I have a really strange question (I know, these types of optimizations are whacky and 99% of the time useless, but this is just an interesting case):
Suppose we have a struct with 1 method and 1 function pointer, that is assigned at RT. Considering when the functions are identical, the call to function pointer will require an additional pointer resolution and thus little-bit slower (where a method call is just a static offset).
Can we somehow eliminate this pointer resolution? (given that our dynamically assigned function pointer will never change afterwards)
The only solution I've thought of was to declare this "function pointer" as a static array of bytes, copy the code there, set memory to be executable and call it. That way the call also be identical to a "method" call.
Are there any other ways to achieve this strange run-time "linking"? (if you can call it this way :))
Related
As per my understanding when we call a non-inlined function like foo() program control will shift to called function address then store the location of caller and return bank to the caller to another statement after previous function class. But when I implement the class with operator definition will the same process occur or something different happens in favor for operator function?
An operator overload is just a function with a peculiar name.
The compiler translates use of the operator into a function call.
That is, a + b becomes a.operator+(b) or operator+(a, b), depending on how the overload is defined.
(You can also write those out yourself, and it will behave exactly the same but miss the point.)
Note that function call overhead is something I haven't seen anyone worry about during this millennium. It only takes nanoseconds on a reasonably modern machine, unless you make very expensive argument copies – but then you get rid of the copying, not the function.
You will very likely never encounter a situation where getting rid of function calls is your top-priority speed optimisation.
Virtual function calls can matter in very time-sensitive situations, for instance in a tight loop, but those instances are rare.
(And the overhead for that is not the function call per se, but is caused by the late binding.)
In a recursive function in C++, one of its argument is reference type. I just want to know what will happen during the recursive call of the function.
Without reference type, I believe every time the function is called recursively, a new variable will be created in the stack. So with the reference, every time what has been created in stack now is some kind of pointer pointing to the address of the original variable where it is declared,right?
So by using reference in such scenario, I believe sometimes we can save some memory.
Yes, you've got the right idea. Note, of course, that you only save memory if the parameter type is larger than a pointer. A reference to an integer (or maybe even a double) won't save any memory on the stack.
Usually parameter values change during recursion. You can't simply share those across all levels.
Furthermore, when a function is not inlined (and recursion interferes with inlining), passing an argument by reference costs as much space as a pointer.
I have a simple question. I know that after compile a program when I call a function a call stack is generated with the arguments, space for local vars, return point and the registers that i'm charged.
But in object-oriented language like c++, where the compiler stores the reference to the current object? object->instanceMethod() will store the object pointer like an argument in the call stack?
I know the question is generalist and thanks for the answer
It's implementation-defined but in practice you will find that most (all?) C++ compilers generate code which passes the this pointer as a hidden first argument to the function, so you can access it without explicitely specifiying it in the method signature.
In C++, when a member function is called the pointer to the instance on which it will operate (i.e. what will be this inside the function) is implicitly passed alongside the other function arguments/parameters. Actually, different systems use different conventions, so some number of such parameters could be packed into registers and never placed on the stack (this tends to be faster), but your conception is basically sound.
With C++ how do i decide if i should pass an argument by value or by reference/pointer? (tell me the answer for both 32 and 64bits) Lets take A. Is 2 32bit values more less or equal work as a pointer to a 32bit value?
B to me seems like i always should pass by value. C i think i should pass by value but someone told me (however i haven't seen proof) that processors don't handle values not their bitsize and so it is more work. So if i were passing them around would it be more work to pass by value thus byref is faster? Finally i threw in an enum. I think enums should always be by value
Note: When i say by ref i mean a const reference or pointer (can't forget the const...)
struct A { int a, b; }
struct B { int a; }
struct C { char a, b; }
enum D { a,b,c }
void fn(T a);
Now tell me the answer if i were pushing the parameters many times and the code doesn't use a tail call? (lets say the values isnt used until 4 or so calls deep)
Forget the stack size. You should pass by reference if you want to change it, otherwise you should pass by value.
Preventing the sort of bugs introduced by allowing functions to change your data unexpectedly is far more important than a few bytes of wasted stack space.
If stack space becomes a problem, stop using so many levels (such as replacing a recursive solution with an iterative one) or expand your stack. Four levels of recursion isn't usually that onerous, unless your structures are massive or you're operating in the embedded world.
If performance becomes a problem, find a faster algorithm :-) If that's not possible, then you can look at passing by reference, but you need to understand that it's breaking the contract between caller and callee. If you can live with that, that's okay. I generally can't :-)
The intent of the value/reference dichotomy is to control what happens to the thing you pass as a parameter at the language level, not to fiddle with the way an implementation of the language works.
I pass all parameters by reference for consistency, including builtins (of course, const is used where possible).
I did test this in performance critical domains -- worst case loss compared to builtins was marginal. Reference can be quite a bit faster, for non-builtins, and when the calls are deep (as a generalization). This was important for me as I was doing quite a bit of deep TMP, where function bodies were tiny.
You might consider breaking that convention if you're counting instructions, the hardware is register-starved (e.g. embedded), or if the function is not a good candidate for inlining.
Unfortunately, the question you ask is more complex than it appears -- the answer may vary greatly by your platform, ABI, calling conventions, register counts, etc.
A lot depends on your requirement but best practice is to pass by reference as it reduces the memory foot print.
If you pass large objects by value, a copy of it is made in memory andthe copy constructor is called for making a copy of this.
So it will take more machine cycles and also, if you pass by value, changes are not reflected in the original object.
So try passing them by reference.
Hope this has been helpful to you.
Regards, Ken
First, reference and pointers aren't the same.
Pass by pointer
Pass parameters by pointers if any/some of these apply:
The passed element could be null.
The resource is allocated inside the called function and the caller is responsible should be responsible for freeing such a resource. Remember in this case to provide a free() function for that resource.
The value is of a variable type, like for example void*. When it's type is determined at runtime or depending on the usage pattern (or hiding implementation - i.e Win32 HANDLE), such as a thread procedure argument. (Here favor c++ templates and std::function, and use pointers for this purpose only if your environment does not permit otherwise.
Pass by reference
Pass parameters by reference if any/some of these apply:
Most of the time. (prefer passing by const reference)
If you want the modifications to the passed arguments to be visible to the caller. (unless const reference is used).
If the passed argument is never null.
If you know what is the passed argument type and you have control over function's signature.
Pass by copy
Pass a copy if any/some of these apply:
Generally try to avoid this.
If you want to operate on a copy of the passed argument. i.e you know that the called function would create a copy anyway.
With primitive types smaller than the system's pointer size - as it makes no performance/memory difference compared to a const ref.
This is tricky - when you know that the type implements a move constructor (such as std::string in C++11). It then looks as if you're passing by copy.
Any of these three lists can go more longer, but these are - I would say - the basic rules of thumb.
Your complete question is a bit unclear to me, but I can answer when you would use passing by value or by reference.
When passing by value, you have a complete copy of the parameter into the call stack. It's like you're making a local variable in the function call initialized with whatever you passed into it.
When passing by reference, you... well, pass by reference. The main difference is that you can modify the external object.
There is the benefit of reducing memory load for large objects passing by reference. For basic data types (32-bit or 64-bit integers, for example), the performance is negligible.
Generally, if you're going to work in C/C++ you should learn to use pointers. Passing objects as parameters will almost always be passed via a pointer (vs reference). The few instances you absolutely must use references is in the copy constructor. You'll want to use it in the operators as well, but it's not required.
Copying objects by value is usually a bad idea - more CPU to do the constructor function; more memory for the actual object. Use const to prevent the function modifying the object. The function signature should tell the caller what might happen to the referenced object.
Things like int, char, pointers are usually passed by value.
As to the structures you outlined, passing by value will not really matter. You need to do profiling to find out, but on the grand scheme of a program you be better off looking elsewhere for increasing performance in terms of CPU and/or memory.
I would consider whether you want value or reference semantics before you go worrying about optimizations. Generally you would pass by reference if you want the method you are calling to be able to modify the parameter. You can pass a pointer in this case, like you would in C, but idiomatic C++ tends to use references.
There is no rule that says that small types or enums should always be passed by value. There is plenty of code that passes int& parameters, because they rely on the semantics of passing by reference. Also, you should keep in mind that for any relatively small data type, you won't notice a difference in speed between passing by reference and by value.
That said, if you have a very large structure, you probably don't want to make lots of copies of it. This is where const references are handy. Do keep in mind though that const in C++ is not strictly enforced (even if it's considered bad practice, you can always const_cast it away). There is no reason to pass a const int& over an int, although there is a reason to pass a const ClassWithManyMembers& over a ClassWithManyMembers.
All of the structs that you listed I would say are fine to pass by value if you are intending them to be treated as values. Consider that if you call a function that takes one parameter of type struct Rectangle{int x, y, w, h}, this is the same as passing those 4 parameters independently, which is really not a big deal. Generally you should be more worried about the work that the copy constructor has to do - for example, passing a vector by value is probably not such a good idea, because it will have to dynamically allocate memory and iterate through a list whose size you don't know, and invoke many more copy constructors.
While you should keep all this in mind, a good general rule is: if you want refence semantics, pass by refence. Otherwise, pass intrinsics by value, and other things by const reference.
Also, C++11 introduced r-value references which complicate things even further. But that's a different topic.
These are the rules that I use:
for native types:
by value when they are input arguments
by non-const reference when they are mandatory output arguments
for structs or classes:
by const reference when they are input arguments
by non-const reference when they are output arguments
for arrays:
by const pointer when they are input arguments (const applies to the data, not the pointer here, i.e. const TYPE *)
by pointer when they are output arguments (const applies to the data, not the pointer)
I've found that there are very few times that require making an exception to the above rules. The one exception that comes to mind is for a struct or class argument that is optional, in which case a reference would not work. In that case I use a const pointer (input) or a non-const pointer (output), so that you can also pass 0.
If you want a copy, then pass by value. If you want to change it and you want those changes to be seen outside the function, then pass by reference. If you want speed and don't want to change it, pass by const reference.
Is it possible to get the virtual address as an integer of a member function pointer?
I have tried.
void (AClass::*Test)();
Test = &AClass::TestFunc;
int num = *(int*)&Test;
But all that does is get me the virtual address of a jmp to the function. I need the actual functions virtual address.
I know this is old, but since there's no meaningful on-the-subject answer, here I go.
Some things need to be taken into account first.
Member-function calling convention in C++ is called __thiscall. This convention is almost identical to __stdcall, the only significant difference being that, before the effective call is made, ECX is set to be the pointer this of the object of which's method is called.
To illustrate this and answer your question at the same time, let's say that the class AClass has a member function declared like this: int AClass::myFunction(int a, int b) and that we have an instance of AClass called aClassObject.
Here's a rather hackish way to do what you initially asked for AND 'simulate' a AClass::myFunction call on the aClassObject once you obtain the raw pointer:
// declare a delegate, __stdcall convention, as stated above
typedef int (__stdcall *myFunctionDelegate)(int a, int b);
// here's the 'hackish' solution to your question
char myFunctionPtrString[10];
sprintf(myFunctionPtrString, "%d", &AClass::myFunction);
int myFunctionPtr = atoi(myFunctionPtrString);
// now let's call the method using our pointer and the aClassObject instance
myFunctionDelegate myFunction = (myFunctionDelegate)myFunctionPtr;
// before we make the call, we must put a pointer to aClassObject
// in ECX, to finally meet the __thiscall calling convention
int aClassObjectPtr = (int)&aClassObject;
__asm{
mov ecx, aClassObjectPtr
}
// make the call!
myFunction(2, 3);
And of course, the instance can be any instance of type AClass.
No, member function pointers can have a variety of sizes (from 4-16 bytes or more depending on platform, see the table in the article) and cannot reliably fit inside the space of an integer. This is because virtual functions and inheritence can cause the compiler to store several pieces of information in order to call the correct function, so in some cases there is not a simple address.
While I can't say definitively whether there is a portable way to do this, I generally recommend making a static wrapper function to provide this type of external access to a class method. Otherwise even if you succeeded you would be creating very tight coupling of the application to that class implementation.
If this is what I suspect it is, just switch off incremental linking. In the mean time, you're getting the right answer.
My other suspicion is that TestFunc may be virtual. For virtual functions whose address is taken, VC++ fakes up a little thunk that does the vtable lookup, and gives a pointer to that thunk as the address. (This ensures the correct derived function is found when the actual object is of a more-derived type. There are other ways of doing this, but this allows such pointers to be a single pointer and simplifies the calling code, at the cost of double jump when they're called through.) Switch on assembly language output for your program, and look through the result; it should be clear enough what's going on.
Here, too, what you're getting is the correct answer, and there's no way to find out the address of the "actual" function. Indeed, a virtual function doesn't name one single actual function, it names whichever derived function is appropriate for the object in question. (If you don't like this behaviour, make the function non-virtual.)
If you REALLY need the genuine address of the actual function, you've got two options. The annoying one is to write some code to scan the thunk to find out the vtable index of the function. Then look in the vtable of the object in question to get the function.
(But note that taking the address of a non-virtual function will give you the address of the actual function to call, and not a thunk -- so you'd have to cater for this possibility as well. The types of pointers to virtual functions and pointers to non-virtual functions are the same.)
The easier one is to make a non-virtual function that contains the code for each virtual function, then have each virtual function call the non-virtual function. That gives you the same behaviour as before. And if you want to find out where the code is, take the address of the non-virtual function.
(In either case, this would be difficult to make work well, it would be annoying, and it would be rather VC++-specific -- but you could probably make it happen if you're willing to put in the effort.)