Suppose I have a function in a single threaded program that looks like this
void f(some arguments){
char buffer[32];
some operations on buffer;
}
and f appears inside some loop that gets called often, so I'd like to make it as fast as possible. It looks to me like the buffer needs to get allocated every time f is called, but if I declare it to be static, this wouldn't happen. Is that correct reasoning? Is that a free speed up? And just because of that fact (that it's an easy speed up), does an optimizing compiler already do something like this for me?
For implementations that use a stack for local variables, often times allocation involves advancing a register (adding a value to it), such as the Stack Pointer (SP) register. This timing is very negligible, usually one instruction or less.
However, initialization of stack variables takes a little longer, but again, not much. Check out your assembly language listing (generated by compiler or debugger) for exact details. There is nothing in the standard about the duration or number of instructions required to initialize variables.
Allocation of static local variables is usually treated differently. A common approach is to place these variables in the same area as global variables. Usually all the variables in this area are initialized before calling main(). Allocation in this case is a matter of assigning addresses to registers or storing the area information in memory. Not much execution time wasted here.
Dynamic allocation is the case where execution cycles are burned. But that is not in the scope of your question.
No, it's not a free speedup.
First, the allocation is almost free to begin with (since it consists merely of adding 32 to the stack pointer), and secondly, there are at least two reasons why a static variable might be slower
- you lose cache locality. Data allocated on the stack are going to be in the CPU cache already, so accessing it is extremely cheap. Static data is allocated in a different area of memory, and so it may not be cached, and so it will cause a cache miss, and you'll have to wait hundreds of clock cycles for the data to be fetched from main memory.
- you lose thread safety. If two threads execute the function simultaneously, it'll crash and burn, unless a lock is placed so only one thread at a time is allowed to execute that section of the code. And that would mean you'd lose the benefit of having multiple CPU cores.
So it's not a free speedup. But it is possible that it is faster in your case (although I doubt it). So try it out, benchmark it, and see what works best in your particular scenario.
Incrementing 32 bytes on the stack will cost virtually nothing on nearly all systems. But you should test it out. Benchmark a static version and a local version and post back.
The way it is written now, there is no cost for allocation: the 32 bytes are on the stack. The only real work is you need to zero-initialize.
Local statics is not a good idea here. It wont be faster, and your function can't be used from multiple threads anymore, as all calls share the same buffer. Not to mention that local statics initialization is not guaranteed to be thread safe.
I would suggest that a more general approach to this problem is that if you have a function called many times that needs some local variables then consider wrapping it in a class and making these variables member functions. Consider if you needed to make the size dynamic, so instead of char buffer[32] you had std::vector<char> buffer(requiredSize). This is more expensive than an array to initialise every time through the loop
class BufferMunger {
public:
BufferMunger() {};
void DoFunction(args);
private:
char buffer[32];
};
BufferMunger m;
for (int i=0; i<1000; i++) {
m.DoFunction(arg[i]); // only one allocation of buffer
}
There's another implication of making the buffer static, which is that the function is now unsafe in a multithreaded application, as two threads may call it and overwrite the data in the buffer at the same time. On the other hand it's safe to use a separate BufferMunger in each thread that requires it.
Note that block-level static variables in C++ (as opposed to C) are initialized on first use. This implies that you'll be introducing the cost of an extra runtime check. The branch potentially could end up making performance worse, not better. (But really, you should profile, as others have mentioned.)
Regardless, I don't think it's worth it, especially since you'd be intentionally sacrificing re-entrancy.
If you are writing code for a PC, there is unlikely to be any meaningful speed advantage either way. On some embedded systems, it may be advantageous to avoid all local variables. On some other systems, local variables may be faster.
An example of the former: on the Z80, the code to set up the stack frame for a function with any local variables was pretty long. Further, the code to access local variables was limited to using the (IX+d) addressing mode, which was only available for 8-bit instructions. If X and Y were both global/static or both local variables, the statement "X=Y" could assemble as either:
; If both are static or global: 6 bytes; 32 cycles ld HL,(_Y) ; 16 cycles ld (_X),HL ; 16 cycles ; If both are local: 12 bytes; 56 cycles ld E,(IX+_Y) ; 14 cycles ld D,(IX+_Y+1) ; 14 cycles ld (IX+_X),D ; 14 cycles ld (IX+_X+1),E ; 14 cycles
A 100% code space penalty and 75% time penalty in addition to the code and time to set up the stack frame!
On the ARM processor, a single instruction can load a variable which is located within +/-2K of an address pointer. If a function's local variables total 2K or less, they may be accessed with a single instruction. Global variables will generally require two or more instructions to load, depending upon where they are stored.
With gcc, I do see some speedup:
void f() {
char buffer[4096];
}
int main() {
int i;
for (i = 0; i < 100000000; ++i) {
f();
}
}
And the time:
$ time ./a.out
real 0m0.453s
user 0m0.450s
sys 0m0.010s
changing buffer to static:
$ time ./a.out
real 0m0.352s
user 0m0.360s
sys 0m0.000s
Depending on what exactly the variable is doing and how its used, the speed up is almost nothing to nothing. Because (on x86 systems) stack memory is allocated for all local vars at the same time with a simple single func(sub esp,amount), thus having just one other stack var eliminates any gain. the only exception to this is really huge buffers in which case a compiler might stick in _chkstk to alloc memory(but if your buffer is that big you should re-evaluate your code). The compiler cannot turn stack memory into static memory via optimization, as it cannot assume that the function is going to be used in a single threaded enviroment, plus it would mess with object constructors & destructors etc
If there are any local automatic variables in the function at all, the stack pointer needs to be adjusted. The time taken for the adjustment is constant, and will not vary based on the number of variables declared. You might save some time if your function is left with no local automatic variables whatsoever.
If a static variable is initialized, there will be a flag somewhere to determine if the variable has already been initialized. Checking the flag will take some time. In your example the variable is not initialized, so this part can be ignored.
Static variables should be avoided if your function has any chance of being called recursively or from two different threads.
It will make the function substantially slower on most real cases. This is because the static data segment is not near the stack and you will lose cache coherency, so you will get a cache miss when you try to access it. However when you allocate a regular char[32] on the stack, it is right next to all your other needed data and costs very little to access. The initialization costs of a stack-based array of char are meaningless.
This is ignoring that statics have many other problems.
You really need to actually profile your code and see where the slowdowns are, because no profiler will tell you that allocating a statically-sized buffer of characters is a performance problem.
来源：https://stackoverflow.com/questions/3730000/can-static-local-variables-cut-down-on-memory-allocation-time