相对运营成本 [英] Relative cost of operations
问题描述
嗨！
我一直在寻找一个C ++相对成本的比较表，因为它对所有人都非常有用。我找到了
这个：
printf / scanf 1000
new / delete 800
trig 。功能500
浮点数100
等等...
但它没有说明一些C ++功能，像虚拟
函数。
与
普通函数相比，调用虚函数的成本是多少？
有谁能告诉我在哪里可以找到C +
+的运营成本表？
谢谢。
PD：对不起我的英语。
Hi!
I have been looking for a comparison table of the relative cost of
operations in C++ because it can be very useful for all. I have found
this:
printf/scanf 1000
new/delete 800
trig. function 500
floating point 100
and so on...
But it doesn′t tell anything about some C++ features, like virtual
functions.
What is the cost of calling a virtual function comparing with a
ordinary function?
Could anyone tell me where can I find tables of operations cost for C+
+?
Thanks.
PD: Sorry for my English.
推荐答案
：
拨打电话的费用是多少虚拟功能与
普通函数相比较？
What is the cost of calling a virtual function comparing with a
ordinary function?
另一个间接级别。
而不是：
PointerToFunction（）;
你有：
PointerToVTable-> PointerToFunction（）;
-
$ b $bTomásóhéilidhe
One more level of indirection.
Instead of having:
PointerToFunction();
you have:
PointerToVTable->PointerToFunction();
--
Tomás ó héilidhe
Tom ?? s ?? h ?? ilidhe写道：
Tom??s ?? h??ilidhe wrote:
：
:
>调用虚函数的成本是多少与普通功能相比？
>What is the cost of calling a virtual function comparing with a
ordinary function?
另一个间接级别。
而不是：
PointerToFunction（）;
One more level of indirection.
Instead of having:
PointerToFunction();
实际上，常规函数通常不是通过指针调用，而是直接
。
Actually, a regular function is often not called through a pointer, but
directly.
你有：
PointerToVTable-> PointerToFunction（）;
you have:
PointerToVTable->PointerToFunction();
实际上，常见的实现更像是
PointerToObject-> PointerToVTable-> PointerToFunction（），所以它是3额外
间接水平。
Actually, the common implementation is more like
PointerToObject->PointerToVTable->PointerToFunction(), so it''s 3 additional
levels of indirection.
Rolf Magnus：
Rolf Magnus:
>而不是：
PointerToFunction（）;
>Instead of having:
PointerToFunction();
实际上，常规函数通常不是通过指针调用的，而是直接调用
。
Actually, a regular function is often not called through a pointer,
but directly.
我不确定你的意思是直接打电话。当一个函数在程序中调用
时，CPU的程序计数器被设置为该函数的地址
。我不知道如何在不使用
的地址的情况下调用该函数。
..
I''m not sure what you mean by "called directly". When a function is
called in a program, the CPU''s program counter is set to the address of
the function. I don''t see how you could call the function without using
its address.
..
>你有：
PointerToVTable-> PointerToFunction（）;
>you have:
PointerToVTable->PointerToFunction();
实际上，常见的实现更像是
PointerToObject-> PointerToVTable-> PointerToFunction（），所以它是3
额外的间接级别。
Actually, the common implementation is more like
PointerToObject->PointerToVTable->PointerToFunction(), so it''s 3
additional levels of indirection.
如果我们在堆栈上有一个对象，那么它的地址只是从堆栈指针偏移的一个
。所以没有：
跳到（address_of_function）;
我们有：
JUMP TO（（stack_pointer + offset） - > vtable-> address_of_function）;
或更简单：
跳到（address_of_object） - > vtable-> address_of_function）;
我认为这相当于另外两个间接级别。在
a CPU上，其时钟频率为2 GHz，周期约为4个时钟，b $ b脉冲，而且间接需要2个周期，我想我们正在寻找在
a延迟8纳秒之后。
-
$ b $bTomásóhéilidhe
If we have an object on the stack, then its address is simply an
offset from the stack pointer. So instead of having:
JUMP TO (address_of_function);
We have:
JUMP TO ( (stack_pointer + offset)->vtable->address_of_function );
Or more simply:
JUMP TO ( address_of_object->vtable->address_of_function );
I think this equates to two more levels of indirection in total. On
a CPU whose clock runs at 2 GHz, and where a cycle is about 4 clock
pulses, and where an indirecton takes 2 cycles, I think we''re looking at
a delay of something like 8 nanoseconds.
--
Tomás ó héilidhe
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