否有办法实现这一点？
示例代码显示我的意思，借用其他问题和简化。有关详细信息，请阅读链接的问题及其接受的答案：
struct T {
uint8_t * target;
void unpack3bit（char * source，int size）{
while（size> 0）{
uint64_t t = * reinterpret_cast< uint64_t *>
/ **`this-> target`不能被缓存在这里的寄存器中，但
被重载16次，因为编译器
认为`this-> target`可能是别名与this本身。
我想要的是一个特殊的uint8_t类型，不会触发
这种行为。 * /
this-> target [0] = t& 0x7;
this-> target [1] =（t>> 3）& 0x7;
this-> target [2] =（t>> 6）& 0x7;
this-> target [3] =（t>> 9）& 0x7;
this-> target [4] =（t>> 12）& 0x7;
this-> target [5] =（t>> 15）& 0x7;
this-> target [6] =（t>> 18）& 0x7;
this-> target [7] =（t>> 21）& 0x7;
this-> target [8] =（t>> 24）& 0x7;
this-> target [9] =（t>> 27）& 0x7;
this-> target [10] =（t>> 30）& 0x7;
this-> target [11] =（t>> 33）& 0x7;
this-> target [12] =（t>> 36）& 0x7;
this-> target [13] =（t>> 39）& 0x7;
this-> target [14] =（t>> 42）& 0x7;
this-> target [15] =（t>> 45）& 0x7;
Source + = 6;
size- = 6;
target + = 16;
}
}
};
您可以使用 struct ：
<$
类型p $ p> struct X {uint8_t i：CHAR_BIT; X（uint8_t i）：i（i）{}};
这似乎消除了别名pessimization（用gcc 4.9.0测试）： http://goo.gl/S1NbsB
即使 [t] bit-field属性不是类成员类型的一部分（ [class.bit] / 1）使成员位字段似乎必要de -pessimize the generated code。
I recently asked this question:
Using this pointer causes strange deoptimization in hot loop
The problem was that I was writing to an array of type uint8_t and the compiler treated it as if it could alias with the this pointer of the method (of type struct T*), because void* and char* (=uint8_t*) can always alias any other pointer in C++. This behaviour caused a missed optimization opportunity. I want to avoid this, of course. So the question is: Can I declare an uint8_t array that enforces strict aliasing, i.e., that the compiler treats as never aliased with any pointer of another type? I.e., I am looking for something like a strict_uint8_t type that is an uint8_t with special aliasing behaviour. Is there a way to achieve this?
Example code to show what I mean, borrowed from other question and simplified. For more details, read the linked question and its accepted answer:
struct T{
uint8_t* target;
void unpack3bit(char* source, int size) {
while(size > 0){
uint64_t t = *reinterpret_cast<uint64_t*>(source);
/** `this->target` cannot be cached in a register here but has
to be reloaded 16 times because the compiler
thinks that `this->target` could alias with `this` itself.
What I want is a special uint8_t type that does not trigger
this behaviour. */
this->target[0] = t & 0x7;
this->target[1] = (t >> 3) & 0x7;
this->target[2] = (t >> 6) & 0x7;
this->target[3] = (t >> 9) & 0x7;
this->target[4] = (t >> 12) & 0x7;
this->target[5] = (t >> 15) & 0x7;
this->target[6] = (t >> 18) & 0x7;
this->target[7] = (t >> 21) & 0x7;
this->target[8] = (t >> 24) & 0x7;
this->target[9] = (t >> 27) & 0x7;
this->target[10] = (t >> 30) & 0x7;
this->target[11] = (t >> 33) & 0x7;
this->target[12] = (t >> 36) & 0x7;
this->target[13] = (t >> 39) & 0x7;
this->target[14] = (t >> 42) & 0x7;
this->target[15] = (t >> 45) & 0x7;
source+=6;
size-=6;
target+=16;
}
}
};
You can use a struct type (i.e. a strong typedef) backed by a bitfield with base type uint8_t:
struct X { uint8_t i : CHAR_BIT; X(uint8_t i) : i(i) {} };
This appears to eliminate the aliasing pessimization (tested with gcc 4.9.0): http://goo.gl/S1NbsB
Even though "[t]he bit-field attribute is not part of the type of the class member" ([class.bit]/1) making the member a bit-field seems necessary to de-pessimize the generated code.
这篇关于如何创建一个不会破坏严格别名的uint8_t数组？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
